[Illustration: HUMPHRY AND HIS “WONDERFUL LAMP.”—Page 306.]
THE
WONDERS OF SCIENCE;
OR,
YOUNG HUMPHRY DAVY
(THE CORNISH APOTHECARY’S BOY, WHO TAUGHT HIMSELF NATURAL
PHILOSOPHY, AND EVENTUALLY BECAME PRESIDENT
OF THE ROYAL SOCIETY).
The Life of a Wonderful Boy
_WRITTEN FOR BOYS._
BY
HENRY MAYHEW,
AUTHOR OF “THE STORY OF THE PEASANT-BOY PHILOSOPHER,” ETC.
“Majestic steep! ah, yet I love,
With many a lingering step to rove,
Thy ivied rocks among;
Thy ivied, wave-beat rocks recall
The former pleasures of my soul,
When life was gay and young.
Enthusiasm—Nature’s child—
Here sung to me her wood-songs wild,
All warm with native fire;
I felt her soul-awakening flame,
It bade my bosom burn for fame,
It bade me strike the lyre.”
DAVY’S _Ode to St. Michael’s Mount in
Cornwall_. Written at the age of 18.
NEW YORK:
HARPER & BROTHERS, PUBLISHERS,
PEARL STREET, FRANKLIN SQUARE.
1878.
A WORD OR TWO BY WAY OF EXPLANATION AND DEDICATION TO MICHAEL FARADAY.
MY DEAR SIR,—
I inscribe your name on one of the fly-leaves of this little book, with
the same devotion as youths are wont to carve upon the trunk of some
forest tree the name of those whom they admire most in the world; and I
do so for many reasons.
First of all, because Davy was, as it were, the foster-father of your
great genius; and, secondly, because it allows me to tell the lads for
whom this book has been written the graceful story of the way in which
the hero of it first befriended you—the young “bookseller’s apprentice,
very fond of experiment, and very averse to trade”—in your own dignified
language; and, moreover, because I know your zeal in the cause of
education, and that you are, like all generous minds when fired with the
beauty of fine truths and discoveries, unable to rest, as it were, till
you have imparted them to others, and so made them as happy as yourself
with the wondrous knowledge. Indeed, it is the glorious privilege of
intellectual pursuits that they are not marked by that “selfishness,”
which made you as a boy “desirous of escaping from trade;” for wisdom,
happily, can share its riches with the world, and yet be all the richer
for the sharing. None know better than yourself that every science is
built up of an infinity of such contributions; and that, had the human
mind been impressed with a desire to hoard its intellectual gains—to keep
them locked in the coffers of the brain—how little even _you_—who have
added, by your many profound discoveries, more to the knowledge-fund of
the world than any other single philosopher—could yourself have known.
There is no finer instance, perhaps, of human magnanimity than the
chemist working unseen in his laboratory—watching alone for hours the
action of different kinds of so-called dead matter upon each other, in
the hope of being able to add his little mite of truth to that store
of mental riches, which is to benefit not only his own generation, but
all those to come for ages after. Nor can we detract from the natural
greatness of the act by ascribing it to any lower principle of our
soul—such as that petty craving for praise which we call “vanity” in
women, and a desire for fame among poets and philosophers; for every true
scientific mind knows that there is sufficient reward in the intense
beauty of a new discovery—the first flash across the brain of some deep
insight into the mysteries of Nature—to repay him, over and over again,
for all the long puzzling of his thoughts; and that, were he even alone
in the world, without a voice to cheer him on, he must still continue
spelling out passage after passage of the Great Poem of Creation, from
the mere love of the Poem itself.
I can readily understand that it was some such generous purpose that
first rendered you anxious “to enter the service of science, which,”
as you say, you “imagined made its pursuers amiable and liberal,” for
this is impressed in every line of the following letter, which becomes
peculiarly interesting as a record of the circumstances which brought
together two of the greatest chemical geniuses that the world has yet
seen.
“To J. A. PARIS, M.D.
“ROYAL INSTITUTION, _Dec. 23, 1829_.
“MY DEAR SIR,—You asked me to give you an account of my first
introduction to Sir H. Davy, which I am very happy to do, as I
think the circumstance will bear testimony to his goodness of
heart.
“When I was a bookseller’s apprentice, I was very fond of
experiment, and very averse to trade. It happened that a
gentleman, a member of the Royal Institution, took me to hear
some of Sir H. Davy’s last lectures in Albemarle Street. I took
notes, and afterwards wrote them out more fairly in a quarto
volume.
“My desire to escape from trade, which I thought vicious and
selfish, and to enter into the service of Science, which I
imagined made its pursuers amiable and liberal, induced me at
last to take the bold and simple step of writing to Sir H.
Davy, expressing my wishes, and a hope that, if an opportunity
came in his way, he would favour my views; at the same time, I
sent the notes I had taken at his lectures.
“The answer, which makes all the point of my communication, I
send you in the original, requesting you to take great care of
it, and to let me have it back, for you may imagine how much I
value it.
“You will observe that this took place at the end of the year
1812, and early in 1813 he requested to see me, and told me
of the situation of Assistant in the Laboratory of the Royal
Institution, then just vacant.
“At the same time that he thus gratified my desires as to
scientific employment, he still advised me not to give up
the prospects I had before me, telling me that Science was
a harsh mistress; and, in a pecuniary point of view, but
poorly rewarding those who devoted themselves to her service.
He smiled at my notion of the superior moral feelings of
philosophic men, and said he would leave me to the experience
of a few years to set me right on the matter.
“Finally, through his good efforts, I went to the Royal
Institution early in March of 1813, as Assistant in the
Laboratory; and in October of the same year went with him
abroad, as his assistant in experiments and in writing. I
returned with him in April, 1815, resumed my station in the
Royal Institution, and have, as you know, ever since remained
there.
“I am, dear Sir, very truly yours,
“M. FARADAY.”
The following is the note of Sir H. Davy, alluded to in Mr. Faraday’s
letter:
“TO MR. FARADAY.
“_December 24, 1812._
“SIR,—I am far from displeased with the proof you have given
me of your confidence, and which displays great zeal, power
of memory, and attention. I am obliged to go out of town, and
shall not be settled in town till the end of January: I will
then see you at any time you wish.
“It would gratify me to be of any service to you. I wish it may
be in my power.
“I am, Sir, your obedient, humble servant,
“H. DAVY.”[1]
And now let me add, by way of excuse for the many short-comings of the
present volume, that I have found some little difficulty in developing
my object, which was to show youths how one of the greatest natural
philosophers had, when a lad, like themselves, _made himself_ acquainted
with the principles of science, and thus to induce them to “go and do
likewise;” for, assuredly, there is no education like that self-education
which is sure to follow directly a fervent taste is created for any
particular branch of knowledge. To create such a taste was my sole
motive for writing this book. Nevertheless, when I came to deal with the
subject, I discovered that it was impossible to follow _literally_ the
scientific history of Davy’s mind, since he had begun by adopting the
most flighty theories. To have evolved all his visionary notions when
a lad, in a work that was meant to have an educational tendency, would
have been merely to have taught error. I have, however, in adapting the
book to the present state of science, deviated as little as possible from
the biographical facts, and I have, moreover, in all things striven to
be true to the character of my hero, which after all is the great truth
required in “story-books.” Again, by a pardonable license, I believe I
have made the boy foreshadow some of his after-discoveries—such as the
safety-lamp—and for all these deviations I can only plead a desire to
show youths that they have it in their own power to do as the Cornish
apothecary’s boy did, if they will but set about the work quickened with
the same determination to succeed. It is my belief that our present
system of education begets in the minds of youths too great a sense of
dependence, and too little reliance on their own powers, so that it is
thought by a lad on leaving school to be impossible to learn any thing
without the help of a master to teach it. Now my object in such books as
the present is to show boys that some of the greatest minds the world has
yet seen have been self-taught; and by letting the young note how the
great men, when _they_ were young too, set about the task of informing
themselves, thus to breed in youthful minds not only a faith in their
own capabilities, but a taste for the beauties of knowledge, as well as
a strong purpose to number themselves, if possible, among the future
teachers of mankind.
And now, my dear Sir, let me, in conclusion, thank you for your generous
encouragement of my labours when I was engaged in inquiring into the
condition of the “_London Poor_.” Many know your wisdom, but none are
better acquainted with your goodness than
Yours, very truly,
HENRY MAYHEW.
BONN, _Nov. 25th, 1854_.
CONTENTS.
PAGE
CHAPTER I.
THE WIDOWED AND THE FATHERLESS 15
CHAPTER II.
YOUNG HUMPHRY’S RESOLVES 33
CHAPTER III.
HUMPHRY AND HIS MOTHER 50
CHAPTER IV.
THE FIRST DRINK AT THE WELL 70
CHAPTER V.
THE FIRST GLIMMER OF THE SAFETY-LAMP 93
CHAPTER VI.
THE WONDERS OF HEAT: ITS SOURCES 112
CHAPTER VII.
THE WONDERFUL DIFFUSION OF HEAT 134
CHAPTER VIII.
THE WONDERFUL EFFECTS OF HEAT 193
CHAPTER IX.
THE WONDERFUL EFFECTS OF HEAT—(_continued_) 208
CHAPTER X.
THE WONDERFUL EFFECTS OF HEAT—(_concluded_) 241
CHAPTER XI.
HUMPHRY AND HIS “WONDERFUL LAMP” 285
CHAPTER XII.
HUMPHRY PRACTISES AS A SURGEON—ON HIMSELF 315
CHAPTER XIII.
THE FIRST SUN-PICTURES 329
CHAPTER XIV.
THE WONDERS OF THE REFRACTION OF LIGHT 341
CHAPTER XV.
THE WONDERS OF THE REFRACTION OF LIGHT—(_continued_) 367
CHAPTER XVI.
THE WONDERS OF THE REFLEXION OF LIGHT 405
CHAPTER XVII.
THE WONDERS OF COLOUR AND PHOTOGRAPHY 419
CHAPTER XVIII.
CONCLUSION 445
THE WONDERS OF SCIENCE.
CHAPTER I.
THE WIDOWED AND THE FATHERLESS.
“Well, gentlemen, I have gone over your several accounts, and find that
the debts of the late Mr. Robert Davy amount altogether to the sum of
£1300.”
These words were addressed to a little group of tradesmen and others
who were assembled in a small room over a mercer’s shop, in the town of
Penzance. There was Jan Penberthy, the neighbouring miller, who, though
in his holiday clothes, had sufficient of the flour clinging to his black
eyebrows and whiskers to indicate his calling; and Malachy Carteret, the
carpenter and builder, who had slipped on his best coat, as he ran out
from his work, to be present at the meeting, and had still the brass
ends of his foot-rule projecting from the little fob at the side of his
fustian trousers. There was Mr. Trevisky, too, the sporting lawyer,
in his check shooting-jacket, all over pockets, and those at the hips
large enough for game or law papers, and who held in his hand a square,
horny-looking parchment deed; and close beside him sat the village
apothecary, in a long, snuff-brown greatcoat, between the skirts of
which might be seen, shining in the light, his high boots, that reached
well up to his knees; and there were two or three other of the village
tradespeople besides, all seated at the end of the little apartment, and
who, when they heard the amount of their united claims, exchanged glances
with one another in astonishment at the largeness of the sum.
At a table in front of the little assembly sat Mr. John Tonkin, the old
gentleman who had addressed them. Nor was he the least remarkable among
the company, for he was habited in the costume that had been fashionable
in the previous century. Over his shrivelled and veiny hands flapped deep
lace-ruffles, and the top of his head was white as a twelfth-cake, with
the large powdered wig that surmounted it. The straight, stand-up collar
of his Quaker-cut coat was as if mildewed at the back with the powder
that fell from his peruke, and the large fan-like frill that protruded
from his waistcoat—which was long as a modern groom’s—was speckled
brown in places with snuff, as thick as nutmeg on a custard; while from
underneath the table at which he sat peeped a pair of gold shoe-buckles
and black silk stockings. On a chair by his side lay the cocked hat which
completed his antiquated costume, and the underneath part of which was
greyed with hair-powder and long usage.
Peculiar as was the old gentleman’s dress, yet it had more of a quaint
than comical appearance with him; for his features, though creased with
age, and his form, though slightly bowed with his load of years, were
still of too manly a cast to excite any irreverent feeling, even in the
lightest minds; indeed, he had too stern and austere a look to dispose
any to smile at the oddity of his costume.[2]
After a short pause the old gentleman continued his address to the
company before him. “Now to meet this sum of £1300 there is the property
of the farm at Varfell, which is valued at about £150 a year, and upon
which Mr. Trevisky’s client has a mortgage to a small amount.”[3]
At the mention of the name of the attorney, that gentleman proceeded to
open the square deed before him, and to throw back the huge skins, that
looked like large sheets of bladder, as he glanced his eye down them one
after another.
“This little property, gentlemen,” proceeded Mr. Tonkin, “is all that
the widow and her five children—the eldest of whom, you will permit me
to remind you, is but sixteen years of age—have to subsist upon. Mrs.
Davy, therefore, I think, has a claim to some little indulgence and
sympathy at your hands. Some years must pass before her children are old
enough to obtain a maintenance for themselves, and in the mean time they
have to be supported, educated, and apprenticed. How this is all to be
done upon such slender means is a matter that I need not tell you adds
severely to the widow’s distress; for not only has she the grief to bear
on losing the partner with whom she had lived in happiness for nearly
twenty years, but she has the greater grief, if possible, of knowing that
her children are fatherless, and that she herself lacks the means of
providing for them in comfort. Her sorrow, then, gentlemen, has a double
sting. It arises not only from regret for the past, but a dread of the
future. I am sure, therefore, she will, under her great affliction, meet
with every consideration from you.”
At this point Mr. Malachy Carteret—a little man, remarkable for the
blackness and bushiness of his eyebrows, which grew so close together
as to look like one long one, rather than a pair, and who, from his
being considered a “good prayer-maker” at the chapel to which he
belonged, was always glad of an opportunity of displaying his oratorical
powers—ventured to observe, that he was sure all then present felt for
Mrs. Davy under her trials, but they had most of them children of their
own, and it was their duty to look at home first; for who could tell how
soon they themselves might be called away, and their little ones left in
the same distressing situation, unless their bills were duly paid? “Now
my little account,” proceeded the carpenter, “has been standing so long
as the new house at Varfell has been built, and that were the same year
as Dolly Pentreath died—I mean her as were 102 year old—and that’s some
time agone, you know; so I’m sure no one can’t say as I’ve been hard
about my little matter. But we were a thinking among ourselves, Mr.
Tonkin, that as you’d always been suchy kind friend to the family, and as
we hadn’t no wish to trouble Mrs. Davy under her affliction, that maybe
a—a—you—you—you wudn’t mind becoming security yourself for the debts,
and then they cud stand over for another year or two if need be. You’ll
excuse ma making maself so bould, sir; but I’m a plain man, and think
plain speaking is better than double-dealing at ale times.”
The old gentleman’s brow fell suddenly, and looking the carpenter full in
the face, he said, scornfully, “Though you have no wish to trouble Mrs.
Davy under her afflictions, you would not object, it seems, to involve
her friends in her liabilities. You allude, sir, to my past services,
and surely the recollection of the melancholy occasion which, rendered
such services necessary should have made you less eager after what is
due to you, and less anxious to entangle in the family difficulties a
person who, when he found that his assistance was needed, has never
added to the distress by waiting till asked for it. In this very house,
now thirty-seven years ago, it was my sad lot to see three young girls
deprived of both father and mother in the same week. Mrs. Davy was one,
and the youngest of those three—left almost in her infancy without a
friend or counsellor to help her through the world. And now in her
womanhood the same hard fate attends her—bereft of him whose affection
had made him her protector, and finding her children fatherless, as
she herself was, at the very time when needing most a father’s care.
Surely the remembrance of a double bereavement like this, sir—for
hardly is she out of her orphan age before she is doomed to enter on
her widowhood—should teach you that Providence must have some special
design in visiting so much misery upon this poor lady.[4] Nevertheless,
I am happy to say that Mrs. Davy needs no pecuniary assistance from
her friends, and requires but little indulgence from those to whom her
husband was indebted. It is proposed to increase the mortgage upon the
farm at Varfell to the amount of £1300, though this—even if Mr. Trevisky
can obtain the money at 4 per cent.—will reduce the family income to
less than £100 a year; and immediately the mortgage is completed and the
money paid, your accounts, gentlemen, will be discharged. The reason
of my requesting your attendance here to-day was not to seek to make
any compromise with you, nor to crave any unusual indulgence at your
hand, but merely to ascertain the amount of the collected claims against
the late Mr. Davy, and to inform you that steps were being taken for a
speedy payment of them, so that you might not trouble his widow with any
importunities on the matter. She, poor soul, has enough to bear with in
the privation she has recently suffered, and those wordly ones which
threaten herself and children in the future; but, thank Heaven, she is
prepared to meet all her trials with resignation and courage.”
Then, rising from his seat, he bowed haughtily to the company as he said,
“Now, gentlemen, I wish you a good day.”
The words were no sooner uttered than Mr. Trevisky, who had previously
folded up the deed, and sat fidgetting on his chair for the last quarter
of an hour—now twisting a piece of red tape round and round—then paring
his nails—and then twiddling the brass fox’s-head buttons of his
shooting-jacket—the words were no sooner uttered, we repeat, than the
lawyer started to his feet, and, pulling out his watch, said, half to
himself, “Egad, I shall be in time for the cock-fight yet;” and then,
waving his hand rapidly in the air, shouted as he darted from the room,
“I’ll see to that directly, Mr. Tonkin—I’ll see to it directly.”
Malachy Carteret was the last to leave, for he purposely remained behind
to plead his excuse to Mr. Tonkin for the use he had made of his name.
“I hope no offence, I’m sure, sir,” began the little carpenter. “I always
thoft ma money safe enow, but ya see times is hard, and there’s the men
to pay every Saturday night, and that’s a great pull. No one feels more
for Mrs. Davy than your humble servant M. C. does. Her husband always
behaved honourable to me, and M. C. is ‘ever grateful for past favours,’
as my card says. That there wor a bit of poor Mr. Davy’s handiwork,
worn’t it, Mr. Tonkin?” added Malachy, pointing to the oak mantel-piece,
that was elaborately and beautifully carved with birds, fruit, and
flowers.
“Yes,” returned the old gentleman, dryly: “Mr. Davy presented it to me
when he used to come courting here, at the time his wife and her sisters
were under my care.”
“Ah, he wor very clever with his tools,” continued the carpenter. “‘The
last of the carvers,’ we used to call him. I remember him afore he went
to London to larn the business—he lived with his uncle Robert then. I
used to work for the uncle; indeed I sarved my time with the late Mr.
Davy’s father—the builder, ya know—so, of coose, I cudn’t mean anything
but kindly to the family—only money’s very scarce just now, sir, and I’ve
a-mashes of bills to meet this quarter: so I hope no offence, I hope no
offence, sir. You doan’t want nothing in my way, do you, Mr. Tonkin?”
The old gentleman shook his head.
“Very well, sir,” proceeded the tradesman; “when you do, M. C. will be
proud to take your orders—ever grateful for past favours, sir, and hoping
for a continuance of your kind support. Allow me to give you one of my
new cards, sir; you’ll see I says as much there.” So saying, the pushing
little carpenter thrust one of the printed bits of pasteboard into the
old gentleman’s hand.
Suddenly a loud shout was heard in the street beneath.
Malachy and Mr. Tonkin looked vacantly at one another, as they both
inwardly wondered as to the cause of the noise.
In a few minutes the shouting was repeated, and this time the ear,
quickened by curiosity, could distinguish the shrill cries of the village
boys and women among the rest.
The carpenter, in the excitement of the moment, forgot his customary
obsequiousness, and rushing towards the window threw open the little
diamond-paned casement, making its metal frame twang again as he did so;
and as he craned his neck over into the street he cried, “Oh, Mr. Tonkin,
Mr. Tonkin! here, you never saw suchy thing in ale your life!”
The old gentleman was sufficiently curious to be unable to resist sharing
in the excitement, and proceeded to join the little carpenter, who was
still eagerly surveying the mob that was gathered round about the “Star
Inn,” on the opposite side of the street.
“It’s Squire Giddy’s new conveyance, sir!” exclaimed Malachy. “The
gardener told me last night it had come down from London the day afore,
with all its wheels done up in haybands like an Irish reaper’s legs.
It’s the fust as has ever bin seen in these parts, and so of coose the
whole town has come to have a peep at it.”
Ay, so it had! All Penzance was out to behold the first carriage that had
ever appeared in its streets.
There were the fishwomen from the neighbouring villages of Mousehole and
Newlyn, who had stopped in their rounds to join in the throng, and who
had their “cowals” or panniers of fish slung round the crown of their
broad-brimmed hats, with the load of pilchards glittering like lumps of
silver at their backs. And there were the ruddy-faced oil-girls, who
had put down their heavy pitchers and ceased for a while their cries of
“Buy ma traa-in! Buy ma traa-in!” to eye “the big box upon wheels,” as
they called it. The portly town-crier, too, was there, with his huge
dustman-like bell turned upwards, and looking like a big tulip in his
hand; for having found that his audience had suddenly deserted him, and
were more attracted by the sight of the new conveyance than with his
announcement that the grocer “had just received several chests of the
best tea from London,” the bellman had himself helped to swell the crowd,
and was now as eager as any to obtain a view of the new wonder. Mingling
with these might be seen the forms of the boatmen—half-smugglers,
half-fishermen—from the neighbouring shore, with their tight-fitting
blue “Guernseys,” their yellow, greasy-looking, fan-tail hats, and their
large high jack-boots, that bagged about their legs and looked as rusty
as if they had been made out of brown paper; whilst at the outside of the
motley group the eye fell upon the news-boy, mounted on a podgy pony,
with his long tin horn in his hand and pad of “SHERBORNE MERCURYS” under
his arm; and he, in his eagerness to catch sight of the strange-looking
vehicle, was leaning over on one side of the saddle, as a butcher-boy
loves to ride. All the shop people were at their doors: some in long
aprons, and others with white sleeves on their arms over their coats; and
the boys kept darting across from the houses towards the mob; while the
upper windows all down the street were knobbed over with heads, each bent
towards the one grand focus of attraction.
“It’s a queer-looking consarn, aint it, sir?” inquired Malachy; “and
there’s a good bit a work in it, I’ve no doubt.”
“It’s a hideous lumbering affair,” responded the old gentleman, as he
turned away from the window, “and it’s a great pity that persons haven’t
something better to excite their admiration than the follies of the rich;
but there’s such a love of luxury coming over our people, that soon we
shall become as effeminate as those of the East, who are borne about upon
couches when they journey from one place to another. In times past we
were a sturdy, energetic race, inured to hardship, and loving, rather
than avoiding, exercise; but now we must have soft, easy seats, and
beds of down, or we cannot rest. Not many years back the floors of our
nobles’ houses were strewn with rushes, but at present even our gentry
are beginning to find a sanded room unpleasant to their feet, and so they
must needs have soft carpets to tread upon—as if they had all at once
grown as tender-footed as negroes. There’s Squire Austell has already
carpetted his best sitting-room; and mark my words! there’s sufficient
of the monkey in our natures to make his great and little neighbours ape
the Squire’s manners. Ugh! We shall be as unmanly as fiddlers before
many years have passed over our heads. Haven’t we got to drink slops for
breakfast instead of a horn or two of good strong ale, as they did in our
fathers’ time? and do you think, sir, strength, and courage, and energy
are to be got out of teacups? Soon we shall find it impossible to eat
without silver forks, as they do in London already; and, by and by, the
dinner-bell will ring at the same hour as the curfew-bell used to toll in
olden times—for what’s called fashion is setting nearer that way every
day. But, thank goodness, we still dine at noon here, and our parties are
limited to tea and Pope Joan at three o’clock, instead of grand dinners
or dances with Frenchified gavottes, and minuets that begin with the
owls and end with the lark. I hate such new-fangled customs! they would
put John Bull into stays like a Frenchman, and exchange his top-boots
for dancing pumps. Take my word for it, sir, since that four-wheeled
aid-to-laziness has appeared in our town we shall shortly find every
one of our would-be fine ladies unable to stir a yard from their homes
without one.”[5]
“You’re quite right, Mr. Tonkin, _quite_ right, sir,” chimed in the
little carpenter; “our rich folk are getting more proud and fond of
luxuries and vanities every day of their lives. Why, what do you think?
they’re talking of putting cushions to all the seats of the pews in our
chapel, sir, just because the gentlefolks has ’em to their sittings in
Madern church! and I give you my word, sir, I’ve only just finished
setting a bright polished steel grate in the withdrawing-room, as they
call it, at Castle Horneck. It’s just bin had down from London, and I
declare one might see to shave one’s self in any part of it. It never
was made to put a fire in, I’m sure. But there’s nothing I can do for
you in my little way; is there, Mr. Tonkin? I’ve got the newest designs
for furniture just arrived from town by the pack-horse as came in last
Monday.”
Mr. Tonkin shook his head, and turned, towards the window.
“I hope no offence, sir,” continued Malachy; “another time, maybe, I
shall be honoured with your commands, and then I can only say that your
orders shall be punctually attended to by your humble servant, M.
C.;” and, having delivered himself of this speech, the pushing little
carpenter bowed himself backwards out of the room.
CHAPTER II.
YOUNG HUMPHRY’S RESOLVES.
“THE FIRST AND LAST INN IN ENGLAND, KEPT BY RICHARD BOTHERAS,” was the
inscription recorded on both sides of a sign-board that swung backwards
and forwards outside a little lonely homestead—more like a cottage than a
tavern—standing at the extreme western point of Cornwall.
The open door revealed a room without a visitor; the floor was white with
sea-sand, and you could tell at a glance, from the evenness with which
the sand was strewn, how scanty were the customers in that part of the
world, for it was plain that no foot had trodden it that day. Above the
ample chimney-board was ranged a row of bright tin mugs, that had been
worn more by polishing than use. The top of the little round deal table
that stood in the centre of the room was as clean as if it had been newly
planed; and over the painted chest of drawers, in one corner, stood what
appeared like a _quire_ of tea-boards, which, together with the written
paper pasted against the wall, and informing the stranger that “parties
were supplied with hot water,” gave one a notion of the many visitors who
came in summer to take tea at the Land’s End.
The host, from lack of custom, was busy in the garden at the back,
digging in refuse fish as manure for his next year’s crop; and the
hostess might be seen in the adjoining out-house, with her arms half
buried in a cushion of dough, preparing the week’s bread for the humble
family.
Suddenly the innkeeper paused, with his foot resting on his spade. His
quick ear had caught the clink of a horse’s hoofs on the neighbouring
granite. The man put his hand across his brows, and looked under it in
every direction to see who was coming.
In a minute or two afterwards he ran to his wife, crying, “Come, tidy
thaself a bit, dame! Here’s Master Davy on his pony Derby jist at hand:
he’s ale in deep black, too.”
The good wife was not long in scraping the dough from her hands, and,
having invested herself in a clean apron, was quickly at the door beside
her husband, awaiting the arrival of the youthful visitor.
[Illustration: HUMPHRY AT THE LAND’S END.—Page 37.]
As the innkeeper had stated, the lad was dressed in deep mourning, and
the beaver of his hat was completely hidden by the broad crape band that
encircled it, while the gloss upon his clothes indicated the recent loss
he had met with. To a casual observer there was but little in the youth’s
appearance to mark the budding genius which inspired him, excepting the
ample forehead and the full black eyes beneath it. He was, however,
generally considered an “extraordinary-looking boy.” He was of diminutive
stature, while the roundness of his shoulders gave him somewhat—as it
has been termed—of a “bucolic aspect.” His hair was chestnut brown, and
hung in neglected curls about his brow; his eyes were dark and piercing,
but the rest of his features were anything but finely chiselled. His
complexion appeared paler than ordinary, from the contrast of the suit of
black in which he was habited, and the dejected air and wet-looking eye
gave a melancholy tone to his appearance that immediately enlisted the
heart towards the boy.[6]
The lad said but a word or two in answer to the greeting of the couple,
and jumping from the saddle, gave the reins to the innkeeper, who
forthwith led the plump little animal round to the stable.
The instant after the youth had disappeared among the rocks.
“Poor lad! he seems deeply cut up; doan’t he, Richard?” said the wife,
following her husband to the shed.
“Ah, that he doa,” returned Richard, as he stooped down under the pony
to loosen its girths. “He’s not tha maze-gerry boy he was a little
while agone, when he used to come over here, Greace, with a hammer a
cracking all the stones that lay in his way into ‘midjons and jouds,’ and
scrambling, like a young goat, over the rocks after some trumpery bit of
stone as took his fancy—just as our Jan do after daws’ eggs.”
“Yes, that he used to,” returned the wife, taking off her clean apron,
and carefully folding it up as she talked; “and I’ve seed Master Humphry
come back, after being out all day among the rocks, with his cap full of
old stones, as he seemed to prize like as if they was lumps of goold,
but such as I wodn’t a picked out of ‘a stomps’—not I. Ah! there’s sad
trouble at Varfell now, take my word for it, Richard. Mistress Davy,
poor thing! has seen enough sorrow in her time to ha’ broke many a stout
heart; and here she’s left with five young ones, and not a ‘cheeld-vean’
among ’em as can get a penny to help her. Master Humphry’s a good
scholard, they say; but larning won’t fill the cupboard, Dick; and they
tells me, down at Penzance, that Mr. Davy (rest his soul!) was too fond
of wasting his money in mines—as we’ve seen many a family ruined with in
our day—to leave his wife anything to fall back upon at this time; though
I’m sure I pity the poor widow and her little ones from the bottom of my
heart, for it isn’t none of their bringings on. Sometimes, do you know,
Dick, I fancy as there’s a spell on that poor woman.”
“Go along with you and your spells!” indignantly shouted the husband, as
he held the pail of water for the pony to drink from; “you’ve always got
some stuff of that kind in your head, Greace.”
“Well, you may talk as you like, but I shall b’lieve in such things to
my dying day,” retorted the superstitious little body. “Didn’t I go to
Madern Well and drop some pins into it; and didn’t they fall with their
pints together, I should like to know? And wasn’t our old sow took ill
the very week afterwards, and died on the very day as we’d settled to
kill her—eh? Oh! you’re as unb’lieving as a Jew; you are, indeed, Dick.”
The innkeeper treated his wife’s argument with a hearty laugh, whereupon
the dame proceeded to cite to him a hundred and one such instances as
were current throughout the county, and in the midst of which we must
leave the worthy couple for the present.
* * * * *
The restless, pensive boy, had wandered to the extreme point of the land,
and here, resting upon a shelf of crag far above the sea, that roared
and dashed against the base, he sat for a while, with his tearful eye
peering across the Atlantic, vacantly gazing at the huge watery disc that
heaved like a giant breast before him. Behind the lad towered tremendous
pinnacles of granite, some with their monster blocks ranged in cubes
one above the other, like Nature’s solid masonry; others with massive
stones standing right on end; while some seemed tossed about in such
confusion as if the “sixth seal” itself had opened, and the heavens had
rained rocks upon the land. Here a large square lump protruded like a
bond-stone from the straight sides of some tall pile; there a huge mass
was scored through at the top, leaving the blocks standing up on either
side of it, as if castellated; and there again was seen a ponderous lump,
so delicately balanced on some high peak, that it seemed as if the least
gust would topple it over into the sea beneath; while the outline of the
whole was as jagged as if it had been gnawed, or as if the entire granite
pile were some immense crystal that the sea was gradually dissolving
away. Below the height the crags stretched far out into the sea, their
black and bluff heads peeping up at different distances through the waves
which compassed them with a ring of the whitest foam; while at the end
of the winding, broken line, there rose one rock higher than the rest,
and on top of this glistened the silver tower of the light-house, that,
like a star-tipped wand, pointed the way, as it gave the first glimpse
of home to the returning mariner.[7] Far beyond this, again, the eye
could just trace, in the mist of the distance, the cloud-like islands[8]
studding the crystal ring of the horizon, while all the rest was one wide
desert of water stretching away to the Western World, that even the fancy
was weary-winged in its struggle to reach.
Nor was the scene behind the narrow tongue of land that forms the very
end of our island less grand and solemn than that which lay before it. To
the southward the pathway was along the edge of a precipice that the sea
beneath had scooped into a curve, and here, at one extremity, the ocean
had drilled huge caldron-like cavities in the rock,[9] and pouring into
these boiled with a roar that made the cliffs boom again with the noise.
Beyond this rose the majestic headland of “_Carn-y-Voel_” its summit half
veiled by a light scarf of clouds, and its tall sides, built of granite
cubes, rising straight as a fortress wall from out the sea. Here the
ocean had worked for itself a little bay, where the smooth green water
lay like a mirror, with the shadows of the yellowish-red cliffs above it
reflected deep into the pool, and trembling, as the surface rippled,
into zigzag lines that played with a thousand lights and shades. On the
other side of this bay a low granite cliff jutted out like a buttress,
the green ground above sloping abruptly down; and against this the waves
beat and dashed till the spray played around the rocks like a cloud of
smoke, and sparkled in the sun delicately tinted with many a prismatic
hue. Here, again, the ocean had burrowed into the thick granite wall,
while near the verge of the cliff there was a perpendicular shaft, the
sides of which were smooth and circular, as if they had been drilled out
of the solid rock; and looking down these, as down a dark well, the eye
could see the white waves tumbling and tossing below with a terrible
fury.[10]
On the northern side of the promontory, called in Cornish “_Antyer
Deweth_,” or the Land’s End, the headlands were higher than those even
on the south, for there one tall rock rose out of the waves towering
high into the air, and formed also of granite cubes, which looked in the
distance so like a suit of mail, that it had acquired the name of the
“_Armed Knight_;” and here at the very top of one of the craggy summits a
singular cross of rock was to be seen, while as the eye travelled along
the curved and crumpled shore, far away to the north, it rested on the
point of land known by the name of “Cape Cornwall,” the outlines and
tints of whose slate cliffs, seen through the atmospheric veil, appeared
soft and blue with the haze of distance.
Despite the blocks of granite that protruded through the land, like the
bones of the earth itself, the ground roundabout was rich in parts with
flowers. Now the soil was purple with the richest heaths, and now it was
yellow as a plate of gold with the bloom of the dwarf-furze, the latter
filling the air with a perfume like apricots, while the green patches of
grass were almost iridescent with the various wild flowers that peeped
with their delicate blossoms from out the blades. The air, too, was
savoury with the odour of the sea, and fresh with the spray that, like
a dew, brushed against the cheek. Still amidst the solemn convulsion of
rocks and the vast belt of water which encompassed the beholder as far as
the sight could stretch, a feeling of overpowering loneliness—a sense of
one’s own insignificance and helplessness, such as travellers are said
to feel in deserts—oppressed the mind there—_there_, at the very brink,
as it were, of one’s native country—the last bit of the land with which
all one’s affections and associations were linked—and rapt in a ghastly
silence, that was broken only by the moan-like booming of the monster
sea, as it beat into the cavities of the cliff far beneath the feet, or,
now and then, by the shrill shrieking of the cormorant, or the whirr of
some passing sea-mew’s wing.[11]
The boy sat, as we said, for a while staring vacantly at the waves
that pranced, like curvetting steeds, before him, and as he did so a
heavy teardrop fell now and then on the moss that spotted the rock at
his feet. Sometimes his lips would move, though not a word escaped
them, and he would strike the air with his clenched fist as though
a sudden resolution had crossed his mind; then a shudder would pass
over his frame, and he would clasp his forehead with both his hands,
and sway his body to and fro, as he bent his head almost to his knees.
Presently, after a slight pause, he would raise his head, and with his
neck stretched back, look steadfastly at the heavens as if gazing at some
spirit there. And when this fit was passed, he would resume his seat,
and clasp his hands as in prayer.
Suddenly the boy started to his feet, crying, “Yes, I’ll do it—that I
will. I will rescue them all—every one—from the poverty that threatens
them. I have promised my dead father to do so. I have prayed God to give
me the strength and firmness to carry out my purpose, and in a few years
our home shall be as happy as it’s wretched now. I feel as if I had
just woke from a long dream. What a thoughtless, idle fool, I’ve been!
but it’s past—never to return. Poor mother! I’ve cost her many a tear, I
know, of late. How have I wasted this last year, when I should have been
at some business seeking the means of adding to my mother’s comforts,
instead of lessening the little that is left her at such a time as this!
If I had only worked instead of squandering my time in silly pleasures,
I might now have been a help to her rather than the wretched burden I
am—without the power to earn a crust for myself. What do I know that is
of use to any one? Who would give me a penny for anything that I can do?
and yet I’m old and strong enough to get my own living. Oh, shame! shame!
that I, at my time of life, should have to take from poor mother’s little
store. If I’d had a proper spirit, I should have felt this long ago: but
no matter, it’s ended now; and I’ll go to work and so fill my mind with
knowledge that mother shall soon be as pleased about me, as I know, poor
thing! she has been pained of late. Yes, and father will watch over me;
I know he will. Oh! if I could only have changed before he died, what a
comfort it would have been to him in his last moments: but as it was, I,
like a wretch, let him leave me in doubt as to what was to become of me.
Why didn’t I wake up before? If it had only been a month ago, he might
have felt no pain on my account. Oh, shame! shame! but, thank God, I’m
different now, and I’ll go back home and tell mother all I mean to do.
I’ve the power in me—I know I have. I’ll go back and tell her not to
grieve, and that I’ll do all I can to help her and my brother and sisters
for the future.”[12]
CHAPTER III.
HUMPHRY AND HIS MOTHER.
The resolution once formed, young Humphry hastened to convey the glad
tidings to his mother, and as he rode along he kept talking to himself
all the way, running over the many “fine things” he meant to do for the
future, and dreaming that some day he might perhaps become distinguished
for his learning and wisdom. He amused himself, too, by speculating as
to what he would do with his money if he ever got to be rich. He would
have his little brother John well educated then, and comfortably started
in life. Yes! and he would give up his share in the property at Varfell
to his mother and sisters—_that_ he would do first of all; and if he
grew to be a very wealthy man, he would give a certain sum of money to
the Grammar-school, so that the boys might have a holiday every year on
his birthday—he would like to be able to do _that_, for then he would be
remembered by them as long as the school lasted. Further, he would give
something a year to his aunt Sampson’s Phillis, and his aunt Millett’s
maid as well. Poor Mary Launder and Betty White should get something,
too; and he would have a number of old pensioners besides, that had known
him when he was young. He would take care, moreover, that his pony Derby
and his dog Chloe wanted for nothing in their old age, and wherever he
might be he would have a box of apples sent him at Christmas from the
tree he had planted in the garden when he was a little fellow.[13]
On reaching the humble farm at Varfell, Humphry found his mother seated
beside a table, the top of which was black with a hillock of little
skirts and bodies that she had been busy making up for the children’s
week-day wear. The quick eye of the boy could distinguish as he glanced
at the pile of mourning that the gloss of the bombazeen was dulled in
places with the tears that had fallen upon it.
Humphry, from a sense of the grief that pervaded the house, had entered
the room so softly that his presence was unperceived by the widow, and
for a minute or two he stood watching his mother as she sat there with
her flooded eyes fixed intently on the large carved oak-chair (her late
husband’s handiwork) that stood beside the mantel-piece. Her cheek rested
on her hand, and it was plain by the fixedness of her gaze that the seat
was no longer empty to her, and that her mind was far away in the past.
The sight of that sad wife, widowed almost in her youth, was sufficient
to have touched many a stouter heart than young Humphry’s. The widow’s
hair was still unsilvered by age, and its blackness contrasted forcibly,
and even painfully, with the close white muslin cap that half concealed
it. The dead black of the crape made her cheek as pale as marble, while
the tears that dewed her eyes gave them an almost glassy look, so that
they seemed jettier than usual. Her face, though young in years, was
prematurely old in expression, for the features, which were naturally
well formed, were pinched; and there was an air of mild resignation over
the countenance that told you the poor woman had long ago learnt to bear
affliction, almost without complaint. Nor did it need a second glance to
discern the tenderness and affection of her nature[14]—for though there
was a settled melancholy in her face, there was still so much kindliness
in its expression that the heart could not help extending to her the
sympathy that it knew she would be the first to afford to others who had
seen as much trouble as she herself had in the course of the few years
that had passed over her head.
Humphry drew towards his mother’s chair, and resting against the back
curled his arm gently about her neck. So unexpected, however, was the
embrace, that the widow shrieked with alarm as she was suddenly roused
from her melancholy reverie.
The next moment, pleased at the idleness of her fright, she clasped the
pet boy to her; and while the tears gushed from her eyes she kissed him
again and again, as though she loved him the more now that he and her
other children were all that she had left to engross her affection.
“What! in tears again, mother?” said Humphry, in a tone of kindly
remonstrance. “Nay, do not grieve,” he added, as the widow rested her
head on his bosom, “I have come to promise you that I will do all in my
power for my brother and sisters.”[15]
“But what can _you_ do, Humphry, my good lad?” asked the mother, as she
looked up through her tears and smiled at the youth. “It will take you
some years before you can earn a livelihood, and even then perhaps you
will gain only sufficient for your own wants. What is to become of my
little ones is more than I can bear to think of. How you, too, Humphry,
are to be put out in the world, I’m sure I cannot say. My means, when all
the debts are paid, will be only £100 a-year, if that.”
“There, there; have no trouble on my account, mother,” returned the lad.
“I’ve made up my mind to lay aside all my idle habits, and to set hard to
work at something directly—though I cannot tell what, just now; and you
shall see I won’t be long before I make you all happy here.”
The mother half laughed at the sanguineness of her son, and said, when
she had kissed him for his kindness, “But you talk like a boy, Humphry.
You don’t know how hard it is to earn money yet.”
“Yes I _do_, mother,” replied the determined youth, as he pressed her
hand. “But I feel I have the power in me, and I’ll _do_ it, you shall
see—all by myself too—aye, _that_ I will, if I have to study night and
day. You don’t know what a lesson poor father’s death has been to me. I
never saw you in grief before, and all this last week my mind has been
at work, for your tears were more than I could bear. Not a night of late
has past but I have reproached myself over and over again that I had
wasted the last year of my life, instead of doing something that would
have given me the power to help you at such a time as this. When I heard
Mr. Tonkin, too, talking with you the other day about the money you would
have to live upon, and heard him say that it was high time I should cease
being a burden to you—yes, those were his words, mother—a _burden_” (and
the boy would have turned away from the widow, but she held his hand), “I
felt the blood rush into my face with shame, and a new spirit came over
me. I didn’t say anything to you, mother, at the time, because I thought
I could hardly trust myself; but I went on thinking I _was_ a burden to
you, and the heaviest burden of all, too. So I kept brooding and brooding
it over, until at last I made a solemn determination that, instead of a
burden, I would be a _help_ to you and my brother and sisters for the
future. I have sworn it, mother! I have promised my poor father to do
so this very day—alone among the rocks I made the vow, and I’m sure he
heard me, for I feel as I never felt before, and I _know_ I’ve the power
to do as I have said.”
The mother in her delight hugged the boy passionately to her bosom, and
as her tears fell thick and hot upon him she said through her sobs, “You
_have_ the power, I know, Humphry; and if this sad bereavement which has
come upon us all does but stir you to make use of the genius that is in
you, it will be indeed almost a recompense for the heavy loss we have
sustained. When you were but a child, I used to tell your dear father of
the bright hopes I had of you, Humphry, and that I was sure you would be
very clever some day; though he, poor man! only smiled at my words, and
thought it was my over-fondness that made me fancy as much, saying all
mothers did the same. But _I_ knew differently, Humphry; I could see you
were not like other children, and even from an infant there was hardly
anything babyish about you. When you were only five years old you made
rhymes of your own, and used to recite them in the Christmas gambols, and
I knew there was no little thing of that age that could do the same thing
in these parts.”
“Yes, I’ve often heard you say so, mother,” added the boy, smiling at the
youthful reminiscence.
“You were a very forward child—from a baby I may say, Humphry,” continued
the proud mother, as she passed her fingers through the lad’s hair,
and brushed it from his forehead—for she half forgot her sorrow as the
recollection of her pet boy’s feats stole, one after another, across her
mind. “Why, you were only nine months old when you walked off, all by
yourself; and you could speak as well, and fluently, as a little man,
before you were two years of age. Shall I tell you, too, what you said
when your sister Kitty was born? little sharp thing as you were! The
servant had been assuring you day after day, that when the baby came
you’d be no longer petted in the way you had been—for then, as the maid
said, ‘your nose would be put out of joint.’ This seemed to make a great
impression upon you, for directly you saw little Kitty you put your
chubby fat hand up to your face, and cried, ‘Mamma! my nose not out of
joint at all.’”
“Did I?” laughed Humphry.
“Yes, that you did,” said the mother. “Ay, and before you had learned
to write you used to copy the figures in ‘Æsop’s Fables,’ and print
the names of them in big letters underneath. I really think, too, you
couldn’t have been more than four years old when you could recite a
good part of ‘Pilgrim’s Progress.’ All I know is, you did so before you
could read well the book; for your memory was so great, that anything
you had heard once or twice you could repeat, almost without a mistake,
afterwards; and when you were sent to Mr. Bushell’s school—you remember
old Mr. Bushell, Humphry—you made such rapid progress there in your
reading and writing, that though you were only six years of age, the old
gentleman, against his own interest, recommended your poor father to
remove you to the Grammar-school.[16]
“When you could read, too, I often watched you at your books, and saw
you turn over the pages so fast, that I fancied you were merely counting
them, or hunting for pictures; but on talking to you about the book I
used to find, to my astonishment, that you had read it through in that
short time, and that you really knew all about it, and could give a much
better account of it than children who might have taken hours, or perhaps
days, to get through it.”
Humphry drew closer to his mother, and pressed her hand between his palms
as he looked up in her face, and smiled with delight to hear her run
over all the feats of his youthful genius; for with the history of each
little wonder he felt the faith he wished to have in his own powers grow
stronger in him, and he shook his head proudly as he inwardly thought of
the greater wonders he would achieve in the time to come.
“Go on, mother,” he said, as he seated himself on the stool at the
widow’s feet, “tell me some more things I used to do when I was a little
fellow—tell me some more, they fill me with the same hope as you say
they did you, and I want to have all the trust I can in myself; for I’ve
made up my mind to be a great man, and if I doubt my power to accomplish
the task I have set myself, I shall, perhaps, give it up almost at the
first difficulty. Tell me something more, mother; I want all the faith in
myself you can give.”
“I wish I could give you as much confidence in your own powers, Humphry,
as I have in them,” returned the widow, “though now you begin to speak
with all the aspirations I have longed to see coming upon you; and for
the last year I cannot tell you how grieved I have been to behold one, of
whom I had formed such high hopes, giving himself up to pleasures that
serve to breed only habits of thoughtless amusement rather than wise
reflection.”
“I know I have pained you, mother,” added Humphry, “but it’s all at an
end now. You remember when I was at the Grammar-school, how Mr. Coryton
used to pull my ears for not minding the lessons he set me. But do you
know, mother, I have often thought, that though I learnt little at Mr.
Coryton’s school, it was, perhaps, better after all that I should have
been left to teach myself; for what we learn from our own liking, we
seldom forget; and I am sure I remember more about the books I have had
from Mr. Tonkin, and that I used to read through one after the other as
fast as I could get them, than all the Latin and Greek I was forced to
get by heart at school.”[17]
“Ah, but Mr. Coryton,” interrupted the mother, “was a man little fitted
for teaching youth, Humphry. He was careless about the boys’ studies,
and often very severe for the slightest faults. I remember once you went
to school, unknown to me, with a large plaster on each ear, and when Mr.
Coryton asked you ‘what was the matter with your ears,’ you told him
‘that you had put the plasters on to prevent a mortification.’ But if you
didn’t stand very well with the master, you were at least in high favour
with the boys, Humphry, for you used to do the Latin and English verses
for half the school; and as for writing valentines and love-verses, why
I am sure your play-time was mostly taken up with scribbling rhymes to
first Miss This and then Miss That for some little urchin in a jacket,
who fancied himself to be smitten with the young lady. Then of an evening
you were always to be found under the balcony of the Star Inn—for you
were sheltered there—with a group of boys round about you; and, if there
happened to be a cart on the spot, you would be sure to mount it, and
there you’d remain narrating all kinds of romantic stories to the little
mob of school-fellows who came regularly to listen to you. I never knew
such a boy for story-telling as you were, Humphry! I have many a time
heard you make up the strangest kind of tales out of your own head; and
while I was in the parlour at work, I used to listen to you, as you
and young Batten sat out in the porch, with your arms curled round each
other’s necks, and you would be there hour after hour; for you were never
tired of inventing, nor he tired of listening to the stories of wonder
and terror you both delighted in.”
“I can remember it all well, mother,” added Humphry. “And do you
recollect how fond I was of making fire-works, and how Kitty used to
help me till her fingers were as black as sticks of liquorice with the
gunpowder; and how we used to mix up the composition for our squibs and
crackers on the spring-boards of old Dr. Tonkin’s chamber-horse that
stood in the empty room, when we lived at Penzance, and that the poor old
gentleman used to take his exercise upon in wet weather?”
“Yes, _that_ I do, Humphry,” smiled Mrs. Davy; “and many a time you have
nearly frightened me out of my wits with your ‘thunder-powder,’ as you
called it, which you used to delight in putting under the chairs, so that
the moment any one sat down, there was such an explosion that everybody
in the room felt as though all their bones had been suddenly broken.
Your poor father only perceived in such tricks an idle, thoughtless
disposition; but women see more keenly into character than men, and I
not only recollected, but knew, the quick boy you were, and how rapidly
you could acquire anything to which you applied yourself; besides, I
had noticed your inventive turn from a child, and the force of your
imagination in the stories you made up and the poems you had written—for
at twelve you had composed an epic that I have by me still—and all these
things gave me assurances that one day you would take a foremost place
among the great men of the country. A mother’s heart may have led me to
have these hopes of you, Humphry; my understanding, however, convinced
me that they were not mere dreams begotten by affection, but conclusions
calmly come to after narrowly watching—as a mother only can watch—every
little turn and trait in your character.”
“No, mother!” burst out the boy; “they are _not_ dreams, but clear
foreseeings; and you yourself shall witness the realisation of them
before many years have passed.”
“God grant that I may live to do so, my boy,” murmured the widow, as
she raised her eyes to heaven. “‘Life,’ as some wise man says, ‘has few
better things to give than a talented son,’ and it seems to me there
can be no greater pleasure to a mother’s heart than to witness the
genius which she has watched bud and expand from year to year ripen into
excellent wisdom, and come to be acknowledged and reverenced by the world
at large—no joy more exquisite to a woman’s nature than that which she
must surely experience on finding that the mind which she had tended
from its very dawn—catching up the first glimpses of intellect, and
garnering them in her bosom as household wonders and bright things of
promise—has fulfilled all her best hopes, and that the visions she had
formed of the fame and honour that were to attend her boy in after-life
have not been mere dreams of her admiration or her pride. But rather,
that the being whom she has loved, and wished to have loved by others,
lives to be at length praised and esteemed by all, for the talents and
virtues that she was the first to notice and to foster. This, Humphry,
is the brightest and sweetest reward a woman can meet with in her old
age; and, having reaped it, she parts from life with a sense of duty
fulfilled, and a feeling that the affection with which she welcomed her
child into existence, and the care with which she tended him in his
youth, have not been unprofitably bestowed, but repaid her in the richest
coin the world can offer to a parent.”
Humphry for a while remained in silence, while his mother’s words sank
deep into his soul; then he said, softly, “May it be my proud lot,
mother, to render you such a reward. I am thankful to the Creator that
I have passed through the most dangerous portion of my life with few
errors, and I hope to devote myself for the future to pursuits useful to
mankind, and which in after years may perhaps obtain for me the applause
of enlightened men.”[18]
The widow laid her hand on the boy’s head as he sat at her feet, and she
said, solemnly, “My blessing be on you, my son. May God give you strength
to maintain your noble purpose!”
Then she threw her arms about him, and bursting into a flood of tears,
cried, “Oh, my boy! my boy! you know not how happy you have made me.
Your words are like oil to my wounded heart. Sometimes, of late, I have
wondered why it should please Providence to visit me so sorely—_me_, who
never knowingly injured any one in thought or act. And yet, even almost
in my infancy, I was deprived of father and mother at one blow, so that
the very features of my parents are unremembered by me, and the blessing
of their love a joy I was scarcely allowed to taste. And now, before my
own children are able to help themselves, he who would have been their
best protector is snatched from me, and I again am alone in life, bereft
of the love and care I had hoped to share for years to come, and left
with five young children, and only a woman’s arm to shield them from the
buffetings of the world. It needs no little faith in the goodness of God,
Humphry, to believe that there is a _mercy_ in all this; and often, in
the bitterness of my tribulation, I have been wicked enough to doubt it:
for I, with my mind distempered by suffering, could discover no trace of
kindness in it all. But _now_ I see the purpose of my affliction. It was
to stir you, Humphry, to be a protector to your brother and sisters—to
develop the high and noble nature with which you had been gifted, and
to raise up to me a son, the glory of whose future renown should be
something like a recompense to me for the partner I have lost—a son who
should be the means of contributing not only to the comfort and happiness
of my children, but to the welfare of mankind at large. Yes! I understand
the reason of my trials now: and look you, my dear boy, how good comes
of evil. The first privation I and your aunts suffered was the means of
creating for us such a friend as is seldom met with in this world; I
mean Mr. Tonkin, who was not only a father to me and my sisters, but has
extended his goodness to our children—for you, Humphry, have passed more
of your time with him than under your poor father’s roof. And now, no
sooner is my husband taken from me than _you_—the giddy boy, who had of
late been so absorbed in pleasure that I had almost begun to think the
hopes I had formed were nothing but a mother’s vanity—become quickened
in an instant with a new nature, as if suddenly exalted into manhood,
instinct with generous purposes and noble determinations; and, though you
are but a mere youth in years, ready to supply the place of a father to
your brother and sisters, and a friend and protector to me.”
CHAPTER IV.
THE FIRST DRINK AT THE WELL.
A few months had wrought a great change in the household at Varfell.
The widow, when the stupor of her grief had passed away, and the mental
absorption of her first sorrow had given place to the calm reflection
of melancholy, soon began to see that the comforts and education of her
children demanded energy rather than tears from her.
Then came the struggle. What could _she_ do to help them? And what would
the people think and say if this or that were done?
But Mrs. Davy was not the woman to be daunted by the petty exultation
of neighbours; so that when an opportunity offered for her to embark in
business as a milliner in the neighbouring town, it cost her hardly a
pang—free as she was from all silly pride—to sink from the worldly rank
of the gentlewoman into the humbler station of the trader.
Accordingly, after consulting with Mr. Tonkin upon the matter, she was
duly installed, in conjunction with a young French lady, as dressmaker
and milliner, in a little shop in the town of Penzance.
Nor was Humphry long in finding a fitting occupation. Mr. Tonkin, to
whom the youth had communicated all his determinations, and who loved
the youth almost as if he had been a child of his own (for the greater
part of Humphry’s life had been passed with the old gentleman), was as
pleased as the widow had been to hear of the new spirit that had come
upon the lad; and although the boy’s foster-father was not so sanguine
as his mother had been of the world-wide renown that awaited Humphry
in after life, he had, nevertheless, sufficient faith in the talents
of the youth to believe that he might, by application, ultimately win
his way to competence and respect among the circle of his native town.
Accordingly, when the ardent boy spoke to the calm old man of the fame
and honours he had made up his mind to gain throughout Europe—saying,
with all the fervour of a boy-poet’s nature, that he was resolved his
mind should become a light to all nations, and that his name should be
linked with noble associations in every enlightened country, Mr. Tonkin
smiled incredulously (but still with good humour) at the ambitious
dreams of the lad, and told him he was afraid one so young as he knew not
how difficult it was to excel, even in the most trivial thing, when we
had the entire world for rivals; and that powers which appeared great in
the narrow circle of our own family, grew less and less as the arena of
competition was widened. Therefore, if the youth, instead of regarding
the whole of Christendom as the theatre in which his future powers
were to be displayed, would but limit his views to the humble town of
Penzance, Mr. Tonkin said he thought Humphry might, with industry and
prudence, some day attain a reputable position in the neighbourhood;[19]
adding, that he should consider himself well rewarded for the care and
affection he had bestowed upon Humphry if he should live to see him
settled as a surgeon in his native town.
* * * * *
It was a lovely autumn evening—such an evening as, at the decline of
the year, is known only in those parts of our island which, from the
mildness of their climate, have been styled “the Florence of the North.”
Mr. Tonkin and Humphry had strolled out by Marazion towards St. Michael’s
Mount, journeying along the curved shore of the magnificent bay, with the
ocean spread out on one side, in a broad expanse of unsullied azure, and
fringed with a thin border of silver foam, as the waves came rippling
lazily over the yellow sands. As they sauntered along, the breeze at
sundown began to set from the land towards the ocean, and, sweeping
across the warm earth, it came laden with the perfumes of the many
exotics that bloom in the open air in that part of the world—the garden
of England; for it was just the hour when the flowers love to pour their
odours into the lightened air, like incense from a thousand chalices. The
rays of the declining sun gave a faint tint of purple to the atmosphere,
and the green sward, that was still lustrous with the slanting light,
was striped, here and there, with the long shadows that streamed from
every object intercepting the beams; while the outlines of each form
were growing more and more definite, and the sides and peaks of the
rocks glittered towards the west, as if they were blazoned with red
gold. The brown cattle were quiet in the fields, and the tranquil flocks
on the distant hillsides rested there like clouds; the branches of the
trees beside the roadway were shaggy, almost to the tops, with the long
stalks of wheat that dangled from the twigs, telling of some high-laden
harvest-waggon that had lately swept by them. The white-bellied swallows
skimmed low over the earth in zigzag lines, twittering as they went; and
there was a soothing stillness all around that bathed the soul in balmy
quietude.
Towards the sea the scene was no less beautiful. The ocean was like a
huge green gem, and here and there on its surface tiny boats seemed to
revel in the sundown breeze, now that it had sprung up, and leant over
on one side, as they went ploughing through the liquid field, turning up
the white surf on their way, and leaving far behind them a long trail,
that looked in the distance like a seam upon the water; while in the
offing tall ships stood against the sky, with their sails pouting and
shining white in the sun, like a pigeon’s breast. Nearer the shore rose
the majestic rocky mount of St. Michael, towering above the sea like one
of Nature’s pyramids, with the broken outline of its ivied sides showing
sharp and clear against the grey, ariel distance; one half of it, towards
the east, was dusked in deep rich shadow, while the other, towards the
west, was bathed in such a glory of ruby light, that the Mount shone
as if it had been one huge carbuncle studding the bright shield of the
ocean. Then, in the far west, the sky and the sea were as a sheet of
molten gold; and, almost resting on the ring of the horizon, was seen the
round, liquid orb of the sun, trembling like a well of light, with the
broad beams streaming upwards from it, and tinting the distant masses of
cloud, now ruby and now purple, till they looked like islands of garnet
and amethyst in the heavens.
It was low water, and the couple crossed by the sands from Marazion to
the Mount; and here, after passing the little cluster of fishermen’s
houses that skirted the base of the rocky pyramid, and mounting a short
distance up the cliff, they sat for a while enjoying and discoursing of
the many beauties of the majestic scene that encompassed them on every
side.
And the prospect thence was indeed of the grandest character. The shore
stretched away, revealing headland after headland, to where the Lizard
shot out far into the wave, the rocks there seeming almost phosphorescent
in the sun. Then appeared St. Clement’s Island and the coast towards
the Land’s End, forming a shorter cape, and completing the horn of the
crescent of land towards the west, that looked, as the waves grew crimson
in the sunset, as if bathed in a sea of wine. The ocean here wore its
most imposing attribute of uncontrollable immensity; for the Atlantic,
across the Bay of Biscay to the most western land of Spain, lay on the
south, and melted into distance there; while, beyond the extremity of
our own island, no shore intervened on the north between the line of the
horizon and the land of the New World.
It was a sight that Humphry loved as deeply as old Mr. Tonkin to look
upon, and the couple sat for some time silently watching both the seas
rolling there towards the far distant Spanish and American shores.
The boy, however, less capable of continuous attention to the same
subject, got to weary of the scene sooner than the old man; and when Mr.
Tonkin noticed Humphry’s admiration begin to flag, he availed himself of
the quietude of the time and place to incite a taste in the lad for the
profession he wished him to follow.
Presently the old doctor caught sight of one of the little transparent
zoophytes that had been left on the rocks by the receding tide. In size
it was not larger than a bird’s egg, of a globular form, with several
transparent ridges ranged along it, from pole to pole, as it were, and it
was nearly as pellucid as the purest rock-crystal.
[Illustration: THE FIRST DRINK AT THE WELL.—Page 76.]
“Look, my boy,” said Mr. Tonkin, turning to Humphry, and pointing to the
little ball of jelly at his feet; “here is an orb almost as wonderful
as the sun we have been lately gazing at. It gives light, too, like it;
and though it looks there as if it were only a few drops of the ocean
gelatinised, it is quickened with life, and performs motions that our
wisest engineers can but clumsily imitate.”
The eager boy was about to seize the wonder, so that he might examine it
more minutely.
“Nay, if you touch it,” cried the old man, hastily, as he grasped the
youth’s arm, and held him back, “it will immediately dissolve—thaw, as
it were, to death—so frail is its life, and nothing but a little pool of
water will remain of a creature that once could make the sea glow with
its fire. These little things are by some styled the ‘lucid gems of the
waters.’ By daylight, when in the ocean, they are visible only by the
bright rainbow hues that mark their path as they paddle along; but by
night, Humphry, they blaze with phosphorescent fire, so that some have
termed them ‘the stars of the sea.’ In warm and calm evenings they often
look like balls of light rolling on the surface of the water, and the
more rapid their motion the more intense is the glow they emit. Those
eight transparent ridges you see there,” continued the doctor, as he
pointed with his cane to the tiny watery globe, “support as many rows
of broad, pellucid paddles, and these are all instinct with life, and
by their rapid motion cause the animal to glide, meteor-like, through
the waves. We wonder at the recent invention of the steam-boat, and
speak with pride of the paddle-wheels with which we are to walk upon the
waters; but the tiny paddles here, boy, are far more perfect than any
ever contrived by human ingenuity, for in that little aqueous ball the
cumbrous machinery which is required to move our vessels along, is not
needed, since each float, self-moving without even a visible muscle or
nerve to stir it, keeps time with all the rest.”[20]
“What wonder,” cried the poetic boy, “is here packed in a little living
crystal, as it were, that can make fire flash from what looks almost like
a globe of water, and that can perform the most rapid motions without, as
you say, Mr. Tonkin, any visible means of movement!”
“Yes, indeed, my lad,” went on the old man; “there is a large store of
marvels locked in that little glassy casket. How does it get its food?
How digest it? and how is its frail body nourished? for we can trace
no blood-vessels, nor heart, nor glands—indeed, hardly any organs at
all—in the little clot of half-liquid life. All we know, is, that it is
furnished underneath with so many tentacles or filaments, that serve it
for claws, and that these, which are set round an aperture that we call
a mouth, draw the food it lives upon into its body—which is literally
nothing but a stomach. If we were to watch long enough, we should see
the food thus seized and swallowed gradually dissolve and be reduced to
a fluid state, while the more solid and indigestible portion would be
rejected by the aperture through which it entered. The nutritive matter
we know to be absorbed by the walls of the stomach, every part of which
appears to be endowed with equal power in this respect; and it is then
conveyed to the remoter portion of the body by the simple inbibition of
one part from another, without any proper circulation through vessels.
In some animals of this class the external covering of the body and the
lining of the stomach so closely correspond in their structure as to
admit of being changed one for the other—for the animal may be turned
inside out without its functions being in any way deranged.”
“Can it be?” said Humphry, filled with delight. “Where can I learn these
things, sir? Why was I not taught them at school?”
“You _shall_ learn them, my boy,” replied the old man, pressing the lad’s
hand with pleasure to find the taste that he had longed to develope for
his own favourite study springing up in Humphry’s mind. “And think, if
that little lump of jelly—which is, perhaps, the simplest form of life,
where the vital mechanism is seen in its rudest form—can stir you to so
much wonder—think, I say, Humphry, what admiration will be excited in
you when you come to comprehend the beautiful processes and organism
exhibited in complicate animals like ourselves! If a living, digesting
creature—a thing almost without sense—a mere moving mouth—can appear so
wonderful to you in its structure, what marvels shall you not find in the
constitution of a thinking, speaking, reasoning being like man!”
Then the old doctor ran over to the youth the many sources of knowledge
that the study of human life opens up to the thoughtful and inquiring
mind.
He told the eager boy—as they sat there in the subdued light of the
evening, with the hum of the sea that rippled into the caverns at the
base of the Mount, falling almost musically on the ear—how, in the
organism of the nerves and brain, we get our first insight, rude though
it be, into the subtle processes of the senses, and even the mind itself.
He told him, also, how, in the senses themselves, lay the rudiments of
all the sciences; how, without the sense of vision, there could have
been no “optics,” and consequently no astronomy—for to the blind the
movements of the planets, and even the very existence of the stars, must,
of course, have remained unknown: in like manner, without the sense of
hearing there could have been no “acoustics” and no music; and without
the sense of muscular effort no knowledge of weight, and consequently
of “gravitation”—the main-spring, as it were, of the mechanism of the
universe.
Mr. Tonkin explained to the youth, moreover, that had we been formed
without the exquisite organ of the hand there would have been little work
done, and but little art achieved; and without the organs of the mouth,
there could have been no inter-communication of thought—no transfusion of
mind into mind, by which one wise man nowadays contains stored in his own
brain the wisdom of almost all those who have preceded him. And further,
if we had had no appetites, and no pains nor uneasinesses to stir us to
action, we should, even with the beautiful muscular apparatus with which
our frames are fitted, have remained idle and inactive all our time,
starving to death with delight.
“Some persons,” said Mr. Tonkin, “have supposed that plants may be
susceptible of feeling, as well as ourselves and the rest of the animal
race. But that trees and herbs are incapable of knowing either pain or
pleasure” (he added) “is made evident, physiologically, by the fact
that they are supplied with no organs of locomotion, and consequently
deprived of the means of avoiding the one and seeking the other. For,
so benevolently is the world arranged, that wherever feeling is given,
the power of acting is immediately associated with it; indeed, it
requires hardly a moment’s thought to perceive that it would have been
incompatible with All-Kindness to have made creatures sensible to pain,
and yet have denied them the means of escape from it.”
After this the old man pointed out to Humphry, that in the comparison of
one system of life with another, and so tracing the delicately interwoven
chain of animal creation, we perceive that the first type of sentient
existence was a mere stomach—a life of pure appetite—susceptible of no
other feeling than hunger, and fitted only with organs for seizing and
assimilating its food; while as we advance gradually in the scale of
development, we find nerve after nerve added, and a new set of feelings
and actions brought out, with each new set of fibres. “We discover,
besides,” he continued, “that when a little kernel of nervous matter was
superadded to the previous sentient apparatus, the wondrous sense of
vision was first awakened in animal life, and how the addition of another
such little kernel made an animal for the first time hear, and another
gave the first sense of odour to the world, while another added taste to
the food and drink.”
And when he had thus briefly explained to the youth the uses and
characteristics of the several organs in man and the lower animals,
the old gentleman went on to point out to him how these same organs
were nourished, and the destruction that was continually going on in
the body—“for,” said he, “we cannot move a muscle, not even wink our
eyelids, without wasting some tissue or other”—was being as continually
repaired by the food consumed. Pursuing this subject, he then proceeded
to explain to young Humphry how the blood was made to circulate by means
of the cunningly-wrought chambers of the heart through the veins and
arteries, distributing health and vigour to the different organs in
its course—now renovating the tissues, now depositing little specks of
bone, then extending the filaments of hair, and then exciting thought
and developing feeling in the nerves and brain, stimulating action in
the muscles, and diffusing warmth throughout the whole frame—all these
different functions being performed by the one wondrous substance in
which, even when examined by the highest microscopic power, it was
impossible to detect even the rudiments of the many various tissues it
formed.
“Such,” said Mr. Tonkin, “is a part of the marvellous process of
secretion—a process so subtle that even the wisest can only wonder
in their ignorance concerning the function; for it is a mystery to
them how, by means merely of little glands, so many different things
can be produced from one and the same fluid. How, for instance, skin,
cartilages, muscles, hair, nails, bones, tears, and the infinite variety
of products which our bodies are made up of and evolve, can all come from
the same ruby stream, and that a small nut-like organ only shall be
necessary to eliminate each different substance from it. Then, again,
there is the beautiful process of breathing, by which the vital air is
combined with the blood, and the blue fluid of the veins changed into the
crimson stream of the arteries;” and he recounted to him the while how
respiration among animals was merely a process of burning, accompanied
with the evolution at each exhalation of so much invisible smoke from the
lungs—the same smoke, indeed, as comes from burning charcoal: and he told
Humphry that he would one day come to see how the rotting wood underwent
precisely the same chemical change as the breathing man, and that what is
a process of death and decay in the one is a process of life and health
in the other.
“Indeed,” concluded the old gentleman, “there is, perhaps, no sphere of
knowledge so replete with wonder and beauty as that which unfolds to us
the mysteries of our own existence—no science which gives us greater
wisdom or deeper insight into the constitution of our natures, as well
as that of the elements around us. To comprehend such a subject, even
vaguely, requires an intimacy with almost every branch of learning,
dealing as it does at once with the material and spiritual; while a just
appreciation of the wonders it reveals cannot fail to inspire us with the
highest regard for life, even in its rudest forms, and render us more
keenly alive to suffering than the rest of humanity, from the greater
sense it gives us of the causes of pain, while it arms us, at the same
time, with the means of relieving anguish, restoring health, and often of
prolonging existence.
“Some there are,” he added, “who prefer poetry to philosophy; but
science, Humphry, rightly understood, is merely the translation of the
Great Poem of Nature—that which the Almighty himself conceived when he
designed Creation. There may be high beauty in music, boy; but, to my
mind, there is even higher beauty still in comprehending the phenomena
by which the Creator has fitted us to enjoy it. In the rich glories of
colour there is, certainly, an exquisite feast of visual delight; but
what array of tints, be they ever so beautifully blended—what tracery of
form, be it ever so cunningly put together—can fill the mind with ecstasy
equal to the contemplation of that splendid little translucent globe,
the eye—a crystal world in itself, filled with an infinity of wonders—by
which we are enabled to perceive the light, and to tell one hue from
another? What work of art, however consummate the execution—what picture,
however choice the painting or grand the composition—what architecture,
however commanding the mass or harmonious the details—and what poem, even
though the verse be mellifluent as music on the water, though the imagery
be luminous and profuse as the stars in winter, and the thoughts subtle
as the mountain air, can bear the least comparison, either as regards
the skill of its art, the craft of its design, or the nice adjustment of
its parts, with the organism of the smallest animalcule fashioned by the
Great Artist, Architect, and Poet of All?”
The sentence was barely finished when the sharp report of a gun rattled
amidst the rocks, and Humphry, whose eyes had been turned upward as he
listened to the wonders recounted by the doctor, saw the gull, which but
a moment before he had noticed almost lying on the air, poised on its
white outstretched wings, bound suddenly upwards with a shriek, and the
instant afterwards it tumbled heavily on the crag at Mr. Tonkin’s side.
The old gentleman stretched out his hand and grasped the still warm and
quivering form of the bird. “If the wings of this body had been moved by
some piece of curious mechanism, Humphry,” he said—“if by some cunning
combination of cogwheels, and levers, and springs, it had been made to
beat the air and to rise by clock-work into the sky, how would men have
prized the marvellous apparatus! Monarchs would have given immense wealth
to possess it: and yet the machine would have been, at best, but a clumsy
toy compared with the exquisite arrangement here; for in this wonderful
piece of divine mechanism the force was supplied by means of little
threads of nerves that the unaided eye can scarcely trace—the movement
given by muscles so beautifully elastic that no artificial fabric can
imitate their play—and the bones jointed together so aptly, that when
our wisest engineers wish to get movements in all directions, they can
only copy their arrangement, instead of designing any such hinges for
themselves. Then, again, to give lightness to the whole, these same
bones were filled with air, and the living, flying machine, so made more
buoyant in the thin fluid in which it was destined to soar. But let us
suppose, Humphry, that it might be within the compass of art to reproduce
such an apparatus as this by mechanical means; still what mechanism,
however skilful, could have supplied the wonderful motive power that
lately quickened it? What spring, or arrangement of weights, could
imitate the action of life? Could steam even, or electricity itself, have
moved the wings and guided them, like the subtle principle that stirred
and directed this body only a few moments past? And then, what cunning
engineery could ever have performed the function of the senses? Could
mechanism have made the animal see? Could the galvanic fluid—the most
spiritual, perhaps, of all our motive powers—have made it love its young,
or know when to repair its strength with food? Ah! had the thoughtless
fool who, for wanton sport, Humphry—who for the mere sake of hitting
a moving mark in the air—known and pondered over all this, do you not
think he would have found more pleasure in watching the performance of
all its wondrous functions than in destroying the beautiful principle
which animated them? Had he needed its body for food, hunger would have
excused him. But no! It was simply the petty pleasure—the little spasm
of exultation—that we derive from success in trials of skill which led
him to put an end to the life of the poor bird, that had surely as much
_divine_ right to its place in creation as even a king himself.”
* * * * *
Humphry was overjoyed with the lesson of kindness and wisdom he had
learnt. He had been so enraptured with the knowledge that Mr. Tonkin had
poured into his mind that he sat almost like one entranced, with his
spirit lulled in a dream of bright things he had never heard or thought
of before.
The boy till now had been more smitten with the beauty of creation
than curious as to its mysteries. True, in his romantic visits to the
extremity of the island, as well as to the Mount of St. Michael, he had
been often led to wonder how the huge masses of rock had come there;
and he had many times pondered over the origin of metals, as in his
rambles he had passed the openings of the mines that perforated the
surface of his native country, wondering as he went along why a vein
of one ore should be deposited here, and another there. As he noticed,
too, the Atlantic waves lashing the Land’s End, he would repeatedly
question himself as to what became of the rocks that the sea was for
ever crumbling into sand; and he would form fanciful theories in his own
mind, as to how the detritus of one ancient country became at last the
substratum of some new one. Again, the ebbing and flowing of the ocean
had led his mind to ruminate vaguely upon the mighty pulsation of the
tides, while the sight of the liquid orb of the sun sinking below the
ring of the horizon, away towards the invisible shores of America, had
often turned his thoughts to the revolutions of the planets, and set him
rudely speculating as to the source of the light and the heat of the sun
itself.
Still the youth as yet had found more pleasure in contemplating the
golden glories of sunset, than in seeking to comprehend the wisdom that
designed them. The sea, too, to him had been more an object of grandeur
than a stimulus to thought, while the sight of the rocks had filled his
mind with admiration far oftener than they had quickened it with inquiry.
The mystery, and even the beauty, of the principle of life, however, had
never before been heeded by Humphry; so that, when he heard Mr. Tonkin
relate the many wonders wrought in the changes that were continually
going on in his own frame, the boy was almost overwhelmed with the flood
of new thoughts that poured through his brain, and he felt as if he could
have sat and listened to the old man the long night through.
Accordingly, when Mr. Tonkin came to a conclusion, Humphry begged him to
proceed, saying he had begotten emotions in him that he had never known
before; and he felt as if a burning thirst had come upon him for the
truth, and he could drink of such knowledge for ever without quenching it.
Mr. Tonkin was pleased to find he had stirred the boy’s thoughts so
effectively; and he promised him that, before long, he would place him in
a position where he should be able to pursue the subject as far as his
powers could carry him.
* * * * *
Not many weeks after the above conversation, Humphry, to his exceeding
delight, was articled to Mr. Bingham Borlase, the surgeon and apothecary
of Penzance; and there, alone in his little chamber, at night, he wrote
the following passage in his note-book:
“I have neither riches nor birth to recommend me; yet, if I live, I trust
I shall not be of less service to mankind and to my friends than if I had
been born with these advantages.”[21]
CHAPTER V.
THE FIRST GLIMMER OF THE SAFETY-LAMP.
Humphry was hardly at home in his new quarters, when an incident occurred
that directed his mind towards the investigation of one of the most
subtle and mysterious principles in nature.
Mr. Borlase had returned from his day’s rounds, and as he was busy
unfastening the long leggings that covered his black silk stockings, he
informed the family, who, with the boy, were gathered round the tea-table
in the little parlour adjoining the shop, that he had heard that day of
a fearful explosion which had occurred, during the last month, in one of
the Welsh coal-mines.
“It seems,” said the doctor, as he took his place at the table, “that
there were two ‘shifts,’ or sets, of men employed at the pit. The first
went to work at four in the morning, and were relieved by the next set
at eleven; and so secure was the mine considered—so little thought of
danger, indeed, entered the minds of the pitmen—that the second shift of
men often entered the mine before the first had left it. This happened
to be the case, they tell me, at the time of the accident; for shortly
after the second set of hands had descended the shaft, the people
above-ground were alarmed by a terrible report, followed by others so
quickly, that it sounded like the firing of infantry, and a sheet of
flame was seen to flash from the mouth of one of the shafts. The ground
shook as if with an earthquake, the tremor being felt for half-a-mile
round the workings; while the dull, subterranean boom of the explosion
was heard, they say, nearly four miles off. Vast clouds of dust rose high
in the air, in the form of an inverted cone, and large masses of timber
and fragments of coal were shot straight up from the pit-mouth, as from a
huge piece of artillery, and fell with a heavy crash near it; while the
dust, borne by the wind, descended in a shower upwards of a mile from
the spot, and as it did so, it caused a gloom, I am assured, like early
twilight, in the neighbouring villages, inhabited chiefly by the families
of the miners.
“The boom was no sooner heard,” continued Mr. Borlase, “the tremor of
the earth felt, and the darkness from the shower of ashes perceived,
than the wives and children of the miners rushed frantically towards the
pit. Horror and dismay were painted on every face. The crowd thickened
from all sides, so that in a short time several hundreds of women
and children were gathered round the shaft. The air, the people say,
resounded with shrieks and cries of despair for the fate of husbands,
fathers, and sons, from many a bursting heart.
“The machinery, it was then found, had been rendered useless by the
explosion, so that it was near upon an hour before thirty-two persons—all
that survived that dreadful catastrophe—had been brought to daylight, and
of these twenty-nine only lived to relate what had occurred in the mine
below.
“It was now discovered that one hundred and twenty-one, men and boys,
had been in the pit when the accident happened, so that eighty-nine
poor souls still remained entombed in the workings. Those who had their
friends restored to them appeared, it is said, to suffer for a while
as much from an excess of joy as they had, a short time before, from
the depth of despair; while those who were yet in the agony of suspense
filled the air with shrieks and howlings, and ran about wringing their
hands and throwing their bodies into the most frantic and extravagant
gestures.
“After some little time, it appears that nine persons volunteered to
descend into the pit, with the faint hope that some engulfed below might
still survive. As the fire-damp, however, would have been instantly
ignited by candles, those who went to search the mine lighted their way
by ‘steel-mills,’ as they are called, which,” added Mr. Borlase, turning
round to Humphry, “are small machines for giving light, by turning a
cylinder of steel against a piece of flint; for it has been found, I
should tell you, that though the fire-damp is immediately ignited by
flame, it is not explosible by sparks.”
The remark evidently sank deep into the boy’s mind, for he knit his brows
and bit his lips as if a sudden thought had flashed across his brain. But
Humphry was too much interested in the narrative to interrupt the doctor,
so he said not a word, and waited anxiously for Mr. Borlase to proceed.
“The men who had descended the pit,” continued that gentleman, “attempted
to make their way towards the spot where they knew the miners must
have been at the time when the explosion happened. Their progress,
however, was soon intercepted by the prevalence of what is called the
‘choke-damp’—an atmosphere which it is suffocation to inhale—and the
sparks from the steel-mill, they say, fell into this like dark drops of
blood.
“Deprived of light, therefore, and nearly stifled, they were forced to
grope their way back to the shaft.
“As each came up he was surrounded by a group of anxious inquirers,
but not a ray of hope could be elicited. It was impossible, they told
the people, for any breathing thing to live in the mine. At first, the
assertion seemed to obtain some credit, but hope still lingered. All
there recollected how persons had survived similar accidents, and stories
were told how, upon opening a mine forty days after an explosion, men had
been found still alive, having subsisted during the time on horse-beans
and candle-ends. Then distrust began to enter the minds of the crowd, and
some suggested that want of courage or bribery had induced the men who
had descended to magnify the danger; so that when it was proposed by the
owners to close the mouth of the pit, and so shut out the air from it—for
the most experienced ‘viewers’ had pronounced the mine to be on fire—the
proposition was received with cries of ‘Murder!’ and with expressions of
determination to oppose such a proceeding with violence!
“All that night, they tell me,” the doctor proceeded, “many of the widows
lingered about the mouth of the pit, with the hope of hearing the cries
of a husband or a son.
“The next morning it was again proposed to exclude the air; still the
populace, made furious by their misery, would not allow the project to
be carried out until some others had again descended the shaft. But none
could now be found hardy enough to enter the jaws of the burning cavern.
At length, however, two brave fellows were induced to make the perilous
attempt, and they nearly lost their lives in so doing.
“The account given by these adventurers (for they confirmed the opinion
as to the pit being on fire) ultimately convinced the people of the
impossibility of their friends surviving in so deadly an atmosphere, and
reconciled them to the plan of excluding the air. Accordingly the shaft
was closed, with the eighty-nine poor souls entombed in it, and more than
a month elapsed before the mine was opened again and in a state to admit
of an examination.
“During this interval, I leave you to imagine,” went on the apothecary,
“what must have been the terrible suspense of those whose love made it
impossible to eradicate all hope from their bosoms. The widows, anxious
to believe that their husbands still lived in the closed mine, gave a
ready credence to the idle tales of escape that were continually being
circulated through the country. These inventions, however, had the effect
of daily harrowing up afresh the sorrows of the people; so that when the
morning came that had been appointed for the re-opening of the pit, the
distress of the neighbourhood burst forth once more with almost redoubled
fury.
“A great concourse of people assembled round the mine on that sad day:
some came out of curiosity, others out of public sympathy, but the
greater part came there with broken hearts and streaming eyes, intent on
once more beholding the loved form of a father, brother, husband, or son.
“Soon a message was despatched for a number of coffins to be in
readiness at the pit-mouth. Upwards of eighty of these had been ready
prepared, and they had to pass by the miners’ villages on their way to
the shaft. As soon as a cart-load of them was seen, the howling of the
women, who had not yet found their way to the melancholy spot, floated
on the breeze in low, fitful gusts, presaging a scene of the greatest
distraction and confusion; and as each load of coffins came to the pit,
it brought a long train of wretched mourners in its wake.
“The bodies of the ill-fated men were found under various circumstances.
One, from his position, must have been asleep when the explosion
happened; others were huddled together in ghastly confusion—twenty-one
were found in a heap in one spot. The power of fire was visible upon all:
some were scorched; others almost torn to pieces; while others, again,
appeared as if they had been stifled at their work.
“Then came the heart-rending scene,” added Mr. Borlase, “of mothers and
widows examining the mangled remains for marks by which to identify
the bodies of their lost sons and husbands. Few, however, were able to
recognise their relatives by their features; their clothes, their shoes,
and—when these were too much burnt to be known again—their tobacco-boxes,
or some token of affection, were often the only indications by which the
lost friend could be singled out from the rest.
“Every family had made some arrangements for receiving the dead bodies
of their kindred, but the doctor had very properly stated that, in
his opinion, such a proceeding might spread a putrid fever through
the neighbourhood, and the first body, when exposed to observation,
presented so horrible and corrupt an appearance, that the people were
induced to consent that each corpse should be interred as soon as it was
discovered—on condition that the hearse, in its way to the chapel-yard,
should pass by the door of the deceased.
“And the condition was duly complied with,” concluded the doctor,
solemnly. “Hour after hour, and day after day—for the finding and removal
of the bodies continued for upwards of a week—the funeral carriage
might be seen slowly wending its way through the half-desolate miners’
villages, passing first by the door of one closed cottage, and then by
another, while at the hatches of the others stood groups of women, the
greater part of whom were habited in black, with little things by their
side, and some with infants in their arms, mostly wearing some humble
mark of mourning. As the hearse moved on, the women, with tears in their
eyes, would tell one another whose body was then on its way to its last
home, and each would have some little story to recite of good done and
charity bestowed by the ill-fated man, while all would sigh to think what
would become of the wretched widows and little orphans who, as the bier
stopped at the cottage, might be seen, with streaming eyes and dejected
heads, to issue forth and follow the funeral carriage slowly and sadly to
the grave.
“For ten long, melancholy days,” said Mr. Borlase, mournfully, “were the
shutters of the houses closed in the miners’ villages, and for ten days
did the bell of the neighbouring chapel continually toll—for the finding
of the bodies lasted all this time: and by this one terrible accident
there were no less than ninety-two pitmen hurled into eternity, while
as many as forty widows and one hundred and six orphan children were
deprived of their protectors and ordinary means of subsistence.”[22]
Mr. Borlase, on finishing his melancholy story, turned to Humphry, and
saw the tears trickling from his cheeks.
There was a silence among all present, as if the awe of the calamity was
still pressing on their hearts.
Presently the impulsive boy started to his feet and cried, “I’ll put an
end to this shocking misery, please God I will, some day.”
The quick eye of young Humphry saw a smile play faintly on the doctor’s
lip, and he added, “I know, sir, you have reason to doubt my power to
do as I say, and, perhaps, it may take me years of hard study to gain
the knowledge to enable me to compass my end; but though it cost me
a lifetime I will master it at last. I have sufficient faith in the
goodness of the Creator, to believe that these terrible afflictions come
upon us only through our ignorance, and that if we but study His will, as
expressed in the laws of the universe in which He has placed us, He has
given us the faculty to avert misery, and to turn the current of Nature
to our own welfare rather than injury.”
Mrs. Foxell (Mr. Borlase’s sister), who was presiding at the tea-table,
and who had already learnt to esteem Humphry highly for the generous
qualities of his nature, was moved almost to tears with the benevolent
impulses of the boy; for she was naturally of a kindly disposition, and
the melancholy details of the accident had so affected her, that when she
heard the youth vow he would one day put an end to such calamities, the
transport of joy she felt was too much for her woman’s heart, and though
she would have cheered him on, there was an hysteric spasm in her throat
that prevented her utterance for a time.
Presently the lady said, “Do not be discouraged by what my brother may
say to you, Humphry. He has lived too long in the world to be as hopeful
as you are, and he is so accustomed to scenes of anguish that suffering
is, with him, almost an everyday occurrence. But you and I, boy, are,
thank Heaven, unused to such sights, so that the mere recital of them
stirs us to the depths of our natures. Besides, it is only a woman who
can fully comprehend the distress wrought by such a catastrophe as my
brother has recounted to us; for the real suffering in all such cases
falls lighter on those who are even destroyed by it than it does upon
those who are left behind. It is not so much the dead husbands I grieve
for as the living widows; the lost fathers felt but a momentary pang, but
the fatherless children have years of misery to pass through: and it is
because my sex teaches me to understand these things deeper than yours,
that I, for the sake of the poor living victims—the wives and babes,
beggared in heart as well as in means—would not have a word said that
would take away one spark of hope from your noble purpose. Though the
prospect of success may appear barren to some minds, nevertheless if you,
Humphry, can, in the ardour of your sympathy, imagine such an object to
be barely possible of attainment, I say to you, Go on; and God speed you
in your good work. You wish such a result to be possible, and therefore
believe it to be so, and believing it, perhaps you may find it to be as
you fancy; whereas if you had no faith in it you would never work at it,
and consequently could never accomplish it. Think, too, if you should one
day gain your end, what honour would await you—how many thousand poor
creatures would hail you as their preserver—what evils you would be the
means of preventing—ay, and even what wealth you might reap from such a
discovery, for you could secure it to yourself, and so derive a large
income from the profits of it.”
“No, my good madam,” replied Humphry, half indignant at the idea of
enriching himself by such means, “I would never think of such a thing. My
sole object would be to serve the cause of humanity, and, if I succeeded,
I should be amply rewarded in the gratifying reflection of having done
so. All I desire is a competence, and this, I hope, my profession will
yield me; more wealth might be troublesome, and distract my attention
from pursuits in which, even now, I delight. Riches,” he added, “could
not give me either fame or happiness; they might, undoubtedly, enable me
to put four horses to my carriage, but what would it avail me to have it
said that Humphry Davy drives his carriage and four?”[23]
The noble disinterestedness of these sentiments produced a deep
impression upon all present, for they were uttered, not in the same
passionate tone as that in which the boy had previously spoken, but
calmly and almost gravely, as if they were the result of long reflection,
and showed that the youth had already learnt to prize fame more than
wealth—that his mind was bent on winning an honourable reputation rather
than amassing a worldly fortune.
Old Mr. Borlase, the venerable father of Humphry’s master, looked with
wonder and admiration at the youth, and drawing him closely to him,
exclaimed, “There’s a brave lad! You remind me, Humphry, of my poor
brother the clergyman, who is dead and gone now, rest his soul!—I mean
him, you know, that wrote the ‘History of Cornwall;’ a wonderful book it
is, too!—he’d just the same notions when he was a youngster, and used to
say that money was only of value for the happiness it could bring, and
that there was more real pleasure to be found in seeking and discovering
the truth than the richest fortune could purchase. I am sure, for my
part, lad, I hope you may succeed in your noble object; and I have seen
quite enough changes in my time to think nothing impossible now. Why, I
have heard my grandfather say that, when he was a boy, coal itself was
seldom used as fuel, and now see what wonders are being worked by it.
Haven’t we just had one of those wonderful steam-engines, which have
been of late years invented by Mr. Watt, put up at the Wherry Mine close
by?”[24]
The boy nodded quickly, as if he was well acquainted with the locality.
“And there,” continued the old gentleman, “that great monster of brass
and iron goes on, day after day and night after night (though the
shaft of the mine, you know, is in the sea, and the workings entirely
underneath the sands), acting at a distance over the surface of the
ocean, and drawing up the water from beneath its bed; and all, too, by
means of a few bushels of coals. I am sure when I first saw the engine
lifting up its arms, and snorting away as if with the heavy labour it
was doing, it put me in mind of the old fable, I learnt at school, of
Prometheus, who stole fire from the sun, you know, boy, and made men with
it out of the materials of the earth. For it struck me as being a huge
steam man—a kind of monster labourer, as it were, that would work on for
ever, without needing any sleep, and without knowing any fatigue; and
that wanted only coals, instead of bread and meat, to keep it going. Ah!
we live in wonderful times, my lad, that we do; and whatever the world
will come to in a few years, when I am dead and gone, is more than I can
say. Why, Mrs. Foxell here was reading to me the other day, out of the
‘Sherborne Mercury,’ a paragraph, saying, that a Mr.—Mr.—What was the
name, my dear?”
“Symington,” answered the lady appealed to.
“Yes; that’s it!—Mr. Symington,” proceeded the old gentleman, “had been
making some experiments on the Clyde to propel a vessel, without sails or
oars, over the water—what do you think of that?—and that he had actually
got a large boat to move some three or four miles an hour by means of
paddles worked by a steam-engine on board the vessel.[25] Dear, dear!
What shall we come to next, I wonder! They say in the paper, too, that
the experiment was perfectly successful; so that, I dare say, in a few
years our sailors will be no longer at the mercy of the winds, and if
they have only a stock of coals aboard, they’ll be able to traverse the
seas which way they like. Ah! coal is a wonderful thing, that it is,
Humphry. But I’m afraid that when we sit and warm ourselves by the fire,
we seldom give heed to the dangers and hardships suffered by the poor
creatures who are far away underground, digging it out of the bowels of
the earth for us.”
“That’s true enough,” interposed the doctor, “and it’s long been an
opinion of mine that the greatness of England will soon depend, not
so much on the energy of its people as the extent of its coal-fields.
You have heard, doubtlessly, that Mr. Murdoch, in our own county here,
has, within the last year or two, made a successful application of the
gas from coal to the purposes of illumination; he has produced by it a
light much more brilliant than that of any lamp, and which requires no
feeding nor trimming, nor has it any wick; and I am told that he speaks
confidently of its being possible to light our streets and houses by
such means.[26] But I must confess, that I myself can hardly go with the
gentleman so far as that.”
“Well, for my part, Bingham,” interrupted the father, “I am ready to
believe anything. I have lived to see mail-coaches introduced throughout
the country for expediting the post, and letters that used to take
near upon a fortnight to go from here to London, now carried the same
distance in little more than two days.[27] So nothing they could do
would astonish me after that! No, not even if I was to hear that the
mail-coaches themselves were driven by coals, and at twice the rate they
go at now.”
This was considered so wonderful a stretch of imagination on the part
of the old gentleman, that the whole company laughed heartily at the
apparent impossibility of such an achievement.
“You may smile,” went on the old man, “but steam is only in its infancy
yet, depend upon it; and the engineer at the Wherry Mine, when I was
talking to him about the machine there, told me that there was force
enough in a bushel, or eighty-four pounds of coals, when properly
consumed, to raise 70,000,000 pounds weight one foot high. Now, the
ascent of Mont Blanc, from the valley of Chamouni, is said by travellers
to be the most toilsome feat that a strong man can execute in two days;
nevertheless, I find by calculation” (and the old gentleman drew a bit of
paper from his waistcoat pocket) “that the combustion of only two pounds
of coal would be sufficient, by means of a steam-engine, to lift a man
to the summit. Again, the great Pyramid of Egypt is composed entirely of
granite, it stands on eleven acres of ground, and is 500 feet high, so
that its entire weight has been calculated to be about 13,000 million
pounds; consequently about 180 bushels of coal would be sufficient to
raise the entire mass twelve inches from its base.[28] So that you see,
Humphry, what a wonderful thing coal is, and the large amount of force
that lies locked up in every pound of it.”
“I do, sir,” said the boy, “and it is this which makes me wish to
decrease the suffering attendant upon the working of so valuable a
mineral. The account of the accident which Dr. Borlase has just told us,
has so harrowed my feelings, that I shall spare neither time nor labour
in seeking to discover the causes upon which such calamities depend,
so as to find out the means by which to prevent them for the future.
It may be some years before I shall be able to perfect my plans, but
perfected they _shall_ be one day if my life be spared; and then I have
no fear that a discovery, having for its object the preservation of human
life and the diminution of human misery, will be either neglected or
forgotten. However high the gratification of possessing the good opinion
of society, there is a still more exalting pleasure in the consciousness
of having laboured to be useful.”[29]
CHAPTER VI.
THE WONDERS OF HEAT: ITS SOURCES.
Humphry was so full of his project, that all the day long he could think
of little else, and at night he lay awake in his bed for many hours,
planning an infinity of rude schemes for accomplishing the object he had
in view. He devised and fashioned a number of odd contrivances, too, for
the purpose, but each in its turn was found to be of little or no avail.
Nevertheless, the idea was too great to be hastily abandoned, and the
sense of the fame that success in such an undertaking would assuredly
give to his name had entered so deep into the boy’s mind, that he got to
crave more and more for worldly honours; and he would sit of an evening,
alone among the rocks, dreaming of the time when he, a poor Cornish
boy, was to be ranked among the intellectually noble, and reverenced
throughout Europe for his genius and his benevolence.
Nor did the lad fail, when he visited his mother, to confide to her all
his hopes of success and renown; and when the widow heard that he was
bent on discovering the means of saving the lives of the poor miners—a
class whom she had long learnt to pity, for she had been brought up in
the midst of them, as it were—she felt prouder than ever of her darling
boy, and shed many a tear of joy over him.
Mrs. Foxell, too, when she beheld Humphry’s vexation at the repeated
failures of the models he constructed, encouraged the lad in every way,
reminding him that _he_ himself had said the project would cost him years
of hard study to accomplish.
Accordingly, after many disappointments, Humphry himself began to see
that he could only hope to attain the result he desired by making
himself acquainted with all that was already known upon the matter. He
was ignorant even of the laws of combustion in general, and the rude
experiments he had made had set his mind craving for knowledge on the
subject. “Why did this thing burn, and that not?” he would inwardly
inquire. “How came it that one body, as gunpowder for example, went
alight all of an instant, and another, like tinder, took a long time
to smoulder away? And why was phosphorous so easy to kindle, and wood,
comparatively, so difficult, that the slightest friction would inflame
the one, whereas it required a long time to light the other by such
means? What mysterious process,” he would ask himself, “went on when any
substance burst into flame, and whence came the light and heat that were
then given out from materials that, a few moments before, had been dark
and cold? Could the light and heat have been imprisoned in the substance,
and were they set free during the combustion; or were these powers
generated merely by the burning of the bodies?”
Then Humphry’s mind darted off to the deeper question, “What were the
principles of light and heat themselves? Were they one and the same, or
two distinct powers in nature? In a winter’s day,” he mused, “we have
the same light from the sun, and but little heat; whilst in all cases
of artificial illumination there is great heat and but little light,
compared with that of the solar rays.”
These and many other such puzzling inquiries passed through Humphry’s
brain, and left his mind in such a state of perplexity that he could
not rest without a clearer insight into the subject. He soon saw, too,
how silly he had been in setting to work before he had availed himself
of the discoveries of those who had gone before him; for, he would say,
how could he think of finding out, by himself, all that was known of the
science of heat and light, when each of those sciences, as Mr. Tonkin
had told him, had taken thousands of the wisest minds—ay, and thousands
of years of intense study—to build up; for in them was contained the
accumulated experience of all mankind from all time. Yet, because the
truths were free to the world, he had refused to avail himself of them,
and, like a proud fool, had though he could compass his end without any
such knowledge at all.
* * * * *
It was not long before Humphry was hard at work making himself master
of the laws of heat. He had borrowed of Mr. Tonkin the best book then
extant upon the subject, and often when the streets of the little town
in which he lived were silent as the tomb—and the distant bell of Madern
Church, as it tolled the morning hours, was heard to boom upon the
still air, almost like a moan—and the sound of the waves that rippled
upon the neighbouring shore stole on the ear softly as the murmur of a
sea-shell—the candle might be seen burning in the boy’s chamber, making
the little diamond panes of the casement shine like plates of amber in
the darkness (for every other window in the street looked black from the
want of light), while the observant eye could trace on the white wall on
one side of the room, the huge distorted shadow of the lad bending over
his books.
As Humphry read on, and got to see clearer and deeper into the nature
and properties of the subtle principle he was studying, he grew more and
more enraptured with the wonders and the knowledge that were opened up to
him at every step; and often when some new discovery burst upon his mind
he would, in the fervour of his admiration, fall upon his knees, there
alone in his chamber at night-time, and thank God that he had come to
know so much of His goodness and glory; then he would rebuke himself,
too, for having remained so long ignorant of the many beauties that lay
concealed in the wisdom and exquisite fitness with which the phenomena
of the universe are linked together. “What fairy tale of enchantment,”
he would say to himself, “can display magic like this? What work of
human invention can fill the mind with such amazement and delight at the
subtlety of the art, as the mind feels when it first learns the wondrous
story of Creation?”
When Humphry had read through the books that he had obtained of Mr.
Tonkin and Mr. Borlase, he proceeded to repeat the most striking of the
experiments in connexion with the subject, so as to impress the knowledge
more firmly on his mind. But before doing this he reviewed the whole
matter, and arranged it after his own manner—for he was not the boy to
follow in a beaten track, and found no little delight in the exercise of
his own genius.
* * * * *
“First,” said Humphry, as he pondered over the science of heat in
general, “_the sources of it_ have to be considered; that is to say,
whence is the heat of the earth derived? The universe is a vast reservoir
of caloric, and it is capable of being evolved, by some means or
other, from almost every substance that surrounds us; and though its
production artificially is now so common that it has lost all wonder
with us, there must have been a time when the elimination of it from
substances on the earth must have been a matter of such amazement as to
have produced a feeling of awe, on the part of the multitude, towards
those who first discovered the art. This, perhaps,” the boy went on,
“is the origin of the fable of Prometheus, who was, probably, the first
man who found out the way to kindle a fire, and so was thought to have
stolen the heat from the sun. Tradition says, the first artificial fire
was produced by lightning striking a decayed tree. Our minds can, even
now, almost conceive the terrible awe of the people who first witnessed
the liberation of fire on the earth—who beheld, for the first time, the
transparent red flames burst forth from the combustible and lick the
air like burning tongues, while the smoke rolled upwards from them in
dense leaden clouds. Then the intense pain felt on touching the fire
must have made the populace almost believe that they had been stung by
demon serpents, while the roar of the wind, as it rushed towards the
blazing mass to supply the place of the lighter air that had been driven
upwards by it, must have sounded to the people like spectral voices,
and the ultimate dissipation of the huge solid substance into invisible
gases must have appeared like the most marvellous magic to them. Thus it
came that men at last got to worship the fire, for its wonders and its
terror, though we kindle it nowadays almost without a thought or a fear.
“The sources of heat at present known to man,” continued the boy, as he
wrote down the divisions of the subject in his note-book, “are many.
First, there is the _heat of the sun, and, perhaps, that of the moon_;
philosophers, however, have concentrated the moonbeams upwards of 300
times, by means of a burning-glass, nearly 3 feet in diameter, and yet
the most delicate thermometers have shown not the least increase of
temperature. This is said to arise from the feebleness of the light of
the moon, as compared with that of the sun; for the lunar rays have
been calculated to possess 300,000 times less illuminating power than
the solar ones, whilst the light of the sun itself has been shown
by experiment to have 12,000 times the intensity of the flame of a
wax-candle, so that a little fragment of the great luminary the size of
such a flame would possess the illuminating power of 12,000 wax-candles;
and, since the diameter of the sun is nearly four times greater than
the distance of the earth from the moon, this may give us some notion
of the vast flood of light and (if the two are connected) of heat that
are being continually streamed forth from the sun into the universe.
Of the intensity of the _solar heat_, the law of the decrease of all
radiant matter enables us to form some conception; for this teaches us
that the heat of the sun’s rays, after travelling to the distance of
the earth, must be diffused over at least 300,000 times a greater space
than it is at the sun itself, and consequently that the intensity of the
heat must be that number of times more highly concentrated at the sun’s
surface than it is on reaching our atmosphere. Now, one of the largest
burning mirrors that have ever been constructed, and which had the power
of concentrating the sun’s rays rather more than 17,000 times, melted a
piece of Pompey’s pillar in less than a minute, a piece of cast-iron in a
quarter of a minute, a copper halfpenny in sixteen seconds, and fragments
of slate and tile in three and four seconds. A lens that increased the
intensity of the sun’s heat about 10,000 times fused pieces of platinum,
gold, asbestos, quartz, &c., in three seconds; so that—as the solar fire
must, at the sun itself, be 30 times more intense than the calorific
power of its rays, even when thus concentrated, at the surface of the
earth—it is evident that the fury of the sun’s heat must at its source be
sufficient to dissipate the most obstinate metals in vapour, and to make
the most infusible of the earths as liquid as glass.
“Then, again, there is the _heat of the stars_; for if each of these be
suns, and they, like our own sun, give off heat, together with light,
while the heat radiated by them decreases in the same proportion as
their light, it is clear that the united beams of the starry host
must give a certain general temperature to the realms of space. This
temperature philosophers have calculated to be as low as that at which
quicksilver freezes, and which degree of cold appears to be attained in
the Arctic regions, during the long absence of the sun through a polar
winter. According to the principles which regulate the radiation of
light and heat, it is demonstrable that the starbeams can only maintain
a temperature in infinite space which, when compared with the heat we
derive from the sun, must be as much inferior to it as the light of a
moonless midnight is to the light of mid-day at the equator; and it is
plain, that the rate at which the earth cools down or radiates back
into space the heat it receives from the sun must have its limit in the
temperature of planetary space itself, so that, had this been higher or
lower, the earth’s surface must have been hotter or colder than it is.
“But, besides the preceding _celestial_ sources of heat in nature, we
must also (if we suppose that such things as give light to the earth
radiate heat as well to it—though in ever so minute a degree) enumerate
that peculiar cone or pyramid of luminous mist which is seen an hour
or two after sunset, at certain months of the year, in the line of the
ecliptic, and to which astronomers have given the name of ‘_the zodiacal
light_.’ Travellers in tropical regions tell us that this is sometimes so
brilliant that it seems a second sunset, lasting almost to midnight, and
that the clouds which are scattered over the deep azure of the distant
horizon appear to flit past the glowing nebulosity as before a golden
curtain, while above these other clouds are seen reflecting from time to
time brightly variegated colours.
“Then, again, there are the brilliant coruscations of the _aurora
borealis_ (or ‘_northern dawn_,’ or ‘_polar light_,’ as it is sometimes
called), though this appears to be rather an emanation from the earth
itself than any celestial phenomenon. According to the best accounts,
the light of the aurora exists almost within the bounds of our own
atmosphere, and seems to stream from one of the poles of our globe, as
if the earth had suddenly acquired the power of becoming self-luminous
like the stars and sun.[30] This brilliant exhalation is rendered more
interesting by the fact, that the great Herschel himself, from repeated
observations of the spots on the sun, came to the conclusion that such
spots are parts of the dark solid body of the sun itself, laid bare to
our view by fluctuations in the solar atmosphere, and that from that
atmosphere alone the light and heat proceed—the shining matter of the
sun being, not a fluid, but a mass of brilliant or phosphoric clouds,
glowing with the beams of the luminous strata of the solar atmosphere far
above them—in the same manner as the aurora with us is said sometimes to
illuminate a stratum of clouds below it.
“It is impossible to say what increase of heat our atmosphere may derive
from the beams of the aurora and the zodiacal light, but as we have no
reason for supposing the rays in these cases to be destitute of all heat,
it is evident that, when enumerating the several sources of caloric in
the universe, some mention should be made of them; for who can tell what
would have been the effect upon the general stock of heat in nature
without such phenomena, or how low the temperature of the earth’s surface
might have been if the heat of the planetary space in which it moves had
been less than it is?
“But a far more important source of caloric to the earth lies in its
_subterranean heat_, or the increase of temperature which is found to
ensue as we descend below the surface of our globe. Carefully conducted
experiments have shown that, in our own climate, the temperature
increases about 1° for every 50 feet that we go down. At 354 feet below
the ground the heat is found to be 60°, in those parts where the surface
of the earth itself has a mean temperature of but 50°. At 792 feet under
the soil the thermometer rises to 70°; at 1434 feet it marks 80°; at
1872 feet it reaches 90°; whereas at 2556 feet it is as high as 100°.
Now, if this rate of increase (viz. 1° in every 50 odd feet) continued
uniformly as we descended, it would follow that, at a depth of rather
less than 2 miles, water would be constantly at the boiling point, and at
9 miles below the surface everything would be red-hot, whilst at rather
more than 20 miles the granite rocks themselves would be in a continual
state of fusion. In the ‘United Mines’ of Cornwall one of the levels is
so hot, that though a stream of cold water is allowed to flow through
it, in order to reduce the temperature, the miners are compelled to work
nearly naked, and will bathe in water at 80° to cool themselves. At the
Tresavean Mine, in the same county, which is nearly 2000 feet deep, the
temperature is greater than the intensest heat of summer in the dog-days.
“There is, however, no evidence to prove that the increase of temperature
beneath the surface proceeds at a uniform rate when we descend to a
greater depth than 1500 feet below the level of the sea. It will be seen
by the rates of increase above given that the temperature underground
rises at first 1° in every 35½ feet, whereas at great depths the increase
amounts only to 1° in every 68 feet; and, as far as our observations have
extended, the subterranean heat appears to bear a close relation to the
thermic condition of the climate at the surface; for it is found, on
descending to a depth of about 60 feet in our own climate, the heat of
the earth no longer fluctuates with the different seasons, but remains
always at the same point during winter and summer, whilst at the tropics
the stratum of invariable temperature is situate at only 1 foot below the
soil.
“But whether the subterranean heat be due to the absorption of the solar
rays, or whether it arises (as some have supposed) from a vast body of
central fire in the earth, it is certain that we have many indications of
prodigious elevations of temperature beneath the soil. These appear to
be due to great chemical changes taking place in the substances forming
the crust of the earth. Every schoolboy knows that a mixture of sulphur
and iron-filings buried under the ground will, in a short time, become
so heated as to burst into flames. Now these two materials form the
mineral called ‘iron pyrites,’ which prevails through every coal-field,
and the moisture acting on these is known to generate heat enough to
inflame neighbouring combustibles—many coal-mines having been set on
fire spontaneously by such means. In the ‘United Mines,’ where the heat
is found so oppressive, it is undoubtedly owing to the decomposition of
immense quantities of pyrites that are known to exist at a short distance
from the works.
“Of the prodigious subterranean heat existing in some parts of the
earth we have the most unmistakeable evidence. In some places boiling
_hot springs_—as the Geysers of Iceland—issue from the ground (in these
eggs have been cooked in 4 minutes); and even in our own country the
wells of Bath have a temperature of 115°, while at those of Carlsbad,
in Bohemia, the heat of the water is as high as 167°. In other parts,
‘_fumaroles_,’ or eruptions of steam, burst from the soil; while in
others there are ‘_solfataras_,’ or jets of sulphurous vapour; and in
others, again—as in the valleys of the Eifel, at the lake of Laach in
Germany—‘_mofettes_,’ or vast exhalations of carbonic acid gas, occur.
Further, there are Artesian fire-springs—like those termed ‘_ho-tsing_’
in China, where a province has been lighted (by the gas issuing from
them) for several thousand years. Moreover, eruptions of boiling acid
mud, called ‘_salses_,’ are not unfrequent. The volcano at Carguaraizo,
in Peru, threw up a torrent of hot mud in the year 1698, that covered
nearly 80,000 acres of ground; and in 1797, an entire village near Rio
Bambo was buried under a similar mass.
“All these phenomena are evidences of intense subterranean heat, many
of them being simply the products of underground combustion: but lofty
jets of flame have likewise been known to blaze up from the earth, to
such a height that they could be seen at a distance of 24 miles from the
eruption; as, for instance, at the village of Baklichli, near Baku, on
the Caspian Sea. During _earthquakes_, too, the earth has been known to
open and to vomit forth flames, and gases, and enormous fragments of
rocks, accompanied with a noise of subterranean thunder; while in some
places the heaving soil has been inflated by the force of the compressed
vapours beneath, and expanded like a bladder filled with air. Such
was the case among the plains of Malpais in Mexico, in the month of
September, 1759, when a tract of ground, 3 to 4 miles in extent, rose up
like a huge bubble—flames bursting from the earth the while over more
than half a square league—and the volcano of Jorullo being formed at the
summit.
“In _volcanoes_, or burning mountains, again, we have manifestations of
the intense elevation of temperature existing at certain places within
the crust of the earth. The explosions from the volcano of Guacamayo,
in South America, are heard almost daily at a distance of 80 miles;
whilst the noise of the detonations from one in the Sunda Islands, near
Java, have been distinguished 970 miles from the spot. But the most
remarkable instance on record of the fury and power of the subterranean
fires is to be found in the eruption of one of the Icelandic volcanoes,
called ‘Skaptaa Jokul,’ which occurred on the 8th of June, 1783. During
this eruption the large river Skaptaa entirely disappeared, and the day
after, a torrent of burning lava rushed down the sides of the mountain
and not only filled, but overflowed the channel of the stream, though
in many places it was 600 feet deep and 200 feet broad. Then pursuing
the course of the river, the fiery current poured over a lofty cataract,
and filled up in a few days an enormous cavity that the waters had been
hollowing out for ages. A short while after this another large river, the
Hverfisfliot, disappeared from its bed, and this was filled up by another
fiery torrent, which overflowed the country in one night to the extent
of more than 4 miles. It has been estimated that these two streams of
burning molten rocks were together 90 miles in length by 20 odd miles in
breadth, and, in some places, between 500 and 600 feet deep, while there
were in the aggregate forty thousand million tons of red-hot rock poured
out of the bowels of the earth in the short space of ten weeks, during
which the eruption lasted.[31]
“Another of the natural sources of heat to the earth is to be found in
the _electric discharges_ known under the names of forked and sheet
lightning. Of lightning there appear to be three kinds: (1) The
_zigzag_, which is linear and sharply defined at the edges; (2) the
_sheet_, which illuminates whole clouds, that seem to open and reveal
the light within them; and (3) the _globular_, which appears in the form
of fire-balls. The two first of these kinds last but for the thousandth
part of a second, while the globular form moves much more slowly. Of
the amount of heat contained in each of the different species we have
no precise knowledge, though, from the experiments by which we produce
discharges of electricity artificially, on a small scale, we learn that
it must be considerable. If a spark which is drawn from a small Leyden
jar, and which has force enough to leap only some few inches through
the air, has power to inflame combustibles, and even to fuse the metals
in an attenuated form, what must be the heat evolved from an electric
discharge where the insulating body consists of whole acres of clouds
rather than a few square inches of tin-foil, and whence the fluid has
power to leap through hundreds of feet of the atmosphere towards the
nearest conductor? That the electric flash possesses great heating power,
we have repeated proofs; for it melts all wires that are not sufficiently
substantial to allow it a free passage, inflames decayed trees,
overthrows buildings, and often fuses even the rocks themselves—the tubes
called ‘_fulgurites_,’ which occur in beds of sandstone, and consist of
fused sand (glass), are known to geologists to have been produced by
such means—while at Funzie, in Fetlar, the lightning is recorded to have
torn up a rock 105 feet long from its bed, and hurled the fragments to a
considerable distance from the spot. Moreover, the electric heat is the
greatest that we are enabled to produce artificially, and so much exceeds
that of the strongest furnace, that platinum, which remains stubborn and
infusible in a forge at a white heat, melts in the arc of flame produced
by a powerful voltaic battery—like wax.
“These are the _natural_ sources of heat,” the youth wrote on; “the
artificial methods of generating heat, however, are much more various. We
can evolve heat by _mechanical_ means—as, for instance, by _percussion_
or _pressure_. The blacksmith hammers a nail until it becomes red-hot,
and from it he lights his match; and in coining, the blank piece of metal
becomes greatly heated by the sudden and violent action of the press. By
the compression of air in a small tube, by means of a condensing syringe,
a sufficient quantity of heat may be evolved to light German tinder; and
it has been well said, ‘that, locked in a pint measure of air, there
exists sufficient heat to make several square inches of metal red-hot.’
A piece of Indian-rubber, suddenly and forcibly drawn out, becomes warm
in consequence of the extension, as may be easily perceived by applying
it to the lip the moment it is stretched. Again, by the concussion of a
flint and steel so much heat is produced that the sparks which fly off
consist of small particles of iron that have been fused by it. Moreover,
when a few grains of fulminating silver are struck by a hammer, the
heat produced is sufficient to ignite gunpowder and to cause a violent
explosion. Again, _friction_ is a prolific source of heat. The Indian
ignites two pieces of wood by rubbing them together; and even two pieces
of ice may be made to melt each other by the same means. It has been
truly observed, too, that an unlimited supply of heat seems capable of
being derived by friction from certain materials. Water has been made to
boil in two hours and a half, merely by boring into a mass of metal that
was surrounded by the fluid.
“But not only can we produce heat artificially by _mechanical_ means; we
can do so far more plentifully and easily by _chemical_ action, for it
has been found, that whenever two or more substances rapidly combine,
heat is invariably produced. In _fermentation_ (which is nothing more
than a decomposition of elements loosely united, and their reunion in a
more perfect state of combination) considerable increase of temperature
takes place. During the making of vinegar there is much heat evolved—the
temperature rising, in some processes, from 60° to 85°. Again, in the
process of respiration (which is merely the combination of the charcoal
in the blood, with a certain portion of the air drawn into the lungs) the
heat evolved is supposed to be the cause of our bodies remaining almost
at a constant temperature. A little powder of the metal called antimony
thrown into a jar of chlorine gas spontaneously ignites and burns with
brilliancy, combining with the gas so readily that it takes fire and
produces at first a liquid, and afterwards a soft solid called the
‘butter of antimony.’ Further, there is an oil-like fluid consisting of
two gases (nitrogen and chlorine) that have been made to unite with each
other, and this compound has such an affinity for combustible bodies,
that even if a long rod, the extremity of which has been dipped in oil,
be made to touch only a globule of it—the size of a mustard-seed—confined
under water, it instantly explodes with a flash of light, and with such
violence that it disperses the water in a shower, and breaks into atoms
the vessel in which it is contained; so that experienced chemists always
protect the face by a mask when making the experiment. Again, if oil of
vitriol and spirits of wine, or if aquafortis and spirits of turpentine,
be suddenly mixed, sufficient heat is set free to ignite the spirits.
When caustic potash, too, dissolves in water, a considerable increase of
temperature ensues. So if spirits of wine and water are mixed together,
the mixture becomes much hotter and occupies a smaller space; whilst
if four parts of strong oil of vitriol be mixed with one part of snow
or pounded ice, the heat developed is sufficient to boil water, whereas
if one part of the acid be added to four parts of snow, intense cold is
produced. More than this, if a piece of clean platinum be immersed in a
vessel containing a mixture of oxygen and hydrogen gases, such intense
heat is evolved that the metal becomes suddenly red-hot, and the gases
are made to combine so rapidly that a violent explosion ensues, and the
two gases become water.
“But the most ordinary mode of obtaining heat artificially is
_combustion_, though this is merely a process of rapid combination after
all. It is by the heat evolved during the process of combustion that our
houses are warmed in winter, our food cooked, the steam-engines of our
factories and mines set in motion, our metals smelted, cast, and wrought,
our glass made, our dishes hardened, and an infinity of useful services
rendered to us; indeed, so much do we owe to combustion, that we are
unable to comprehend the state which man must have been in before the
method of producing heat artificially by such means was discovered.
“Lastly, there is the heat generated by _nervous energy_; for if the
sole source of warmth to the animal frame lay in the chemical processes
that are continually going on within it, why should a suddenly excited
emotion (as in states of anger and blushing) have power to produce so
considerable an increase of temperature in the human frame? Indeed we
have personal knowledge, that almost every muscular movement produces
sensible warmth, even as any violent excitement of the mind is attended
with the same result. We know, too, that the injury of one little
thread-like nerve can reduce a member of the body to a state of stony
coldness; and that after death, when the chemical decompositions are
proceeding as actively as during life, the body possesses no animal heat
whatever.” Physiologists, moreover, have shown that, if the respiration
be kept up artificially in an animal after its head has been cut off, the
blood becomes arterialized, and the several chemical changes go on as
during life—_but without the body being in the least warmed by them_.
CHAPTER VII.
THE WONDERFUL DIFFUSION OF HEAT.
When Humphry had written thus far concerning the sources of heat (for
the boy was delighted to note down his thoughts on the various subjects
he was studying),[32] he began to ponder over the several modes in which
heat was _communicated_ to bodies removed from the different sources of
it; for, said he to himself, “if there had been no means of propagating
heat from one part of space to another, the fires could not have warmed
us, and the sun would have been only a moon to our globe, while we should
have been deprived of some of our most agreeable sensations. Hence, in
the consideration of such a subject, it becomes necessary to attend to,
and distinguish between, the several ways in which a body at an elevated
temperature communicates its heat to others that are either in contact
with it or at a distance from it, as well as the several conditions which
determine the reception or absorption of the heat by different substances.
“Now, heat may be communicated from one body to another in three
different ways—
1. By _emission_ of rays of heat from a distance;
2. By _conduction_ along the particles of a solid body;
3. By _convection_ or circulation among the particles of a
fluid.
“The propagation of heat by the emission of heat-rays from a warm or
burning body at a distance, is the one that first demands attention. This
mode of communication is generally styled ‘_radiation of heat_,’ and it
is evident that the heat-rays emanating from one body may be communicated
to another, either directly, by the process of _transmission_ through the
intervening substances, or indirectly, by _reflexion_ from the surfaces
of those opposing them; for the heat-rays, like those of light, always
proceed in a straight line, and are susceptible, likewise, of being
reflected or driven off at an equal angle from polished surfaces.”
Having settled thus much in his own mind, and arranged the subject with
that logical precision which was a marked feature in the genius of the
youth, he proceeded to test experimentally the emissive energies, or, in
other words, the radiating powers of different substances.
For this purpose he provided himself with a square tin canister: one of
the four sides of this he brought to as high a polish as he possibly
could; the second side he coated with a mixture of lamp-black and
gum-water; over the third side he pasted a piece of paper, and the fourth
he covered with glass. Then, having provided himself with a thermometer
from the surgery below, he proceeded to arrange the canister at some
distance from the thermometer, but on a level with it. After this he
filled the canister with boiling-hot water, and then proceeded to note
how the thermometer was affected when the canister was turned round, and
each of its sides successively brought before the instrument. The boy
soon ascertained, to his great joy, that the heat was thrown off most
rapidly from the blackened side of the canister; next to that, he found
the surface covered with paper to radiate heat more rapidly than the
other two; then the glass side was discovered to possess more emissive
power than the polished surface, while the polished surface itself had
the least radiating power of all.
Delighted with the result of the experiment, and pleased with the
knowledge it gave him as to the emissive energies of different substances
for heat, the boy, to assure himself that he was not mistaken, held
his hand at a short distance from the canister, and caused the
differently-coated sides to pass successively before it. As he did so, he
could feel the heat increase gradually as the polished side passed from
before his hand and the blackened one came round in front of it; so that,
had he not been aware of the fact, he would hardly have believed that the
water in the canister was as hot at that part where the bright tin had
been left as it was where the side had been blackened over.
Next the lad tried another modification of the same experiment. Having
blackened one canister entirely over, and brightly polished the outside
of another which was of the same size, he filled the two vessels with
boiling water, and putting a thermometer into each he placed them upon
a table at opposite corners of an empty room, and then found that the
thermometer in the blackened vessel fell much quicker than that in the
polished tin one; so he now saw that the reason why the _black_ side of
the canister in the first experiment felt hotter than the _polished_
surface did to his hand was, that the water there was parting with its
heat at a more rapid rate, so that in the entirely blackened vessel it
necessarily cooled down sooner than in the bright tin one.
Humphry was so delighted with the truths he had thus discovered, that
he tried a number of other experiments as to the radiating power of
different substances, and at last came to the conclusion that lamp-black,
sealing-wax, wool, paper, glass, and black-lead, were much better
radiators of heat than the metals; their power of giving off heat being
in the order in which they are here mentioned—a surface of lamp-black
cooling quicker than one of sealing-wax, and sealing-wax again more
rapidly than writing-paper, and so on down to the metals, which cooled
the slowest of all.
Then, having dealt with _different_ substances, the youth set to work to
ascertain what effect an alteration in the arrangement of the surface
produced in the radiating power of the _same_ substance. Accordingly
he tarnished, by means of acid, one of the sides of the polished tin
canister he had previously employed, and found, on filling the vessel
again with hot water, that the _dulled_ surface parted with its heat
quicker than the _bright_ one. After this, he proceeded to roughen
another of the sides with some emery paper, and then, on re-filling the
vessel, he discovered that the scratched surface cooled at a greater rate
than the smooth polished one.
“So, then,” said young Humphry to himself, “not only have _different_
substances various radiating powers for heat, but also a difference in
the arrangement of the surface of the _same_ substance is attended with
a like effect.”
Still the lad had to examine the result produced by bodies of _different
densities_, and this he did by means of a vessel of cast-iron and one of
wrought-iron, when he found that the _cast_ metal parted with its heat
quicker than the hammered or _wrought_ metal; so that it was evident a
_lighter_ material was a better radiator of caloric than a _heavier_
one—for the particles of the iron in being wrought had been brought
closer together, and the metal thus rendered of greater density.
This done, Humphry made an entry in his note-book, “that not only did
rough or dull surfaces part with their heat quicker than smooth or bright
surfaces, but that light bodies were better radiators than heavy ones.”
The young experimentalist was overjoyed with the progress he had made,
and he would have rambled off into a number of speculations as to the
effect which the principles he had discovered must produce in nature (for
he saw that the different surfaces of different countries must yield a
like result); but he was too intent on pursuing the investigations he had
undertaken to allow himself, yet awhile, to apply them to the explanation
of terrestrial phenomena. Moreover, he had still to learn the different
rates of cooling among bodies in the air and in a vacuum. To do this,
however, an air-pump was necessary, and how he was to obtain such an
apparatus puzzled his ingenuity for a considerable time.
At length the youth remembered to have seen a large syringe among Mr.
Tonkin’s instruments, and having obtained the loan of this, he applied
it to a stand, and used it as the pump for extracting the air from the
receiver. When the instrument was complete, Humphry found that bodies
which took between two and three minutes to cool in the air were as long
as five minutes in parting with the same quantity of heat in a vessel
from which all the air had been exhausted. So he now perceived, that the
same substances gave off their heat twice as quick in the _open air_ as
they did _in vacuo_.
The next step was to ascertain the different amounts of radiation among
different bodies on the earth. For this purpose the boy borrowed as many
thermometers as he could procure among his friends in the town; and early
one evening, after the sun had declined, and when the soil was parting
with the heat it had received in the course of the day, he proceeded to
test, by means of the instruments, the several rates at which the various
substances upon the earth were being cooled down. One of the thermometers
he suspended in the air four feet above the grass-plat in the garden at
the back of the doctor’s house; another he placed on some wool which he
had spread on a raised board; another he deposited on the surface of the
raised board itself; and a fourth he rested on the grass-plat. Shortly
afterwards he proceeded to note the temperatures indicated by the several
thermometers in the different situations, when he found that the one in
the air stood at rather more than 60°, while that resting on the wool
was at 54½°, and that lying on the board at 57°, whereas the one on the
grass-plat marked only 51°. In an hour or two after this, the boy noticed
that the blades of grass were suffused with dew, and that the fibres of
the wool also were beaded over with little drops of moisture, but to
a less extent than the grass, while the surface of the board remained
almost dry.
“So then,” he said to himself, “_wool_ is a better radiator than _wood_,
and, cooling quicker, condenses the moisture of the air more rapidly upon
it; but _grass_ again, as the thermometer showed, cooled quicker even
than the wool, and therefore collects more dew than either.”
This induced young Humphry to try another experiment, in order to
ascertain whether those bodies which cooled most rapidly collected the
most dew on their surfaces. Accordingly he placed a piece of bright
polished metal and a piece of glass (for the surfaces of these substances
were nearly the same) on the gravel-path, and was delighted to perceive
that in a short time the _glass_ was covered with moisture while the
_metal_ remained perfectly dry. A strip of _flannel_ was then put beside
the other two, and, being a good radiator, it soon became spotted with
dew-drops. After this the boy coated the piece of polished metal with
lamp-black, and found it _then_, like the others, capable of condensing
the moisture of the air upon its surface.
“It is as I expected,” cried the lad; “the dew which the ancients
imagined to be shed from the stars is simply the condensation of the
vapour in the atmosphere upon cold surfaces; and, consequently, those
bodies which have the greatest radiating power, and so become cold the
quickest, are found to have the largest deposition of dew formed upon
them, while those which, like polished metals, part with their heat but
slowly, and so remain for a long time at the same temperature, have but
little moisture condensed upon their surface. The deposition of dew,” he
went on musing, “is precisely similar to the condensation of moisture
that occurs on the outside of a bottle of very cold water when brought
into a warm room. The cold surface of the glass abstracts heat from the
vapour in the air of the apartment, and so causes it to be condensed in
the form of little watery globules on the surface. In the same manner the
earth, parting at night with the heat it has received during the day from
the sun, becomes cooler than the atmosphere above it—for thermometers
show, that when the grass is at 51° in the evening, the air only four
feet above it is more than 60°—and accordingly the cold surface of the
blades acts upon the vapour in the atmosphere, precisely the same as the
outside of the cold bottle does upon the air in a warm room.”
So pleased was the lad with the insight that his investigations had
given him into some of the mysteries of nature, that he continued his
experiments on this subject for many nights; and in the course of these
he found, that not only had different bodies different dew-collecting
powers, but that different colours even possessed the same property;
for on exposing a piece of yellow, of green, of red, and of black glass
to the night air, he perceived that the moisture appeared first on the
_yellow_ glass, then on the _green_, but that none at all showed itself
on either the _red_ or _black_ glasses.
To his astonishment, however, he at length discovered, that when the
evenings were _cloudy_, and there seemed to be a greater quantity
of moisture in the atmosphere, the pieces of flannel and glass, and
little piles of swan’s-down with which he had studded the gravel-walk,
remained unmoistened with dew; whereas, when the nights were _clear_ and
apparently _dry_, they, one and all, with the exception of the polished
metals, became rapidly suffused with moisture. This for awhile entirely
baffled the boy’s comprehension. “How came it that more dew was deposited
on dry clear nights than on dull damp ones?” Surely, such being the case,
the dew cannot be said to proceed from the vapour in the atmosphere;
for if it does, reasoned Humphry, it is evident that when there is more
moisture in the air there should be more dew deposited on the earth.
At length it struck the boy that, perhaps, the clouds themselves might
interfere in some way or other with the result; so the next fine
clear night he strewed the gravel-walk, as before, with fragments of
such substances as he had already found to be the best collectors
of dew; and then, at the other end of the path, he placed pieces of
the same substances under a small awning, which he made out of his
pocket-handkerchief, fastened at each corner to a short stick. This he
did in order to see what effect would be produced by _screening_ bodies
from the sky—since the clouds, he fancied, might act in some such manner.
On returning to the garden after a short interval, Humphry was rejoiced
to find that there was a copious deposition of dew on the pieces of glass
and wool that he had left _exposed_ to the sky, while the surfaces of
those which were _screened_ by the little awning above them remained
perfectly dry.
“Yes,” cried the lad, “the clouds _do_ act as screens. They give back,
perhaps, some of the heat that the earth at night is radiating into
space, and so prevent bodies cooling down as rapidly as they otherwise
would.”
However, to satisfy himself that the clouds really _did_ interfere with
the radiation of bodies on the earth, Humphry arranged an apparatus for
testing the point. This consisted of a thermometer, the bulb of which was
first incased in wool (for that substance he knew to part readily with
heat) and afterwards fixed in the focus of a small concave mirror. Then
on the next windy night, when the clouds were drifting swiftly across the
sky—leaving the heavens occasionally clear, and occasionally hiding the
light of the stars—the anxious lad turned the mirror towards the blue
vault above, and, on doing so, he could hardly repress his glee as he
beheld the quicksilver in the tube of the thermometer descend and ascend,
each time the sky became clear or clouded. Though, by means of another
thermometer, he knew the temperature of the surrounding atmosphere to
be 60°, Humphry nevertheless found that, when the sky was _unclouded_,
the mercury in the one attached to the mirror indicated only 45°, whilst
immediately that a _cloud passed over the firmament_, and so prevented
the bulb from parting with its heat, the quicksilver rose rapidly again
to the temperature of the air around. So intensely did Humphry exult
in the result of this experiment, that he remained long, watching the
thermometer rise and fall, as the clouds swept one after another across
the sky.
The next day, Humphry, now that he had made himself acquainted with the
circumstances that regulated the _emission_ or radiation of heat from
bodies, began to turn his attention to the _reflexion_ of it from such
substances as impeded the progress of the rays; “for,” said he, “if
bodies at an elevated temperature have the power of sending out rays of
heat in all directions—in the same manner as luminous bodies emit rays of
light—it follows that substances opposing the passage of the heat-rays
must either _absorb_ them, and so become heated themselves—or they must
_transmit_ them and so allow the rays to proceed in their original
direction—or else they must _reflect_ them and so bend them into another
course.”
For the study of the _reflexion_ of heat the lad procured two concave
mirrors, made of tin-plate and about 1 foot in diameter. These he
arranged so as to slide up and down a pillar, to which they were
respectively attached. Thus provided, Humphry proceeded to place a small
“Florence flask,” filled with hot water, in the focus of one of the
mirrors, while in the other focus he arranged a thermometer after this
fashion:
[Illustration]
Now, though the mirrors were some feet apart, the mercury in the tube,
to the boy’s great delight, rose almost to the heat of the boiling water
in the flask.
After a few moments’ reflection, the lad fancied the effect might perhaps
be due to the radiation of the heat from the flask itself, rather than
to the reflexion of it from the mirrors. So, to satisfy himself whether
or not such were the case, he placed a sheet of pasteboard immediately
in front of the mirror near the thermometer, and thus prevented any rays
being reflected from the one to the other. No sooner, however, had he
done so than the mercury was seen to fall in the tube—even though the
source of heat was as near to the thermometer as before; but directly he
removed the pasteboard from between the mirror and the thermometer, the
quicksilver rose rapidly again, and stood at the same number of degrees
as it previously did.
Having convinced himself upon this point, he then drew the thermometer
away from the focus and nearer to the heated flask, so that, if the
effect were due to radiation, the mercury, as it approached the source
of heat, should rise higher in the tube. The contrary result, however,
was found to ensue; and it will be seen on reference to the preceding
engraving, that by _radiation_ only a few of the heat-rays (which
are indicated by the diverging _unbroken_ lines) would fall upon the
thermometer, whereas by _reflexion_ a much larger number of such rays
become concentrated upon the bulb in the focus of the opposite mirror—as
shown by the _dotted_ lines in the diagram.
[Illustration]
Humphry was now anxious to see whether, by reflexion of the heat-rays, he
could ignite combustible bodies at a distance; but for this purpose he
changed the situation of the mirrors, arranging them vertically one above
the other, instead of horizontally, or each on a level with the other, as
before. Then he made a small basket of iron wire, and having filled this
with burning charcoal, he suspended it below the upper mirror, so that
it hung exactly in its focus, whilst above the lower mirror he fixed a
small piece of phosphorus, and this was exactly in the focus also. Thus,
on the completion of the arrangement, the boy was as astonished as he was
delighted to perceive that the phosphorus was immediately inflamed by
the _reflected_ rays of heat. Some fulminating silver was then exploded
in like manner. After this Humphry boiled some water in a flask that he
substituted for the piece of phosphorus in the focus of the lower mirror,
and finally cooked a chop, by the same means, at some considerable
distance from the fire.
Next, instead of the two mirrors, he rolled up a sheet of bright gilt
paper, with the metallic side inwards, into the form of a long cone or
funnel, so that the opening was larger at one end than at the other; then
holding the larger end towards a clear fire, he found the rays of heat
were concentrated into a focus at a little distance beyond the smaller
end, and there he caused a bit of phosphorus again to inflame, by means
of the reflected heat. The subjoined diagram exhibits the arrangement.[33]
[Illustration]
Humphry now began to wonder what effect would be produced by a piece of
ice placed in the focus of one of the mirrors; and he thought for a long
time whether the rays of _cold_ would be reflected from the ice, as those
of _heat_ had been from the hot water and the burning charcoal. As the
winter had long set in, he found no difficulty in obtaining such a piece
as he required from one of the neighbouring ponds, and then arranging
the mirrors as before, he placed it in the focus of one of them, while
in that of the other he fixed the thermometer which he had previously
employed.
To the lad’s astonishment he discovered that the mercury immediately
began to fall, and at length stood at 32°, or the freezing point. “So
then!” he cried, “it _is_ possible to reflect rays of _cold_ as well as
those of _heat_. And yet,” said he to himself, after musing for a while,
“_is_ it the ice, after all, that is radiating _cold_ to the thermometer,
or the thermometer itself, which, being warmer than the frozen water,
is really and truly radiating _heat_ to the ice?” If, instead of the
thermometer, he had placed a red-hot body in the one focus, while the ice
remained in the other, Humphry knew well enough that the warmer body, as
it became cool, would be giving off heat to the colder one. “Why then,”
he asked himself, “should he fancy that the thermometer itself—because it
was _only a few degrees warmer than the ice_—lacked the power of parting
with its heat to the colder body, in the same manner as the red-hot
charcoal?”
The lad was soon convinced of his previous fallacy; and when he saw
that the apparently contradictory effect was no anomaly after all, he
could hardly refrain from smiling at the simplicity which had led him
to believe at first that rays of cold were reflected from the ice to
the thermometer, instead of the rays of heat being given off by the
thermometer to the ice.
As yet, however, Humphry had experimented concerning the _reflexion_ of
heat with mirrors only of _polished metal_; and one day, when he was
recounting to Mr. Tonkin the curious effects he had produced, the old
gentleman asked the lad what he imagined would have been the effect if,
instead of _metal_ mirrors, he had used _glass_ ones.
Humphry answered confidently, that, as the results were due only to the
reflexion of the rays from the concave surface, a glass mirror, _of
course_, would have given precisely the same effects as the metal ones.
“Try it,” was all the old man said, as he smiled at the positiveness of
the boy’s reply.
Nor was the young experimentalist long in doing so, for he saw by
Mr. Tonkin’s manner that some strange difference in the effect would
ensue—though for the life of him he could not divine what it was to be.
Accordingly, at the earliest opportunity, the boy substituted the glass
concave mirror, which Mr. Tonkin had lent him for the purpose, for
one of the metal ones which he had previously employed; then filling
the little wire basket with red-hot charcoal, as before, and hanging
it in the focus of the upper mirror, he once more suspended a piece of
phosphorus in the focus of the lower mirror, which was now of _glass
instead of metal_. To his utter amazement, however, the phosphorus was no
longer capable of being inflamed in such a manner.
It was but the work of a moment to remove the combustible from the focus
of the glass mirror, and to place a thermometer there in its stead; and
this soon showed that there was now _little, if any_, heat reflected.
“How wonderful!” cried the startled boy. “What _can_ be the cause of it?
I’ll arrange the mirrors differently,” he added, “and see if I can find
it out.”
[Illustration: HUMPHRY’S EXPERIMENTS ON THE DIFFUSION OF HEAT.—Page 157.]
But no sooner did Humphry put his finger on the glass than he drew it
suddenly back, as he exclaimed, “How hot the lower mirror has become!
and I remember when I used the metal one, that I was surprised to find,
on removing it, though the heat was sufficient to boil water and ignite
bodies in its focus, the metal surface of the mirror itself was scarcely
warmed. But now that _glass_ is used,” he went on, “_the mirror itself is
rendered hot, while in the focus of it there is scarcely any perceptible
increase of temperature_. So, then,” he added, “the glass _absorbs_ the
heat-rays, and therefore does _not reflect_ them, while the metal on the
other hand _reflects_, because it does _not absorb_ them. Still it’s
very strange,” mused Humphry, as he proceeded to blacken a small card,
“for the glass mirror must _reflect_ the _light_ of the fire, though it
_absorbs_ the _heat_ from it. I’ll try whether such is the case or not.”
The card was then placed in the focus, and a bright spot of light was
seen shining like silver in the centre of the blackened surface.
“Yes,” cried the lad, “it reflects the light, but not the heat of the
fire. How strange! I wonder whether the same effect would be produced by
the sun’s rays!”
Accordingly the next day, when the sun was shining brightly, Humphry
arranged the mirror in the garden, so that the beams might be
concentrated in its focus; and then, to his greater astonishment, he
found that he could inflame combustibles by the _solar_ heat with
the _glass_ mirror, in the same manner as he had previously done by
_artificial_ heat with the _metal_ ones.
“The _light and heat_ of the sun, then,” said Humphry, as he stood
watching the white fumes of the burning phosphorus rise in the air, “are
capable of being reflected by glass, whereas the _light only_ of an
artificial fire can be concentrated into a focus by it—the heat in the
latter case being absorbed.”
The metal mirror, likewise, was found to possess the power of reflecting
both the solar light and heat, in the same manner as it had been before
made to reflect both the light and heat of an artificial fire.
The experiment with the glass mirror, however, clearly showed that solar
heat differed in some way or other from terrestrial heat; but _how_, was
a source of continual wonder to the lad.
* * * * *
From the _reflexion_ of heat, Humphry proceeded to the _transmission_ of
it.
Light passes readily through certain substances, which are therefore said
to be _transparent_, while others impede the progress of the beams, and
are consequently called _opaque_. “Is there, then,” mused the boy, “such
a property as _transparence_ and _opacity_ for _heat_, as well as light,
among bodies? Are some substances _pellucid_, as it were, to heat like
they are to light? and are some as impermeable to the one as they are to
the other?”
The lad knew well that the heat of the sun was capable of being
transmitted through glass as well as its light, for he had often
concentrated the solar beams by means of a magnifying or “burning” lens,
as it is called: glass, therefore, was transparent to the _solar_ heat as
well as light; but was it so to the rays of _artificial_ heat?
To ascertain this, Humphry borrowed old Dr. Tonkin’s large reading
lens, and held it before the fire so that the focus fell upon the bulb
of a thermometer. But though the light of the burning coals was seen
concentrated into a bright spot upon the bulb, still the mercury in
the tube gave no indications of any increase of temperature. The lens,
however, which was scarcely warmed when the sun’s rays passed through it,
became greatly heated when the rays of the artificial fire were made to
fall upon it—thus showing, that while it _transmitted_ the solar heat it
_absorbed_ the terrestrial.
It was evident, therefore, that though the _heat of the sun_ has the
power of passing freely through glass, _artificial heat_, on the other
hand, is completely stopped by it.
Humphry then thought he should like to try the effect of a piece of black
glass, for this would be perfectly opaque to light, and he longed much to
see whether it would be equally impermeable to heat. On holding a square
piece before the fire, the boy was surprised to perceive the thermometer
he had arranged behind it rise rapidly, thus showing that though black
glass was _in-transparent to light_, it was by no means _opaque to
heat_. That the quicksilver was made to mount in the tube solely by the
influence of the heat-rays which traversed the black glass—and not by any
indirect radiation from the fire—Humphry assured himself, by placing a
piece of white glass, of the same size and thickness as the black one,
before the thermometer: the quicksilver, however, was immediately seen
to fall. “How marvellous is this!” he exclaimed. “Light and heat, then,
are capable of being separated one from the other; and there are bodies
in nature which, like _white_ glass, are _transparent_ to light, but
_opaque_ to heat; while there are others, like _black_ glass, that allow
the heat-rays to _pass through_ them, though they are _incapable of being
traversed_ by the luminous ones.”
The boy was so full of the new truth that had thus become impressed upon
his mind, that he hurried off to Mr. Tonkin to confer with him on the
result. From him Humphry learnt that there were other substances, besides
glass, that gave equally curious effects—the most striking of these,
the old gentleman told the boy, were _alum_ and _rock-salt_, for though
both were transparent to light, they had by no means the same power of
transmitting heat; for it would be found that while a small plate of
rock-salt allowed the rays of heat to pass almost _freely_ through it, a
similar plate of alum was nearly _impermeable_ to them.
The young philosopher was not long in trying the experiment. Having
procured two such plates as Mr. Tonkin had advised, he used them as
small screens in front of the fire, and found that a thermometer behind
the rock-salt rose rapidly; whereas, behind the alum, it was scarcely
affected, for the heat was nearly all stopped by it.
The possibility of separating heat from light made a powerful impression
upon the ardent boy, and he wondered whether he could arrive at the same
result with the solar beams as he had with the rays of an artificial
fire. For a long time he pondered over the matter, and conceived and
tried a number of fruitless experiments in connexion with it.
At length, however, he remembered to have read somewhere, that by means
of a glass prism the beam of white light proceeding from the sun might be
separated into all the colours of the rainbow.
Accordingly he set to work to repeat the experiment. Having darkened his
room he made a hole in the window-shutter, and placed behind it a glass
prism, with one of the sharp edges downwards and one of the flat sides
uppermost, as shown in the annexed illustration:
[Illustration]
Immediately that the arrangement was complete, and the beam from without
fell on the glass within, the wall on the opposite side was iridescent
with a strip of variegated light, as if a slice of a bright rainbow were
clinging to it. The lower end of the luminous band was a rich warm red,
and this passed, by a tint of orange, into a bright yellow, which again
died away, by deepening hues of green, into a narrow strip of dark blue,
while, at the upper end, the indigo tint became warmed into a brilliant
edging of violet.
When the rapture of the boy on first beholding the sight had, in a
measure, subsided, he proceeded, by means of a thermometer, to ascertain
the temperature of the several rays. First he tried the upper end of
the spectrum, and found that in the _blue_ ray the mercury marked 56°.
Then passing downwards Humphry was overjoyed to see the quicksilver
mount as he proceeded towards the middle, where, in the _yellow_ ray,
the instrument indicated a temperature of 62°, _i.e._ 6° higher than in
the blue; while at the lower end—at the extremity of the _red_ ray—the
temperature was found to be as high as 79°, _i.e._ 17° higher than it was
in the yellow.
There was then, altogether, as much as 23° difference between the heat
at the extreme ends of the luminous band—the _red_ ray being upwards of
half as hot again as the _blue_ one—so that light and heat were capable
of being separated even in the solar beams themselves; for the yellow
contained the most light of all, and yet it was 17° colder than the
extreme verge of the red ray, where there was only a faint luminous blush
to be perceived.
* * * * *
The next step was to ascertain the circumstances regulating the
_reception_ or _absorption_ of heat.
Humphry had now investigated the laws which governed the radiation or
_emission_ of the rays of heat from bodies at an elevated temperature. He
had ascertained that these rays not only emanated from heated substances
at different rates, and so caused them to cool down more or less rapidly,
but that—though their tendency was to proceed, like the rays of light,
in a straight line—they were capable of being _reflected_ or bent back
by certain bodies opposing their progress, and that in such cases the
reflecting bodies themselves did not become heated by them. Other bodies,
again, he had found to have the power of _transmitting_ the rays of
heat, that is to say, of allowing them to pass _through_ their substance
rather than reflecting or driving them _back_ from their surface; and
such transmitting bodies, moreover, were likewise scarcely warmed by the
heat that traversed them. Now he was about to investigate the conditions
that determined the _absorption_ of the heat-rays, by bodies upon which
they fell after being given off by _radiation_ from others of a higher
temperature.
The lad’s first experiment upon this subject was to blacken the surface
of one of the metal mirrors that he had previously found to reflect the
heat, without being itself warmed in so doing.
The result proved to be as Humphry had anticipated. The mirror no longer
had the power of concentrating the heat in a focus, at a short distance
in front of it: for now, instead of _reflecting_ the rays and remaining
_cool_ as before, it _absorbed_ all the heat that fell upon it, and
became itself _warmed_ by the neighbouring radiator.
The same effect ensued when the surface of the mirror was whitened with
chalk, and the same again when it was roughened, or scratched, with emery
paper: so that _rough_ and _dull_ bodies proved to be better absorbers of
heat—even as they were better radiators—than _bright_ or _polished_ ones.
Hence there appeared to be some connexion between the radiating and
absorbing powers of different substances—those which cool the quickest
seeming to be capable, also, of being heated in the shortest time.
To test this the lad placed a blackened and a bright-polished vessel in
front of the fire, and found that the thermometer in the _black_ vessel
rose much more rapidly than did that in the _bright_ one.
Humphry then availed himself of these two vessels as a means of testing
the relation between the _absorbing_ and _radiating_ powers of black and
bright-polished surfaces. Into the mouths of the black and the bright
tin vessel he inserted a thermometer, and then placed between them one
of the square canisters he had previously employed, and which, it will
be remembered, had one of its sides bright, while the opposite one was
coated with lamp-black.
Having filled the middle canister with boiling-hot water, he proceeded
first to note the radiating and absorbing effects when the _different_
surfaces were opposed to each other. On arranging the middle canister
so that its black side was turned towards the polished vessel at one
end, and its polished side to the blackened vessel at the other end,
there was no effect produced upon either of the thermometers; for then
the opposite powers of the _different_ surfaces exactly balanced each
other. When, however, the apparatus was so adjusted that _similar_
surfaces were opposed—that is to say, so that the blackened side of the
canister in the middle was turned towards the black vessel, and the
bright-polished side to the bright-polished vessel—the thermometer in the
black vessel immediately indicated a great excess of heat; for then not
only was there a _good radiator_ opposed to a _good absorber_, but, on
the other side, the two bright surfaces were facing each other—that is,
the _bad radiator_ was turned towards the _bad absorber_—so that even the
little heat which was given off from the polished side of the canister
was driven back again to it by the surface of the neighbouring bright
vessel. Hence everything _favoured_ the radiation and absorption of the
heat on the one side, where black was opposed to black, and _prevented_
it on the other, where metal was facing metal; and thus the great
elevation of the thermometer was accounted for.
As it was now winter time and the snow lay thick upon the earth, Humphry
availed himself of the circumstance to test the absorbing powers of
different _colours_. For this purpose he took a number of pieces of
different coloured cloths, and placing them at mid-day upon the snow,
so that the sun’s rays could fall directly upon them, he found that the
_dark_ colours sank the deepest into the frozen mass beneath, while the
_lighter_ hues produced scarcely any thawing effect, and the _white_
remained utterly inactive.
The same result was obtained by means of coloured glasses; for against a
window-pane that was covered with hoar-frost the lad placed some pieces
of black, red, green, and yellow glass, and the consequence was, that the
ice opposite to the _black_ and _red_ pieces was melted long before any
thawing effect was visible upon the frozen film screened by the other
colours.
When the weather grew warm Humphry obtained another very curious
illustration of the power of black substances to absorb the heat of the
sun’s rays. Having filled a glass tube with spirits of wine, he placed
it in the focus of a lens, and found that the solar heat traversed the
transparent liquid without warming it. On immersing a small piece of
charcoal, however, in the alcohol, so great was the absorptive power of
the _black_ surface that the fluid immediately began to boil. By the
same means, too, he succeeded in raising the temperature of water to the
boiling point. This showed that water, as well as spirits of wine, was
a good _transmitter_ and bad _absorber_ of heat; that is to say, that
the rays passed freely through each without warming either, unless some
substance were immersed in the liquid in order to detain and absorb their
heat.
_Air_, on the other hand, the boy knew to have little or no
heat-absorbing power; for the rays emitted by a distant hot body
traversed the atmosphere without sensibly raising its temperature. He had
read, too, that philosophers had calculated that only one-fifth of the
solar heat was absorbed in passing through 1000 feet of the air, and that
but one-third of the entire heat of the sun was taken up by the passage
of the beams through the whole atmosphere.
Humphry, moreover, sought to discover whether the sun’s heat, reflected
from a mirror, would produce the same effect as the direct solar beams.
Accordingly, before the winter passed away, he placed two pieces of
blackened card upon the snow, at a considerable distance apart. One of
these he left exposed to the _direct_ rays of the sun, while upon the
other he caused the sunbeams to fall _indirectly_, by reflecting them
from a polished metal surface. The black card that was submitted to
the direct solar beams sank, after a little time, deep into the snow,
while the frozen mass around—though the beams fell full upon it—was but
slightly thawed. With the black card, however, upon which the sun’s
rays were _reflected_, a precisely opposite result ensued. In that case
the surrounding snow itself was the first to melt, while the blackened
surface seemed to have been deprived of its power of absorbing heat, and
remained high on the unthawed pile beneath it.
Further, Humphry noticed that the snow which lay near the trunks of
trees, or wooden posts, melted much sooner than that which was at a
distance from them, and that the thawing always commenced at the side
facing the sun. Hence it was evident that the solar heat, after being
either _reflected_ or _radiated_ from bodies on the earth, and so made
to fall _indirectly_ upon other bodies, was rendered capable of being
absorbed by substances which, like snow, had but little or no power of
being warmed by it _directly_.
Why this should be, or what alteration the solar rays underwent in
impinging upon terrestrial bodies, so that substances which before
absorbed the sun’s heat with difficulty became afterwards more easily
warmed by it, was more than Humphry’s philosophy could explain—though it
cost him many a day’s hard thinking in trying to account for the result.
* * * * *
Having now investigated the conditions which governed the diffusion of
heat from a _distant_ point, Humphry next proceeded to inquire into the
circumstances which regulated the communication of heat to bodies in
_contact_ with others at an elevated temperature.
This constitutes what is called the _conduction_ of caloric, and occurs
between different bodies, or parts of the same body, immediately
adjoining each other. The communication of heat by _conduction_ is a slow
process compared with that of _radiation_, which is, probably, as rapid
as the diffusion of light itself. Philosophers have calculated, that
even if the crust of the earth were made of cast-iron (which is a much
better conductor than rocks and stones), it would take myriads of years
to transfer the heat from a depth of 150 miles below the surface to the
surface itself; whereas by radiation the solar heat travels from the sun
to us in 8½ minutes.
The laws which regulate the communication of caloric to _distant_ objects
are similar to those which would ensue if the heat really consisted of so
many hot particles darted out from the heated body in all directions;
and colder bodies placed in the neighbourhood of heated ones, either
become hot in the same manner _as they would if_ such particles were
positively absorbed by them, and entered into their substance; or they
_reflect_ the heat to other bodies, while they themselves are unwarmed
by it—_as if_ (according to the hypothesis) the caloric particles
were elastic, and had the power of bounding off from smooth surfaces
interfering with their progress; or else they _transmit_ the heat rays,
_as if_ the imaginary particles of caloric were capable of freely
traversing certain substances, and that, also, without sensibly raising
the temperature of the permeated mass.
But all bodies, or parts of bodies, which are in immediate contact with
some other at a higher temperature, become themselves warmed; _not_ by
rays thrown out from the heated mass, but by _conducting_ or diffusing
the heat from one point to another, and so disseminating it ultimately
throughout their whole substance.
Humphry was thus particular in impressing upon his mind the precise
difference between the _radiation_ and _conduction_ of caloric before
entering upon the study of the latter process; for he knew that without
clear and distinct views upon the subject it was impossible for him to
arrive at any absolute knowledge.
[Illustration]
To illustrate the _gradual_ progress of heat by _conduction_, the lad
took a square bar of iron, about 20 inches long, and he attached to the
under side of it (by means of a little wax) 10 small wooden balls, so
that they were about 2 inches apart from each other. Then he heated one
end of the bar in the flame of a lamp, and found that the balls fell from
under it _one after another_, as the heat found its way along the metal
and melted the wax below. The arrangement of this simple and instructive
experiment is here shown.
[Illustration]
The next step was to learn the different conducting powers of different
substances. For this purpose Humphry had several small metal cones made,
all of the same size; one of these was of copper, another of iron, a
third of zinc, a fourth of tin, a fifth of lead, a sixth of marble, and a
seventh of brick. Then having tipped each of them with a small piece of
wax, he stood them, all a short distance apart from each other, on the
metal plate at the top of the iron stove by which Mr. Borlase’s surgery
was heated. The result was, that the wax at the top of the _copper_ cone
was the first to melt. Some little time afterwards, that at the apex
of the _iron_ began to liquefy; and soon after the iron, that upon the
_zinc_ was rendered fluid; while, shortly following the zinc, the wax on
the _tin_ commenced trickling down the sides. A short interval elapsed,
and then the cerate at the top of the _lead_ became fluid. Again a lapse
of a few moments occurred, after which the wax with which the _marble_
cone was tipped began to flow; and, last of all, that upon the piece of
_brick_ was liquefied.
The different conducting powers for heat among the several substances
employed were thus made evident. The metals were more capable than
either marble or brick of diffusing the caloric from one part of them to
another; while among the metallic substances themselves copper was proved
to be a much better conductor than iron; iron, again, a little better
conductor than zinc; and zinc, too, slightly better than tin. Lead, on
the other hand, was the worst metallic conductor of all.
The limited means of the young experimentalist, however (for Humphry was
obliged to seek Mr. Tonkin’s assistance for any particular apparatus he
required), did not admit of his testing the conducting powers of either
gold or silver. But had he done so, he would have found that the precious
metals were much better conductors than any other—_gold_ being the best
of all, and _silver_ only a little inferior to it. _Platinum_, however,
was a striking exception, its heat-conducting power being only a little
superior to that of iron.
[Illustration]
Humphry after this sought to discover what would be the effect if he
placed a good conducting metal in connexion with a bad one. For this
purpose he employed a short curved bar of copper; and having heated it,
he set it across the top of a small leaden pillar to cool, thus: when, to
his utter astonishment, a series of musical sounds were given forth as
the copper cooled, the tones now rising and now falling like those of an
Æolian harp.
By the same means as Humphry had employed for testing the conducting
powers of the metals, he ascertained that _wood_ was a very bad conductor
of heat, and that the _lightest_ woods were the worst. _Charcoal_, too,
he found to have but little power of diffusing the heat from one part
of it to another. This explained to the boy the reason why a piece of
charcoal, red-hot at one end, may be held—at a short distance even from
the heated part—without burning the fingers.
Humphry now set to work to raise to a considerable temperature several
pieces of such substances as he had ascertained to be good and bad
conductors, so that he might learn what effect they respectively
produced upon the touch when highly heated. As he had anticipated, the
_bad conductors_—such as the wood and brick—could be handled without
pain, whereas the _good conductors_—like the metals—burnt the fingers
immediately they were brought into contact with them.
Pursuing this result, the lad, eager to display his knowledge to the
servant of the house, took the boiling kettle from the kitchen fire, and,
to the amazement of the maid, allowed the sooty bottom of it to rest upon
his palm; for the crust of charcoal with which (by long usage) the vessel
had become coated underneath—being a non-conductor—prevented the heat of
the boiling water within being communicated to the hand.[34]
On recounting to Mr. Borlase the experiments he had performed concerning
the conduction of heat, Humphry was informed by that gentleman that
it was painful to touch good conductors like the metals when they
were heated about 120°. Air, however, he told the boy, might have its
temperature raised even to 300°, without producing any sense of burning;
adding, that some eminent sculptors had large ovens in connexion with
their studios, for drying the moulds they employed in bronze castings;
and though these places were often heated far above the boiling point of
water, the workmen entered, and remained there for some minutes without
much inconvenience; and even persons unused to such high temperatures
might walk in and out of the ovens with impunity, though to such any
attempt to remain occasioned a difficulty in breathing and a painful
sensation about the eyes. It was found necessary, however, under such
circumstances, to carefully avoid _the contact of any good conductor_;
for if, while in the heated oven, a piece of metal were touched, it
would inevitably burn—even the coins in the pocket were sufficient to
produce intense pain. “A story is told,” he continued, “of a person who
once, inadvertently, entered such a place with his spectacles on, and
these, being mounted in silver, soon blistered the parts of his face with
which they were in contact. On the other hand,” proceeded the surgeon,
“it has been found that in high northern latitudes, where the cold is
sometimes sufficiently intense to freeze mercury—though this requires
the temperature to be 72° below that required to freeze water—yet even
such excessive cold may be borne without uneasiness, provided the air
be tranquil, and the persons well clothed in good non-conductors, such
as wool and fur. If, however, _metallic substances_ be touched at this
low temperature, a sensation like that of burning is experienced, and
the part quickly becomes blistered. The reason of this,” the doctor
concluded, “is, that the heat, being as it were free to move in all those
substances which are, like the metals, good conductors of it, is readily
communicated to us by such substances when at a higher temperature than
ourselves, while our heat is as readily abstracted by them when they
are colder than we are. Hence good conductors, like metals, always feel
colder to the touch than bad conductors, like wood or fur—even though
these latter bodies can be shown by the thermometer to be of the same
temperature as the others.”
Humphry’s conversation with the doctor induced him to try another
experiment, illustrative of the conducting power of wood and metal.
He took a small rod of polished brass, about a foot in length, and
stretching a strip of writing-paper tightly over it at one end, he
tried to burn the paper in the flame of a lamp, but discovered that it
was impossible even to scorch it; for the heat, as soon as applied,
was _conducted away_ so rapidly along the metal, that it prevented
the temperature of the paper being raised sufficiently to char it. On
substituting, however, a smooth piece of wood for the brass rod, he found
that the paper stretched over the end of it soon began to scorch in the
flame, and that the wood itself shortly became ignited in consequence
of its bad conducting power, which, opposing the diffusion of the heat
along it, concentrated the effects upon the spot to which the flame was
applied.
After this, the boy began to turn his attention to the conducting powers
of _liquids_, rather than _solids_, with which he had previously dealt.
[Illustration]
That liquids are very _imperfect_ conductors of heat, Humphry made out
in the following manner: He filled a tumbler with water, and in this
he placed a piece of “fusible alloy,” which is a composition of metals
melting at a temperature below boiling heat. Then a thin copper basin was
made to float on the surface; and into this he put some pieces of red-hot
charcoal; so that, after a time, the stratum of water at the top of the
tumbler began to boil; but, even though the upper part of the liquid
was at boiling-point, so slight was the power of the water to _conduct_
the heat from one part of it to another, that the stick of alloy, which
reached within an inch of the top, remained wholly unmelted by it.
[Illustration]
The same effect was found to ensue with heated _oil_, though this the
lad tried in a somewhat different manner. In a thin glass tube a small
quantity of water was frozen by plunging it into a mixture of salt and
snow. Then, upon the lump of _ice_ at the bottom a small quantity of
_oil_ was poured; and, lastly, upon the oil some _spirits of wine_ was
made to float. The tube was now held over the chimney of a lamp, and the
spirit made to boil until the whole was evaporated, when, on plunging a
thermometer into the oil, it was found to be but slightly heated, while
the ice itself had undergone no change, but remained still solid at the
lower part of the tube.
Next, in due order, came the conduction of heat by _gases_ and _vapours_;
and of this Humphry obtained a remarkable illustration in a fact which
he learnt of the engineer at the Wherry Mine, who told the lad that
high-pressure steam did not burn, though its temperature was some hundred
degrees above that of steam at a low pressure. The scalding effect of
the vapour at a low pressure, the man informed the youth, arose from the
small particles of hot water that were diffused throughout it, and which,
indeed, rendered it visible in the air; whereas in high-pressure steam no
such watery particles existed, and the vapour was consequently not only
imperceptible to the sight, but, being a bad conductor of heat, it had no
more power to burn than so much hot air.
Again, that _gases_, in a state of combustion, are bad conductors of
heat, Humphry was aware, from having repeatedly passed his finger
through the flame of a spirit-lamp without burning it, and yet the
temperature of such a flame might be shown to be many hundred degrees
beyond that of a piece of red-hot metal. _Air_, again, he knew to have
little or no conducting power; and he had heard from Mr. Tonkin that
in Russia and other cold countries double windows, with a stratum of
air between them, were used to prevent the heat of the apartment being
carried off. So, again, in furnaces, double walls with a stratum of
confined air in the middle are employed to stop the _egress_ of heat:
even as in ice-houses the same means are adopted to stay the _ingress_ of
it.
* * * * *
The diffusion of heat by the process of _conduction_, however, generally
occurs among _solid_ bodies, in which the particles are more or less
firmly united; but _liquids_ and _gases_ (where the particles, owing to
the want of cohesion among them, are free to move) mostly became warmed
by a very different process; that is to say, the heat applied to them is
spread from one part to another—not by being propagated, as in solids,
from one fixed particle to that which is next to it—but by the _motion
or circulation of the heated particles themselves_, so that each in
its turn receives a portion of the heat applied, and then giving place
to another particle, the whole mass ultimately becomes raised to one
uniform temperature by the _direct_ agency of the radiant body, rather
than by the _indirect_ process of transference from atom to atom along
the entire substance. The one process is termed the _conduction_ of heat,
the other the _convection_ of it; and while the former prevails among the
_cohering_ particles of solid bodies, the latter generally obtains among
fluids whose atoms are _free to move_.
[Illustration]
In order to render visible this same circulation of the particles of
fluids while in a heated state, Humphry bruised in a mortar a small piece
of amber, and then having filled a glass tube with water, he threw in
a few pinches of the powder, which, being nearly of the same specific
gravity as the liquid, neither sank nor floated in it. Then applying a
gentle heat to the centre of the bottom of the tube, the boy saw, by
means of the amber-dust suspended in the fluid, that currents immediately
began to _ascend_ in the middle of the water, and to _descend_ in it
at the sides of the vessel—in the direction of the darts in the above
engraving.
[Illustration]
If, however, he heated the sides of the tube, the currents were found to
take a contrary direction, going _upwards_ at the sides and _downwards_
in the centre.
On continuing the heat, Humphry perceived the currents to become more
and more rapid, till the water boiled, and when the whole of the liquid
had acquired an uniform temperature, he observed that they ceased
altogether. He then endeavoured to ascertain if it were possible to
produce these currents in a liquid by heating it at the top, but the
boy discovered, on applying a spirit-lamp to the upper part of the
tube, thus—that though the top of the water was made to boil, and the
amber-dust there thrown into rapid circulation, the particles at the
bottom remained unmoved, the fluid below being undisturbed and cold.
[Illustration]
The reason of this was almost self-evident. The warm water was _lighter_
than the cold, and therefore _rose_ to the top immediately it became
heated, while the cooler and _heavier_ portions _descended_ to occupy its
place. Hence, in heating the tube at the bottom the current was observed
to go upwards in the middle and downwards at the sides, these being kept
comparatively cool by the action of the external air.
[Illustration]
Pursuing this subject, Humphry took a large and a small Florence flask,
and into the mouth of the large one he fitted two long bent glass tubes,
by means of a perforated cork and cement. These, together with the
large flask, were filled with water and then made to dip into the open
mouth of the smaller flask, which was likewise filled with water, but
tinged a deep blue with indigo. One of the tubes was arranged so as to
dip only about half an inch below the surface of the blue liquid, while
the other descended nearly to the bottom of it, and was slightly curved
upwards at its extremity. The arrangement will be readily understood
by reference to the annexed engraving. On applying the flame of a
spirit-lamp to the lower flask, the blue liquid was seen to ascend by
the tube on the left side; then reaching the large flask at the top, it
there circulated through it, in the direction of the darts, and descended
by the other tube, back again to the small flask at the bottom. Thus a
perfect circulation was seen to be kept up, and the heat, by means of
_convection_, carried from one flask to the other.
[Illustration]
After this Humphry sought for some means of rendering the currents of
heated air visible in the same manner. For such purpose he took a large
glass jar, having a wide opening at the bottom and a narrow one at the
top. Into the upper aperture he inserted a long lamp-glass, and down
this he placed a diaphragm of card, so as to divide the glass chimney
into two channels. Then the lad procured a shallow pan, and having
poured a little water into it, he set a piece of lighted candle in it
and covered it over with the jar and chimney, so that when the whole was
duly arranged it appeared as here shown. Then having lighted a piece of
brown paper and blown it out again, he held the smouldering end over the
chimney, and saw, by the curling of the smoke from the paper, that the
heated air from within was _ascending_ the lamp-glass by one side of
the diaphragm, and _descending_ by the other, in the direction of the
arrows in the illustration; whereas, when the card-board partition was
removed from the chimney, the currents _ceased_, and the light was soon
extinguished.
The boy applied the same simple means, likewise, to learn the direction
of the currents of air on opening the door of a heated apartment,
and found, by the smoke from a piece of smouldering paper, that at
the _upper_ part of the door the heated air from within was rushing
_outwards_, and at the _lower_ part the cold air from without was setting
_inwards_, whilst at the middle scarcely any draught, one way or the
other, was perceptible.
This naturally turned the boy’s attention to the subject of the wind,
which appeared to him to be merely a vast current set up in the
atmosphere by the heating power of the sun’s rays. He had noticed, too,
that, shortly after sunrise, a breeze frequently sprang up at sea and
blew towards the land, increasing as the day advanced, and declining
and ultimately expiring at about sunset; whilst in the evening, after
sundown, a wind often arose in the opposite direction—namely, _from_ the
land _towards_ the sea—and lasted the whole of the night, ceasing only
with the reappearance of the sun.
Humphry was therefore anxious to discover some experimental means of
reproducing these effects on a small scale.
Having procured a large shallow milk-pan, he filled it with cold water,
and then took a metal “hot-water plate,” and having poured some boiling
water into this, he set it in the middle of the pan, saying to himself
as he did so, “the cold water there, in the outer vessel, represents the
ocean, while the heated metal plate in the centre stands for an island
warmed by the rays of the sun; for the land, being a better absorber of
heat than the sea, will have its temperature raised some degrees higher
than the water in the course of the day.”
This done, the eager boy proceeded, by means of the smoke from a piece of
smouldering paper as before, to discover the direction of the currents
that would be set up in the air under such circumstances. As he held
the smoking paper at the edge of the pan, Humphry was delighted to see
the white fumes drawn towards the hot plate in the middle, or, in other
words, _from_ the miniature ocean _towards_ the mimic island encompassed
by it; and this he knew was precisely the current that was found to
prevail throughout the day in tropical countries.
[Illustration: DURING DAY.]
Then, to impress the phenomena firmly upon his mind, the boy drew in his
note-book the annexed diagrams, illustrative of the currents produced
in the atmosphere by the _heating_ of the earth during the _day_, and
the _cooling_ of it during the _night_. “But if the inequality of the
temperature between the land and the sea gives rise to such results, how
much greater,” mused the boy to himself, “must be the effect produced
by the difference between the heat of the earth at the equator—where
the average temperature is said to be 80°—and at the poles, where it is
calculated to be as low as 56° below zero, the difference being as much
as 136°! What a vast aërial current must be set up by such means!”
[Illustration: DURING NIGHT.]
Then the lad made another drawing, illustrative of the effect that would
ensue under such conditions, and he set above it a series of arrows to
show the direction of the currents that would be thus induced in the
atmosphere. For the air, being heated by the vertical sun at the tropics,
rises there, as it does up a chimney, while the colder air from the
northern and southern hemispheres glides in from below, on both sides of
the equator, to supply the place of that which has been made to ascend
by the heat; precisely in the same manner as, when the fire burns, fresh
air is continually rushing in under the door and windows. Then the heated
air, after rising to a considerable height above the earth, at length
flows over, as it were, and forms in the atmosphere an upper current
_from_ the equator _to_ the poles, where it becomes cooled, and is then
drawn down to supply the place of that which has been drafted _from_ the
colder _to_ the warmer regions. “But,” said the boy, as he surveyed the
drawing, “according to this the winds which are found to prevail in the
tropics should blow _north_ and _south_; whereas they are found to come
from the _north-east_ and _south-east_ quarters.”
[Illustration]
Humphry puzzled himself for a long time in endeavouring to explain the
phenomenon, but it was more than his philosophy could accomplish; so he
had to consult his old friend Mr. Tonkin again, and from him he learnt
that the change in the direction of the currents is due to the motion of
the globe and the unequal rates at which different parts of the earth’s
surface revolve. Consequently, as the currents of air which set in
towards the equator from the poles come from parts that revolve about the
axis at a much _slower_ rate than the equator itself, they _hang back_,
or _drag_, upon the surface, in a contrary direction to the rotation of
the earth itself; so that, while the globe turns eastward, they acquire
somewhat of a westerly course, and, appearing to come from the opposite
quarter, assume, therefore, the character of permanent _north-easterly_
and _south-easterly_ winds.
But to make the matter clearer, Mr. Tonkin exhibited the following
illustration to the boy, in which the effect of the earth’s motion in
changing the direction of the atmospheric currents is immediately
apparent.
[Illustration]
The old gentleman, however, informed Humphry that there are other
terrestrial currents produced by the process of _convection_. In the
ocean the same circulation of hot and cold streams is found to obtain;
for the sea, warmed by the heated shores of the tropical regions, is made
by _convection_ to move from the equator like a vast river, while from
the poles an immense current of colder liquid streams forward to supply
its place. For the same reason as was before explained in connexion with
the trade-winds, the polar current, having a slower rate of rotatory
motion, assumes, on reaching the equator, a westerly direction, and
so flows in one broad stream across the globe; then, striking against
the vast continent of America, it divides into two large streams. One
of these flows southward down the eastern coast of Southern America,
and finally enters the Pacific Ocean through the Straits of Magellan.
The other turns northward, enters the Gulf of Mexico, sweeps round the
coast in a powerful current known as the Gulf Stream, and then proceeds
along the Northern American shores to the coast of Newfoundland, where
it crosses the world again, and occasionally extends even to the western
shores of the British Isles.
The direction of these oceanic currents is indicated in the subjoined
chart:
[Illustration]
“There is, however,” continued Mr. Tonkin, “another great heat-stream
traversing the earth, though this takes place within the crust itself,
and is due more to _conduction_ than _convection_, as in the other cases.
For philosophers tell us, that the daily impressions of heat which the
earth receives from the sun, follow each other into the interior of the
mass, like the waves which start from the edge of a canal, and, like
them, become more and more faint as they flow on, one after another,
till they melt into the general level of the internal temperature. The
parts of the earth near the equator,” added the old man, “are more heated
by the sun than other parts, and on this account there is a perpetual
internal _conduction_ of heat from the equatorial to the northern and
southern regions. Then, as all parts of the earth’s surface throw off
heat into space by radiation, it is plain that at the poles, where the
surface receives but little warmth from the sun, a constant waste of
caloric is produced. There is thus a perpetual dispersion of heat _from
the polar parts_ into surrounding space, which is supplied by a perpetual
internal flow of heat _from the equator_ towards the poles. The radiation
from the surface of the earth,” Mr. Tonkin concluded, “has its limit in
the temperature of the planetary space in which it moves (for we may
conceive our globe to be like a heated ball cooling down, _in vacuo_),
and this has been calculated to be not more than 56° below zero,—which
low temperature, indeed, appears to be attained in the long absence of
the sun in a polar winter.”
The poetic boy was lost in wonder at the marvellous results to which
his investigations had led him, and his mind was filled with a sense of
sublimity at the thought of the enormous heat-tides that are continually
flowing through the atmosphere, the ocean, and the solid crust of the
earth itself.
“I’ll work it all out myself,” he cried; “that I will. I’ll not rest
until I know all that is known of Nature and her wondrous ways.”
CHAPTER VIII.
THE WONDERFUL EFFECTS OF HEAT.
The effects of heat are manifold.
In the first place, an increase of temperature _expands_ or enlarges
almost all bodies, while a decrease causes them to _contract_ or become
diminished in bulk.
Secondly. Heat _changes the form_ of bodies, converting solids into
liquids, and liquids into vapours; while cold, on the other hand,
condenses vapours into liquids, and causes liquids again to solidify or
congeal.
Thirdly. Heat causes _ignition_; that is to say, it changes dark opaque
substances to a bright transparent red, rendering them capable of giving
out light, when their temperatures are raised to a high degree, and, when
increased to the highest point, causing them to become even white in the
fire, and then endowing them with the properties of the solar beams, so
that their rays have the same power of traversing plates of glass, and of
producing chemical changes, even as the rays of the sun itself.
Fourthly. _Combustion_, or the burning of bodies, with the evolution of
flame, is another effect of heat. There is also a species of combustion
called _slow_ (_erema-causis_ is the chemical term for it), which is
unaccompanied with flame—as in the rotting of wood and other organic
tissues, the rusting of metals, and even the breathing of animals and
ourselves. In each of these processes there is the same combination of a
combustible body with the oxygen of the atmosphere—but at a much _slower_
rate—than in the more rapid and energetic forms of combustion; and hence
but slight increase of temperature (if any) is discernible, while no
flames or luminous gases that are perceptible to our senses are evolved
under such conditions.
Fifthly. _Phosphorescence_ is likewise produced by heat. During
combustion and ignition, bodies become _temporarily_ luminous; but in
states of what is called phosphorescence they are _permanently_ so; and
there are many substances—such as the compact phosphate of lime, the
dark-blue kind of Derbyshire spar, several varieties of heavy spar, and
powdered quartz—which acquire the property of shining constantly in the
dark after having been made nearly red hot.
Sixthly. _Electricity_ is induced by heat; for it has been discovered
that if a bar of the metal called antimony be heated at one end, while
the other is kept cool, an electric current ensues.
Lastly. Heat promotes both _vegetable_ and _animal life_. For not only is
intense cold destructive of organic existence, but the increased warmth
of the summer invariably calls into being an infinite number of plants,
flowers, insects, and the many forms of organised nature that give
variety and grace to the earth. Moreover, heat produces in ourselves,
and other sentient animals, a _feeling of warmth_, and the absence of it
a sensation of cold; by which we are enabled to measure—though hardly
with perfect accuracy—the different changes of temperature occurring in
the substances around us, and also, by the agreeable impression which we
derive from warmth, induced to seek that degree of heat which is best
fitted for the promotion of our health and development of our faculties.
* * * * *
Humphry began by studying the laws which regulate the expansion of bodies
under the three different forms in which they exist in nature, viz.
_solids, liquids, and gases or vapours_. To determine the expansion of
different solids, the youth procured short bars of the several substances
upon which he had decided to experiment. The bars were all of the same
length and thickness, and were accompanied with a gauge, which measured
their dimensions at ordinary temperatures.
The following diagram illustrates the apparatus employed. The first
step was to test the length and breadth of each bar that was to be used.
This was performed first, by placing it in the gap at the upper part of
the gauge, and seeing whether it exactly fitted between the notches; and
secondly, ascertaining whether it was precisely of the same diameter
as the hole at the bottom part of the plate. This done, the bars were
successively plunged into hot water, when, on applying them once more
to the gauge, they were found to be so much _enlarged_ in all their
dimensions that it was impossible to make them pass through either of
the apertures. After this they were severally cooled down, by immersion
in a mixture of snow and salt, to the temperature of the freezing-point
of water, when they were discovered to have considerably _contracted_
in bulk; so that they could be passed through both of the openings with
perfect ease.
[Illustration]
It was by such means Humphry ascertained that different solids possessed
different degrees of expansibility, and that metals are more susceptible
of change of bulk than other solid bodies. Each solid, however, was found
to have a rate of dilatation peculiar to itself. _Lead_, for instance,
when heated, from the freezing to the boiling-point of water, was
discovered by measurement to have expanded one-350th; _iron_, one-800th;
and _glass_, one-1000th.
_Platinum_, however, was found to be less expansible than iron, and
_copper_ more so. _Silver_, on the other hand, was more expansible than
copper, while _tin_ was more so than silver; _lead_, again, more than
tin, and _zinc_ even more than lead: so that glass was proved to be less
capable of being increased in bulk by heat than the metals; whilst,
among the metals themselves, platinum was ascertained to be the least
expansible, and zinc the most so.
On talking over these matters with Mr. Borlase, the doctor told Humphry
that the expansion of metals was a matter of considerable importance in
many arts. “For instance,” said the gentleman, “coopers put the iron
hoops upon their casks in a heated state, so that they may gradually
contract on cooling, and firmly bind the staves together. With the same
view the wheelwright heats the tire of his wheel, in order that it may,
as it cools, press strongly upon the ‘felly,’ or circumference; and, for
the same reason, the plates of large boilers are united with red-hot
rivets, which, during their contraction on cooling, draw the sheets of
metal closely and securely together.
“In the iron bridge,” continued the doctor, “which was constructed over
the Severn in Shropshire, when I was a lad, it has been found that the
arches are nearly one inch longer in summer than they are in winter; so
that, if due allowance had not been made for the expansion of the metal,
the stone piers, on which the arches rest, must have given way to the
pressure long before this. The same allowance for expansion, Humphry,
has to be made in the clamping together of stones in the construction
of church steeples; for the changes of bulk which occur in metals at
different temperatures, though comparatively small in amount, take place
with irresistible force.”
The perpendicularity of the walls of the Museum of Arts and Manufactures,
in Paris, it may be added, were restored by Molard, upon the same
principle. In consequence of the weight of the roof, the walls were
bulging outward, and, in order to straighten them, iron rods were laid
across the interior of the building, their ends being made to project
through the brickwork outside. These rods were then heated, and, when in
an expanded state, a strong iron plate was passed over each end of them,
and screwed firmly up against the exterior of the walls. As the rods
cooled, they naturally contracted, and drew the walls somewhat nearer
together. The bars were afterwards again elongated by heat, and again
screwed up previous to their contraction; and so, by a repetition of the
process, the walls were gradually brought to a perpendicular position.
Humphry was delighted with the ingenious applications of the expansion
and contraction of metals by heat and cold, and Mr. Borlase, observing
the interest he took in the subject, proceeded to explain to him how, by
the same principle, the alterations in the length of the pendulum of a
clock were “compensated,” and the instrument so made to vibrate seconds
at all seasons. For a pendulum to beat exactly sixty times a minute, he
told the boy, it was necessary that it should be a fraction more than 39
inches long, in the latitude of London. “If, however, the pendulum be
made of metal,” he said, “it will be liable to be _longer_ in summer and
_shorter_ in winter; so that the clock will be slow in the warm weather
season and fast in the cold: for when the bob is let down the one-100th
part of an inch the clock loses 10 seconds in 24 hours, and a change of
temperature equal to 30° (which is nearly the difference between summer
and winter, in our climate), will alter the length of the pendulum-rod
about one-5000th part, and so occasion an error in the rate of going of 8
seconds a day.
“To counteract the expansions of the metal rod of the pendulum,”
continued his preceptor, “there are many ingenious contrivances. The
simplest of these, perhaps, is as follows: A compound bar of two
differently expansive metals, such as steel and brass, is formed by
rivetting or soldering the two metals together; for if such a bar, with
the brass _uppermost_, be placed upon a heated plate, it will be found to
warp or curve _downwards_, in consequence of the expansion of the brass
being greater than that of the steel. If, however, on the other hand,
the compound bar be placed on a plate cooled down by a mixture of snow
and salt, it will be found to warp or curve _upwards_, because the brass
will contract the more with the cold. Now, if two such compound bars,
with the most expansible metal at top, be placed at the upper part of a
pendulum-rod, one on either side of it, and firmly fixed at one end, they
will, as they warp upwards or downwards, tend to shorten the pendulum-rod
when it becomes lengthened by the heat, and to lengthen it when it
becomes contracted by the cold.”
To make this more readily intelligible to Humphry, the doctor exhibited
to him the following engravings:
[Illustration]
“Let us now,” said Mr. Borlase, as he placed his finger on the centre
drawing, “suppose the pendulum, with the compensation-bars perfectly
horizontal, to be vibrating seconds at a temperature of 60°, and that,
some few months afterwards, the heat rises to 80°; in such a case, of
course, the pendulum-rod would be _elongated by the heat_, and the
longer the rod the slower the vibrations, so that it would then vibrate
_less_ than sixty times in the minute. The effect of the increase of
temperature, however, on the compensation-bars (the most expansible
metal being uppermost), would be to warp them _downwards_ (as shown in
the left-hand drawing),” said the doctor, pointing to the illustration,
“and thus they would _shorten_ the pendulum-rod as much as the heat had
lengthened it. In cold weather, however, Humphry, the metal rod of the
pendulum would be _diminished in length_; but then the compensation-bars
would warp _upwards_, and so tend to _elongate_ it, to the same extent as
it had been contracted by the cold (as may be seen on reference to the
picture on the right hand).”
The next day Humphry was busy making experiments concerning the expansion
of _liquids_. He first took a large thermometer tube and poured into it
a sufficient quantity of spirits of wine to fill the bulb at the bottom
and make the fluid rise some few inches in the stem above. Then, having
marked upon the glass with a file the level at which the spirit stood at
an ordinary temperature, the boy plunged the instrument into a vessel of
boiling water, and immediately beheld the liquid rise in the tube till
it stood several inches above its former level. After this he immersed
the tube in a mixture of snow and salt, and found the liquid contract,
so that it fell in the stem almost down to the bulb. On removing the
instrument, however, the fluid immediately commenced rising again, and so
pleased was the youth with the motions that he repeated the experiment
over and over again, being not a little delighted to perceive that each
time, as he plunged the instrument into the hot or cold bath, the spirit
invariably rose or fell to precisely the same place in the tube.
To measure the different rates of expansion among different liquids,
the young chemist provided himself with a long and narrow glass tube,
which was graduated into cubic inches, and into this he poured a certain
quantity of the liquid he wished to experiment upon. Then plunging the
graduated tube into the snow-and-salt mixture, he noted the precise
volume of the fluid at that temperature; after which he immersed it in a
vessel of boiling water, and then noted again how many cubic inches it
occupied in the tube at the higher temperature; so that the difference
told him how much the liquid had been expanded between 32° and 212°, or
the freezing and boiling point of water.
It was thus the lad ascertained that 9 measures of spirits of wine at
32° become expanded into 10 measures at 212°, and 9 measures of strong
aquafortis also become 10, between the same extremes of temperature.
Again, 12 measures of olive oil are increased into 13, while 14 of
ether and the same quantity of oil of turpentine swell each into 15,
with the like increase of heat. Then 17 measures of oil of vitriol, at
the freezing-point of water, are dilated into 18 at the boiling-point,
and 22¾ measures of water are increased to 23¾ within the same range of
temperature, while 55½ of quicksilver become 56½ when similarly treated;
consequently spirits of wine is no less than 6 times more expansible than
quicksilver, so that in the depth of winter 100 pints of spirits of wine
are dilated into 105 in the height of summer.
While making his experiments, however, as to the rate of expansion in
liquids, the boy had been astonished to perceive, when the tube contained
water, that, on placing it in the mixture of snow and salt, the liquid,
as it was cooled down, continued to shrink till it had attained the
temperature of about 40°; and then, _instead of contracting any farther_
(as was the case with other liquids till they froze), _it began to expand
slowly, and kept rising in the tube until it congealed_. He noticed, too,
that, when the water was at its freezing point, or 32°, it was of the
same bulk as it was at 48°; so that it expanded just as much for the 8°
below 40° as it had contracted in the 8° above that point.
Humphry then tried another experiment illustrative of this remarkable
property of water. Having produced two cylindrical glass vessels, he
surrounded one of them at the bottom with a circular tin tray, that
fitted closely to the exterior of the cylinder, and affixed to the other
a similar tray, but this he placed at the upper, instead of the lower
part of the cylinder as before—in the manner represented in the subjoined
engraving:
[Illustration]
A thermometer then having been placed in each of the glass vessels, they
were respectively filled with water at 50°, while a freezing mixture of
pounded ice and salt was placed in each of the trays.
After the temperature of the whole of the water in both vessels had been
reduced to 40°, it was found by the thermometer in the vessel, with
the freezing mixture _at the top_, that the cooling effect would not
proceed downwards, _but was limited to the surface_, where the water
ultimately froze; for the ice-cold water being _lighter_ than the water
below at 40°, necessarily _floated_ like oil upon the surface. In the
other cylinder, however, where the cold was applied at the _lower_ rather
than the upper part of the water, the effect was very different; for
there, the liquid becoming lighter, as its temperature sank below 40°,
_ascended_, whilst the warmer and heavier water at the top _descended_,
until it was cooled, and so expanded in its turn; and thus the _whole_
of the liquid was ultimately reduced to the freezing-point; whereas
in the other cylinder this effect was limited to the _surface_ only.
Humphry now could see the reason why lakes and ponds froze only on the
surface, and why, on breaking the ice (as he had repeatedly done when out
snipe-shooting with his uncle, Leonard Millett), the water underneath was
always found to be warmer than the air above.
* * * * *
The lad had now but to investigate the rate of expansion among _aëriform
bodies_, or _gases_, to complete this part of the subject.
Accordingly, he took the thermometer tube he had before used, and placed
it, with its open end, downwards, in a glass of water, thus:
[Illustration]
The tube was of course filled with air, so he applied his palm to the
bulb, and found the heat of the hand sufficient to expand the air within,
and drive a stream of bubbles up through the water. On removing the
source of heat, however, the volume of air began to contract, and the
liquid to mount in the tube, so that he could see by the height the water
rose in the stem the amount of expansion which the air had undergone.
Humphry then proceeded to ascertain the amount of expansion produced in
a given quantity of air, when heated from the freezing to the boiling
point of water, and discovered that 100 cubic inches of air at 32°
become dilated to 137½ cubic inches at 212°. Air, therefore, at the
freezing-point, expands one-480th for every degree of heat that is added
to it; so that 480 cubic inches at 32° become 481 at 33°, and 482 at
34°, and so on, the volume expanding one cubic inch with each additional
degree of heat. A volume of air, therefore, at 32° would be doubled
at 480°, and tripled at 960°, the latter temperature being that of a
dull-red heat.
Steam, and other vapours, when heated by themselves, are subject to the
same law of expansion as air.
But although the expansion produced in aëriform bodies by heat is great
in amount, the actual force which is thus developed is small when
compared with that of solids and liquids under the same circumstances.
This is owing to the extreme elasticity of aëriform bodies; so that,
although air becomes tripled in volume at a red heat, vessels are easily
found capable of sustaining the pressure of the expanded fluid. It is
only when a portion of liquid is present, so that _volume after volume_
of vapour is added to those already generated—as in the production of
steam—that, on resisting the expansion, the pressure becomes enormous,
and mounts up to a dangerous point.
CHAPTER IX.
THE WONDERFUL EFFECTS OF HEAT—(_continued_).
“But heat,” said Humphry to himself, as he reviewed his previous
experiments, “not only _expands_ the bulk of different bodies, but it
_changes their form_, rendering certain solids liquid, and converting
liquids into vapours. Let us see now what occurs during such changes.”
Accordingly, the youth proceeded to pound some ice, and to cool it down
in a tumbler by means of a freezing mixture to the temperature of zero.
The tumbler was then inserted in a bath of tepid water, the temperature
of which was maintained, by an argand-lamp beneath it, constantly at
60°. A thermometer having been plunged in the ice, the quicksilver was
observed to mount rapidly in the tube until it reached 32°, when the
ice began to liquefy. But, though another thermometer in the water-bath
showed that the liquid there was still 60° hot—the same temperature,
indeed, as when the ice was first immersed in it—nevertheless
the thermometer in the tumbler remained _stationary_ at 32°—the
freezing-point. Nor did it begin to rise until the whole of the ice was
melted; after which it mounted gradually, and ultimately settled at 60°,
the temperature of the surrounding water.
Humphry was astonished at the effect. “Why did the thermometer,” he asked
himself, “remain fixed at the freezing-point until the whole of the ice
was melted? The heat from the surrounding water must have been entering
the ice as much when it began to dissolve, as it did before the thawing
occurred, or even afterwards. How, then, came it that the thermometer was
not affected by it?”
The eager boy was so puzzled with the mystery, that he could not rest
till he had tested the result by another experiment.
Having procured two small glass globes, he filled them both with the same
quantity of water. The liquid in one he froze, and that in the other
he cooled down to 33°, so that it might be as near as possible to the
temperature of the first, without being solid. When the ice in the one
globe had just begun to melt, and the thermometer in that vessel marked
32°, the two globes were plunged into a water-bath, the temperature of
which was kept at 47° throughout the experiment.
The vessels, therefore, were, as near as possible, under similar
conditions of temperature, within and without, and with similar
contents—except that the one contained _ice_, and the other _water_.
The progress of the heating was then noted.
In the globe which contained the _water_, Humphry found the thermometer
rose in half an hour to 40°. In the globe, however, which contained the
_ice_, no less than 10½ hours elapsed before the whole was melted, and
the temperature of the resulting liquid raised to 40°, like the other; so
that the rate of heating in the ice-vessel was 21 times slower than in
that which held the water.
“How can this be?” mused the boy-philosopher. “The actual amount of heat
received by the two must have been uniform during the whole time, and
yet the ice took 10½ hours to have its temperature raised to 40°, while
the ice-cold water needed only ½ an hour to acquire the same heat. How
extraordinary!” he inwardly exclaimed; “what can be the cause of it?
For,” said the lad, as he jotted the figures down in his note-book,
“the water in the globe had its temperature raised 7° in half an hour,
consequently that must have been the rate at which the warmer liquid in
the external bath was giving out its heat to the two globes. Accordingly,
in 10½ hours it must have given 10½ times 7°, or 147° in all, to the one
containing the ice. Consequently the _ice_ required 140 more degrees of
heat to raise its temperature to 40° than the _water_ did.”
Humphry still doubted the accuracy of his conclusions; for they were so
marvellous and unexpected by him, that he sat for a long time considering
by what experiment he could bring the matter to a positive test. “If,”
he inwardly exclaimed, “the ice really received 140° more heat than the
water, what became of them? The two were each, in the end, of the same
temperature; and the thermometer didn’t show, nor could I feel, that the
one had imbibed more heat than the other.”
Presently the boy started to his feet, for a sudden thought had struck
him. He would take 12 oz. of pounded ice, and upon this he would pour
the same weight of hot water, at a temperature of 172°; so that the
difference between the heat of the ice and that of the water would be
172°-32°, or exactly 140°, and this was precisely the quantity of heat
that the ice in the previous experiment had received over and above the
water.
Humphry wondered again and again, as he prepared the experiment, what
the result would be; and when he mixed the warm water with the ice, he
was overjoyed to find that the temperature produced was only 32°—the
same as that of the pounded ice itself. Hence it was plain the water had
lost exactly 140° of its heat, and, moreover, that these had _entirely
disappeared_, since the temperature of the colder substance remained the
same as at the outset of the experiment.
Then came the question—What had become of the lost heat? Where had it
gone? What effect had it produced?
There was but one answer! and this Humphry was not long in divining.
The heat which had disappeared had _combined_ with the ice, and its
effect had been, not to raise its _sensible_ temperature, but simply to
convert the solid body into a liquid one, so that the caloric had become
_latent_, or imperceptible to the senses, as well as incapable of being
detected by the most delicate thermometer.
“So, then,” cried the boy, as the new thought flashed upon him, “a
_liquid is merely a solid, whose particles are kept asunder by so much
heat, which is insensible to us_. In this piece of wax the particles are
held together by a certain force, which is called the ‘attraction of
cohesion,’ so that when I press upon it I am incapable of separating one
part of it from another. If, however, I apply but a little heat to the
substance, the cohesion is soon destroyed, and the particles, instead
of then firmly adhering one to another, become free to move, and are,
consequently, easily separable; so that a rod can be plunged into it when
liquefied, and moved about in it with little or no difficulty.”
“Yes,” he repeated, “a liquid is merely a solid, whose particles are kept
asunder by heat.”
Humphry was so pleased with the result he had arrived at, that he varied
the experiment in a number of different forms.
First, he took some spermaceti (a substance which melts at 112°), and
found that a thermometer plunged in this (as in the ice) remained
stationary at the melting-point until the whole was liquefied, and that
it was not till then that the temperature could be raised above the point
of liquefaction.
Next he tried the same experiment with some lead, and found that, though
he heated a ladle-full nearly red hot, the temperature of the whole was
immediately cooled down again to the fusing-point by the addition of a
piece of the solid metal.
Hence the law was manifest, _that in all cases of liquefaction a certain
quantity of heat, not indicated by the thermometer, is absorbed, or
disappears—this heat being withdrawn from surrounding bodies, and so
leaving them comparatively cold_.
Accordingly, now that Humphry had ascertained that liquefaction itself
was a source of cold, he proceeded to try what degree of cold he could
produce by causing certain substances to melt rapidly.
For this purpose he took some snow and sprinkled a little salt over it,
when he observed that the two solids immediately formed a liquid; and
then, plunging a thermometer into the mixture, he beheld the quicksilver
sink and sink, until it had nearly reached zero, so that it fell from
32° to 0°. Then the boy made a mixture of 5 parts of smelling salts, 5
parts saltpetre, and 16 parts water, and plunging the thermometer into
this, he found that it sank from 50° (the temperature of the room in
which the experiment was made) to 10° (which is several degrees below
freezing-point). With a mixture, consisting of equal parts of saltpetre,
ammonia, and water, the thermometer fell 6° lower than in the previous
experiment—or from 50° to 4°. Again, with 5 parts of Glauber’s salts
and 4 parts of oil of vitriol and water, the temperature sank 1° lower
still—or from 50° to 3°. Further, having finely powdered some of the
crystals of the last-mentioned salt, he drenched them with muriatic
acid, when the salt dissolved to a greater extent than it had previously
done in the water, and the consequence was that the temperature fell
even lower than before—or from 50° to 0°, while the vessel in which the
mixture was made became covered with hoar frost; and when some water in a
tube was plunged into the liquefied salt, it was speedily converted into
a mass of ice.
Humphry was now anxious to see whether the heat which is absorbed, and
becomes latent or insensible during the liquefaction of bodies, really
_remains_ in the liquid, and whether it is given out again or emitted
during their solidification.
To satisfy himself upon this point he prepared two vessels, one full
of water and the other full of brine, the temperatures of the liquids
in each being, at first, precisely 52°. On a very cold day, when the
thermometer stood at 22° (_i.e._ 10° below freezing-point), the boy
exposed the fluids in these two vessels, with thermometers in each, to
the open air, and found that they both gradually parted with their heat
to the surrounding atmosphere, and were soon cooled down to 32°; then the
water began to freeze very slowly, and during this the thermometer in the
water-vessel remained perfectly stationary. The brine, however (which
does not congeal till its temperature sinks to 4°), continued to cool on,
the thermometer in it sinking without interruption, until it gradually
reached the temperature of the external air, or 22°.
Now it was plain that both liquids, in cooling, were alike parting with
their heat to the colder air. Why, then, should one and not the other
_suddenly cease giving out caloric_, and refuse for a certain time to be
cooled down to the same level as the atmosphere around it?
The only explanation of the problem was, that the water, during the
process of freezing, was parting with the 140° of heat which Humphry
had before seen were necessary to retain it in a liquid form, and that
it was the evolution of this amount of caloric which served to keep the
temperature of the water for a considerable time at 32°—notwithstanding
the cooling effect of the surrounding atmosphere.
Still this experiment hardly satisfied the boy. He wanted to have some
_sensible_ proof of the evolution of heat from water during the act of
freezing. It was true, there was no way of accounting for the fact of the
thermometer remaining stationary in one vessel while it was continually
sinking in the other, except by supposing that the latent _insensible_
heat was being given off from the water as it passed into the solid
form. Humphry desired, however, to _see_, as it were, the heat so given
off—that is to say, he wished to behold the thermometer _rise_ instead of
merely _remaining fixed at one point_.
Accordingly it struck the lad, that if he was to keep some water in
a vessel perfectly still, and to prevent even the air from agitating
its surface, he might be able to cool it down some few degrees below
the freezing-point, and then he should be able to see if, in the act
of freezing afterwards, the thermometer would really rise as the water
solidified. So successfully did Humphry perform this experiment, that
he was enabled, by great care, to cool some water in a vessel down to
22° without freezing it. Then he felt a thrill dart through his frame as
he agitated the water, and beheld it immediately shoot into a thousand
transparent crystals—whereupon the thermometer in the vessel instantly
began to rise, and soon stood at 32°, thus showing that the water had
acquired 10° of heat almost in an instant.
Whence came this heat, then?
But one answer was possible. It was manifest that the water, in the act
of solidifying, _gave out_ heat, even as in the act of liquefying ice
_absorbed_ it.
“But do all things,” said Humphry to himself, “give off heat as they
pass from a liquid to a solid form? And is solidification, therefore,
a _heating_ process to surrounding bodies, in the same manner as
liquefaction (from the absorption of heat at such times) is a _cooling_
one?”
The youth had heard that, if a strong solution of Glauber’s salts be
poured, while hot, into a flask, and corked tightly down, it will remain
liquid when cold, and that on removing the cork it will immediately shoot
into a fibrous mass of crystals. He wished to see, therefore, whether, in
the act of solidification, heat is evolved from such a solution.
Humphry was not long in preparing the requisites for the experiment, and
was then delighted to find, as he watched the crystals, immediately that
he withdrew the cork, dart from the surface downwards, the temperature of
the substance became so much increased, that the bottle which contained
it grew sensibly warm in his hand, though it was perfectly cold before.
Now the lad knew, that in making the solution he had added 2 ounces
of water to 3 ounces of the salts, and as the whole of this became
solidified on opening the bottle, it was clear that the elevation of
temperature arose principally from the _solidification of the water_ in
the crystalline mass.
Next Humphry added a saturated solution of tartaric acid to some strong
liquid ammonia, and found, immediately that the two fluids were poured
together, a solid substance was thrown down, and considerable heat
evolved.
Again, the lad was aware that, on mixing powdered plaster of Paris with
water, there is a like increase of temperature at the moment of the
composition “setting;” or, in other words, when the water with which the
plaster has been mixed passes into the _solid_ form.
Further, he could now perceive that the great increase of heat which
occurs during the slacking of lime is due merely to the same cause,
viz. the _solidification of the water poured upon it_. Consequently it
was plain that water, in passing from the _liquid_ to the _solid_ form,
invariably _evolves_ the heat which is necessary to retain it in a liquid
state, and which as it passes, on the other hand, from the _solid_ to the
_liquid_ state, it as invariably _absorbs_.
Not only, however, is there an increase of temperature when water becomes
solid, but Humphry found the same result to ensue, even when the same
liquid is _condensed_. On mixing 4 parts of strong oil of vitriol
with 1 part of water, cooled down to the freezing-point, he perceived
that the two together occupied considerably less space than they did
alone, and that the mixture rose rapidly from 32° to 212°, or from the
freezing to the boiling point. The same result ensued when 1 part of
snow was substituted for the 1 part cold water; but, strange to say,
when the proportions were reversed, so that there were 4 parts of snow
to 1 part of oil of vitriol, _intense cold_ instead of intense heat was
produced—the _increase_ of temperature in the one case arising from
the _condensation_ of the water from the snow, and the _decrease_ of
temperature, on the other hand, being due to the _liquefaction_ of the
snow itself.
Moreover, the boy was aware that a piece of soft iron, when hammered,
becomes intensely heated; and he had heard that when a bar of red-hot
iron is passed through a rolling-mill, its temperature is so much raised
that it is rendered nearly white-hot by the extreme pressure, and the
consequent condensation of the particles.
_Sudden expansion_, on the other hand, Humphry found to be a cooling
process; and this is one of the reasons why high-pressure steam, on
issuing from a small aperture, instead of scalding the hand as ordinary
steam would, scarcely feels warm, even though its temperature be some
hundred degrees higher than the vapour at a low pressure; for as the
compressed steam escapes into the atmosphere, its instantaneous
expansion so far cools it, that it is deprived of all power of burning.
Moreover, at the fountain of Hiero, in Hungary, a part of the machinery
for working the mines consists of a column of water 260 feet high, which
presses upon a large volume of air, enclosed in a tight reservoir, so
that the air within is greatly condensed by the enormous weight of the
water; and that when a pipe communicating with this reservoir is suddenly
opened, the condensed air rushes out with extreme velocity, and then
instantly expanding, absorbs so much heat as to precipitate the moisture
it contains in a shower of snow—a hat held in the blast being immediately
covered with it. So strong, however, is the current of condensed air,
that the workman who holds the hat is obliged to lean his back against
the wall to retain it in its position.
Another illustration of the cooling effect produced by the sudden
expansion of condensed air is afforded in the fact, that if the blast
from an air-gun be directed upon a delicate thermometer, the temperature
will be found to be lowered at the moment of the discharge.
* * * * *
As yet Humphry had dealt only with the effects produced by the conversion
of solids into liquids, or liquids into solids, and there still remained
for him to learn what results ensued when _liquids were changed into
vapours, or vapours into liquids_.
Accordingly he proceeded to heat some water in an open vessel, and found,
as the temperature gradually rose, that the vapour continued to form on
its surface till the thermometer reached 212°, when the liquid became
violently agitated. But then, although the fire was kept up beneath the
vessel as strong as at first, and _the heat continued to flow into it
as before_, the quicksilver in the thermometer became stationary, and
remained so until the whole of the liquid had been dissipated in the form
of steam.
Here, then, was another instance of the absorption of heat during a
change of form; and it was evident, that as water required a certain
amount of temperature in order to convert it from a _solid_ into a
_liquid_ state, so did it need a proportionate supply of heat to change
it from a _liquid_ into a _vapour_.
“The heat absorbed during the boiling passed off, perhaps, in the steam,”
thought Humphry.
On testing the temperature of the vapour, however, it was found to be no
hotter than that of the water during its ebullition.
What, then, had become of the heat which had been added since the boiling
commenced?
Why, it had been rendered _latent_ or _insensible_, being necessary for
retaining the liquid in a more rarified form; for as a liquid is but a
solid whose particles have been separated, and so made free to move, by
the latent heat existing between them, so a vapour is merely a liquid,
whose atoms have been driven farther asunder, and made still more easily
separable, by the heat imbibed during the process of vaporization. It
was evident, therefore, that the production of vapour is attended with
a loss of sensible heat, and that, as in the case of liquefaction,
heat _disappears_ in order to constitute the liquid, so in the case of
evaporation a considerable quantity of heat becomes _latent_ in the
vapour.
To render this part of the subject still more clear, Humphry filled
a flat-bottomed tin vessel with a definite quantity of water, at the
temperature of 50°. Then, having placed the vessel upon a heated plate,
he found that in 4 minutes it had acquired a temperature of 212°, and
began to boil, whilst in 20 minutes the whole had evaporated, having been
dissipated in the form of steam. The water, therefore, had received 212°
- 50°, or 162° of heat in 4 minutes, which is at the rate of 40½° in
each minute. The heat, however, continued to flow into the water at the
same rate during the whole 20 minutes, so that the entire amount of heat
received must have been 40½° × 20, or 810°, and this had become _latent_
in the steam. Consequently the total quantity of heat required to
evaporate boiling water would be sufficient to raise the water—provided
it remained all the time in the liquid state, instead of being converted
into steam—as much as 810° above the boiling-point, or altogether to
1022°.
To verify this conclusion, Humphry heated some water under pressure in
a “Papin’s digester,” so that the liquid was prevented evaporating, and
raised the temperature of it to 400°. Then the lad opened the valve,
and part of the water suddenly rushed out in the form of steam, when
the temperature of that remaining in the digester _sank immediately_ to
212°. Consequently 188° (400° - 212°) of heat had suddenly disappeared,
having been carried off by the steam. It was afterwards found that only
⅕th of the water had gone off in vapour, so that this vapour must have
contained not only its own 188°, but the 188° lost by the 4 other parts
remaining in the digester; that is to say, the steam must have contained
188° × 5, or 940° of heat altogether. This experiment therefore showed,
that steam is water _combined_ with nearly 1000° of heat; and in the
same manner as the water had been previously observed to _give out_ its
heat of liquidity, and to make the thermometer _rise_ at the moment of
its conversion into _ice_, so was it now seen to _absorb_ a considerable
amount of heat, and to make the thermometer _fall_ at the moment of its
conversion into _vapour_.
Evaporation, therefore, should be a means of producing cold, in the same
manner as liquefaction had been previously proved to be; for if it be
necessary, in order to convert liquids into vapours, that a certain
amount of heat be absorbed, it is plain that such heat must be drawn from
surrounding substances, and thus the vaporization of one body will be a
cooling process to others near it.
To test this, Humphry spread out a wet cloth in a keen wind when the
atmosphere was a few degrees above the freezing-point, and found that the
particles of water, as they passed into the form of vapour,[35] carried
off so much heat from the liquid in the cloth (in the same manner as the
steam did in the Papin’s digester) that the remainder became frozen,
while the cloth itself was rendered hard and stiff by the formation of
ice in its pores.
Humphry, however, knew that ether was much more vaporizable than water at
ordinary temperatures. Accordingly he availed himself of this substance
in the production of cold by spontaneous evaporation.
First, he folded a strip of cambric round the bulb of a small
thermometer, and allowed some ether to dribble over it, while he
increased the evaporation by projecting a current of air upon it by
means of a bellows. The quicksilver was immediately seen to fall several
degrees below the freezing-point, and on substituting a thin glass tube
containing a small quantity of water for the thermometer previously
used, the boy was enabled to produce ice by the same means.
Next, the lad made use of the air-pump which he had previously
constructed out of the rudest materials,[36] in order to facilitate
the spontaneous evaporation of water, by removing the pressure of the
atmosphere from the surface of it.
Upon the plate of the apparatus he placed a soup-plate, and this he half
filled with oil of vitriol; above the soup-plate he stood a tin basin,
supported on three pieces of tobacco-pipe, and three parts filled with
water, in which a small thermometer was immersed; while over the whole
he put the glass receiver. The arrangement is shown in the annexed
illustration:
[Illustration]
The pump was now set to work, and the air gradually drawn out from the
glass receiver, whereupon the thermometer was observed to sink; for as
the pressure of the atmosphere was removed the evaporation from the water
was increased, while the oil of vitriol at the bottom served to absorb
the vapour as fast as it was produced: so that, though the temperature
was considerably lowered, the evaporation from the water ultimately
became so rapid that it had all the appearance of boiling, and in the
course of 5 or 10 minutes the liquid was converted into ice.
Before the solidification took place, however, the thermometer was
observed to fall several degrees below the freezing-point, whilst at the
moment of its freezing it rose to 32°, in consequence of the escape of
the heat which had previously served to keep the water liquid.
The explanation of the process is almost obvious. As in the case of the
steam issuing from the Papin’s digester, that part of the water passing
off in the form of vapour abstracted heat from the remainder of the fluid
portion, which, thus losing the caloric that served to keep it liquid,
became solid, or froze.
Humphry was now anxious to see what effect the simultaneous evaporation
of ether and water would produce under the air-pump. Accordingly he
procured a thin glass flask, and this he inserted in a tumbler, so that
it fitted almost close. Having poured a little ether into the flask, and
some cold water into the tumbler, he placed the whole apparatus under the
receiver of the air-pump, as here represented:
[Illustration]
On exhausting the receiver the ether was observed to _boil_, from the
rapidity of its evaporation, while the water in which it was immersed
soon became solidified, or converted into _ice_.
The apparent anomaly of two liquids made to _boil_ and _freeze_ at one
and the same time puzzled the lad for a while. At length, however, he
divined the reason. The ether, in passing into the form of vapour,
required a certain amount of heat to sustain it in that state, and this
it absorbed principally from the water that surrounded it, which soon
became congealed owing to the loss of that portion of heat which was
requisite for its maintenance in a fluid form.
Humphry had now discovered that, by removing the pressure of the
atmosphere, the evaporation of liquids proceeded at a much greater rate,
and he was anxious to learn whether they could be made to boil at a lower
temperature by the same means.
Accordingly, he fitted a stop-cock into the neck of a Florence flask, and
then turning the cock on, so that the vapour might escape, he proceeded
to heat the water, over the flame of a spirit-lamp, till it boiled;
whereupon he removed the flask from the flame and closed the cock. The
liquid then soon ceased to boil; on plunging the flask, however, into a
vessel of _cold_ water, he found the ebullition instantly to recommence,
but to cease again directly the vessel was held near the _fire_ or over
the _lamp_. Now during the boiling, in the first instance, all the air
above the liquid had been driven out of the flask, and replaced by an
atmosphere of steam; this, upon plunging the vessel into cold water, had
become condensed into a liquid form, so that a vacuum being formed above
the water, the fluid boiled at a lower temperature under the diminished
pressure. On removing the flask, however, from the cold medium, a new
atmosphere of steam was generated, and the pressure of this on the
surface of the liquid prevented its boiling any longer; and thus the
water was made to _boil by being cooled, and to cease boiling by being
heated_.
Humphry afterwards ascertained, that if a glass of water of the
temperature of 90° or 100° be placed under the receiver of an air-pump,
and the pressure of the atmosphere removed by exhausting the air, the
water boils violently at that temperature, and continues to do so until
the whole receiver becomes filled with the vapour; which then, pressing
upon the surface of the liquid, again prevents its ebullition. By
continuing to pump out the vapour, however, the boy was enabled to keep
the water boiling at no less than 112° below its boiling-point in the
open air.
With alcohol and ether, however, it was not even necessary to warm them,
for these fluids boil under the air-pump at all ordinary temperatures.
It has likewise been found that water boils at less than 212° upon the
summits of hills and mountains, where the pressure of the atmosphere is
considerably diminished. At the top of Mont Blanc water has been made
to boil at 187°, and even when the air is lighter at the surface of
the earth the boiling-point of all liquids is reduced; so that in this
country, where the density of the atmosphere fluctuates considerably,
water boils sometimes at 2° lower than 212°, whilst on heavy days it
requires to be raised to 214° in order to produce ebullition. Even
the cleanliness of the vessels in which the liquid is heated has been
ascertained to alter the boiling-point. In glass vessels, from which all
chemical and mechanical impurities have been removed by perfect cleaning,
water may have its temperature raised as high as 220° without being made
to boil; whereas a few metallic filings, or other finely-divided or
insoluble materials, have the effect of causing it to boil at a lower
temperature than 212°.
Humphry would now have sought to learn how much water becomes expanded
in passing into the form of steam. But, though he made several rude
experiments on the subject, he was unable, from the want of proper
apparatus, to arrive at any definite result, and so was obliged to rest
contented with the knowledge which his books afforded him; viz. that
a cubic inch of water becomes converted, at 212°, into very nearly a
cubic foot of steam, the expansion being about 1700 times the bulk of the
original fluid.
Spirits of wine, on the other hand, expand only 493 times, ether about
212 times, and oil of turpentine 192 times; each at the temperature of
212°. Steam, however, is lighter than air—whereas the vapours of spirits
of wine, ether, and turpentine, are much heavier than it, the last being
nearly 5 times the density of our atmosphere at the same temperature.
Before quitting this part of the subject, there is one striking anomaly
connected with the production of vapour that deserves mention here,
though it is but a recent discovery.
If a silver, or other metal spoon, be heated to redness in the flame
of a lamp, and some water be dropped into it while red hot, it will be
found that the liquid, instead of passing off at once into steam, will
instantly assume a globular, or spheroidal form, and float about the
heated metal, revolving with rapidity, and evaporating very slowly; while
the _temperature of the liquid will remain constantly below the boiling
point_—so long as the red heat is maintained. If, however, the lamp be
withdrawn, the water, as the spoon cools down, will suddenly be made to
boil with violence, and be dissipated in vapour with almost explosive
energy.
The cause of this singular phenomenon is, that the water is separated
from the red-hot metal by an atmosphere of highly-elastic steam, which
is generated immediately the liquid is projected on the heated surface,
and which, encircling the water, serves to keep it in a spheroidal,
or globular state; whilst the vapour, being a bad conductor of heat,
prevents the temperature of the hot metal being communicated to the fluid
in connexion with it.
This is the reason why water, when accidentally dropped upon the heated
bars, or hobs of a grate, is occasionally observed to run along them like
globules of quicksilver. If these, however, be smartly struck with a
hammer, so as to bring them suddenly into contact with the hot metal, the
globules will be instantaneously converted into steam, and the change of
form attended with a slight explosion.
Another form of the same singular phenomenon consists in plunging a mass
of white-hot metal into a vessel of cold water, when the incandescence
will be found to continue, rather than to be quenched, in the liquid, the
metal still shining with a bright white light, while the water may be
seen to circulate around, though at some distance from, the glowing mass,
being separated from it by an atmosphere of non-conducting vapour, which,
for a time, prevents its heat being communicated to, and so reduced by,
the surrounding fluid. At length, as the metal cools, the water around
it is brought into contact with the heated surface, when it is made
suddenly to boil with energy.
Moreover, if an iron shell containing water be made red hot, and a hole
then drilled in it, no water will be found to flow through the orifice
until the iron has been considerably cooled, when it will suddenly issue
forth with great violence, in the form of steam.
So, again, if water be poured upon an iron sieve, the wires of which have
been heated to redness, it will not pass through the interstices. As the
sieve cools down, however, it will be found to run through rapidly.
Further, if a red-hot cinder be let fall into a pan of water, it will be
seen to swim upon the surface, and then to sink with a hissing sound,
accompanied with a sudden irruption of steam.
But a far more striking illustration of this strange property consists
in heating to redness a silver or platinum capsule (or small crucible),
and filling it while red hot with a freezing mixture, when the whole mass
will instantly be thrown into the spheroidal state, and on introducing
a thermometer therein the temperature of the liquid will be found to be
scarcely increased, so that a small tube filled with water soon becomes
frozen when immersed in it; and _thus ice may thus be produced even in a
vessel, the heat of which is no less than 1000°_. Indeed, by introducing
some ether and solid carbonic acid into an incandescent crucible, even
quicksilver itself has been made to freeze in it, though this requires
a temperature of 82° below the freezing-point of water; and yet this
extreme cold (equal to that of a Polar winter) has been produced, _and
mercury frozen inside a red-hot vessel_.
* * * * *
Humphry, however, was anxious not to conclude his investigations
concerning the changes of form produced by heat without ascertaining
whether all bodies, in passing from a lower to a higher temperature—or,
on the other hand, from a higher to a lower one—absorbed the same
quantities of caloric; that is to say, did one body require a _greater
amount of heat_ to raise it to a given temperature than another, and did
some bodies give off more heat than others in cooling?
First, the lad dealt with equal quantities of the _same_ fluid at
different temperatures, in order to determine whether, on mixing the
two together, the resulting temperature amounted to the _mean_ of both.
He added ½ pint of cold water at 50° to ½ pint of warm water at 100°
and found that the two together gave a _mean_ temperature of 75°
(⁽¹⁰⁰⁺⁵⁰⁾⁄₂); so that the hot water had lost 25°, whilst the cold
had gained precisely the same amount.
Then Humphry proceeded to try whether the result was the same with
equal quantities of _different_ fluids. Accordingly, he took the same
amount of water at 50° as he had previously employed for one portion,
but, instead of the water at 100° for the other, he substituted a like
quantity of _quicksilver_ at the same temperature, and found to his
astonishment, on pouring the one to the other, that the heat of both
together was no longer the mean of the two (or 75° as before), but only
66⅔°. In this case, therefore, the quicksilver had lost as much as 33⅓°
(100 - 66⅔), whilst the water had gained only 16⅔° (66⅔ - 50); so that
_the quicksilver had parted with twice as much heat as the water had
absorbed_. Consequently it was evident that, in order to raise a certain
_measure_ of water to a given temperature, it required just _double_ the
quantity of heat to be added to it that _an equal measure_ of quicksilver
did; or, in other words, the capacity of water for heat was twice that of
quicksilver.
This referred, however, only to equal _measures_ of the two fluids; so
Humphry wished to ascertain whether the effect would be the same with
equal _weights_ of them. He mixed, therefore, 1 pound of water at 50°
with 1 pound of quicksilver at 100°, and discovered that the resulting
temperature was not quite 52°. Here, then, the quicksilver had lost
rather more than 48° of heat, while this amount had served to increase
the warmth of the water only about 2°. There was but one conclusion to
be arrived at, therefore; namely, that the capacity of a given _weight_
of water for heat is about 30 times greater than an _equal weight_ of
quicksilver, whereas the capacity of a given _measure_ of the former is
only twice that of an equal measure of the latter.
After this the boy added 1 pound of water at 50° to an equal _weight_ of
spermaceti oil at 100°, when the temperature of the mixture was found to
be 66⅔°; so that the oil had parted with twice as much heat as the water
had gained.
Thus it was evident that different substances required _different_
quantities of heat to raise them to the same temperature; and that in
order to warm a certain weight of water to the same degree as an equal
_weight_ of oil and quicksilver, twice as much heat must be given to the
water as to the oil, and 30 times as much as to the quicksilver.
Still the cautious boy was anxious to test the truth of this result by
another experiment.
Accordingly, he took 1 pound weight of each of the three substances
above mentioned, and having brought them severally to a temperature of
50°, he placed the flasks in which they were respectively contained in
a large bath of warm water, the heat of which he kept constantly at
100°. This done, he proceeded to note the time and manner in which each
of the fluids was heated, and found that when the thermometer in the
quicksilver had reached 80°, that in the oil stood at 52°, while the one
in the flask of water marked only 51°; and, though the three liquids
ultimately attained the same temperature as the water-bath in which they
were immersed, _the water took 30 times longer to acquire that heat than
the quicksilver, and twice as long as the oil_.
Now it was manifest that each of the liquids in this experiment must have
been receiving heat alike, so that the only feasible explanation was that
the water, in order to have its temperature raised to a given degree,
required 30 times the quantity of heat that the metallic fluid did, and
double the quantity of the oleaginous one.
Nevertheless, to avoid all possible chance of error, Humphry repeated the
experiment in another form, so as to see whether, in cooling, the water
would part with more caloric than either the oil or the quicksilver; and
just as much more, too, as it had been found to imbibe while being heated.
With this view Humphry filled three Florence flasks—one with a pound of
water, another with a pound of oil, and the third with an equal weight
of quicksilver—all at the temperature of 212°, and having placed each of
the flasks in a large funnel, that rested on a graduated glass jar, he
surrounded them one after another with pounded ice, as here shown:
[Illustration]
When the fluids had been severally cooled down to the same temperature as
the ice around them, the lad proceeded to ascertain, by the quantity of
water produced by the thawing of the ice surrounding each flask, how much
heat had been given out by the three liquids respectively, in sinking
from 212° to 32°; and he ultimately found that the hot water, in cooling,
had thawed twice as much ice as the hot oil, and 30 times as much as the
equally hot quicksilver. Hence it was beyond doubt that water, at a given
temperature, _contained considerably more heat than either of the other
fluids_, and hence the reason why it took a longer time than they to be
warmed or cooled to the same extent.
These experiments naturally led the boy to think how great a magazine
of heat the sea must be, and what a beneficial influence its slow rate
of heating and cooling must have in equalizing the temperature of the
atmosphere. Quicksilver, on the other hand, however, having a small
capacity for heat, and, consequently, being quickly warmed and cooled,
becomes of great value as a liquid for the thermometer, since it is this
property that gives great sensibility to the instrument.
It now only remained for Humphry to ascertain the relative capacities
for heat among solids. This he did by cooling down equal weights of the
metals and other bodies under the exhausted receiver of his air-pump, and
noting how long they took to pass each from a like higher to a like lower
temperature.
By such means the youth ascertained that _Lead_ had the smallest capacity
for heat among the metals, cooling more rapidly than even quicksilver
itself, and 34½ times quicker than water. Next in order came _Platinum_,
which again had less capacity than quicksilver, and cooled 32¼ times
quicker than water. After this, _Silver_ was found to cool 18 times
quicker than water; _Zinc_, 10¾ times; _Copper_, 10½ times; and, lastly,
_Iron_, which was ascertained to part with its heat only 9 times quicker
than water. Glass, however, was found to occupy a longer time in cooling
than any of the metals, giving off its caloric but 8½ times quicker than
water; while sulphur, on the other hand, retained its warmth longer even
than glass, but still cooled 5⅓ times quicker than water.
The capacities for heat, therefore, among the above-mentioned substances,
were inversely as their rates of cooling: that is to say, lead, which
cooled the quickest, contained the least quantity of heat, and,
therefore, required less caloric to raise it to a given temperature;
while sulphur, on the other hand, which took nearly 7 times as long to
cool as lead, contained 7 times more heat, and required to be warmed for
just so much longer a period.
The relative capacity for heat among substances is generally termed their
“SPECIFIC HEAT;” for as different bodies are found to possess _unequal
quantities of heat at equal temperatures_, and as this exists in them in
a _latent_ or _insensible_ state, the term specific heat has therefore
been adopted to express _the relative amount of latent caloric existing
in different substances at the same temperatures_.
CHAPTER X.
THE WONDERFUL EFFECTS OF HEAT—(_concluded_).
The young philosopher had now investigated the effects produced by an
elevation of temperature, not only upon the _bulk_, but upon the _form_
of different bodies. He had found, first, that heat increased the size
of certain substances, without destroying the cohesion among their
constituent particles; and, secondly, that it loosened the attraction
between the atoms of other substances, and rendered them free to move:
so that solids became converted by it into liquids, and liquids into
vapours, while the heat which was absorbed and disappeared during the
production of such changes he had ascertained not only to exist between
the molecules of the resulting liquid or vapour in a _latent_ or
insensible state, but to be again evolved in a sensible form when the
vapours became condensed or the liquids solidified.
The next step, therefore, was to study the circumstances regulating the
_ignition_ and _combustion_ of bodies.
That there is an intimate connexion between the principles of light and
heat, Humphry had little doubt. Indeed, it was plain to him that the
two are mutually disposed to produce each other. He had, however, as
yet considered only the laws of heat, divested of luminosity; but, at
present, he was about to examine the one in connexion with the other;
the laws of ignition and combustion being those of the production of
artificial heat, accompanied with light _for the time being_.
Whether substances, when _merely warm_, are capable of emitting rays of
light, it is impossible to determine; “but,” said the lad to himself,
“the slightest increase of temperature is perhaps accompanied with
some kind of luminous power that our sense of vision is incapable of
perceiving, since it is only when the temperature of bodies is raised to
a high point that they acquire the property of becoming luminous to our
eyes.”
It is extremely difficult to ascertain the precise temperature at which
bodies, when heated, acquire the property of giving out light; for the
result is greatly modified, not only by the sensitiveness of the eye of
the observer, but also by the clearness of the atmosphere at the time of
making the experiment.
The amount of heat necessary for producing luminosity, however, certainly
exceeds 650°, since this is the temperature at which quicksilver boils;
and though Humphry heated the metallic fluid to ebullition in a dark
room, it did not become, so far as he could detect, in the least degree
luminous.
Subsequent experiments, however, induced him to place the degree at
which heated bodies begin to emit light in _the dark_ at 810°; though
the investigations which have since been made in connexion with the
subject lead to the conclusion that the first gleam of light which is
given out from a heated platinum wire occurs at a temperature of about
865°. The luminous rays emitted at this heat, however, are not red, but
of a _lavender-grey_ colour (similar to those which exist in the solar
spectrum beyond the violet band), and seem to be the first transition
from darkness to ordinary light.
At the temperature of about 1000° the light emitted by the heated body
becomes _visible in daylight_, and is then of a dull-red hue.
At 1200° the tint of incandescence brightens into a vivid crimson, or
“cherry-red,” as it is termed.
Then, as the temperature increases, the light emitted by the glowing body
assumes partly a _yellow_ colour; so that at 1700° an “_orange_ heat,” as
it is called, is produced.
At length, however, when the heat rises to the highest point, the light
emitted acquires such brilliancy as to be painful to the eye; the
incandescent substances then appearing no longer tinted, but positively
colourless in the fire. This constitutes what is denominated a “_white_
heat,” and occurs at no less a temperature than 3000°.[37]
At this intense temperature a remarkable change is found to occur in
the character of the heat itself, for it has been before shown that the
heat-rays emanating from an ordinary fire are stopped by glass; so that
while the light emitted by the burning coals passes freely through plates
of glass, and is capable of being reflected by glass mirrors, like the
light of the sun itself, the _heat_ radiated by them—_unlike that of the
solar beams_—has neither the power to traverse the transparent substance,
nor is it susceptible of being concentrated into a focus by reflexion
from a glassy surface.
Artificial heat, however, _when at a very high temperature_, is found to
have all the properties of solar heat. Not only does it then admit of
being focussed by burning-glasses in the same manner as the sunbeams,
but the light emitted by it darkens solutions of silver as effectually
as the light of day; so that (as more recent experiments have proved) a
photographic portrait can be taken as well by the rays from coke at a
white heat, as they can by the rays of the sun itself.
* * * * *
But only those substances are capable of being rendered incandescent,
which have power to sustain the high temperatures requisite for ignition,
without being vaporized or decomposed by the heat. Many bodies, however,
are either dissipated or destroyed long before they attain this intense
temperature; while, on the other hand, those termed combustibles, when
heated in the air, _burst into flame_, and undergo what is termed
_combustion_.
“Now what _is_ combustion?” said Humphry to himself, as he thought over
the subject. “What are the phenomena which occur when substances burn
with the evolution of flame?”
The boy knew, that formerly it was supposed bodies owed their
combustibility to the presence of a certain principle called
“phlogiston,” which during combustion, said the philosophers, escaped
from them, producing light and heat; whereas when the bodies had lost
their phlogiston—and had become “dephlogisticated,” as it was termed—they
ceased to be combustible.
Phlogiston, however, Humphry was well aware, was a purely imaginary
principle, of whose existence no proof had been given, and which had
been invented merely to explain a process that appeared to be otherwise
incomprehensible.
Moreover, Humphry had learnt from the books he had already read upon the
subject, that the metals were increased in weight after being burnt; so
that it was impossible to attribute the combustion in such cases to
the escape of phlogiston, since it was inconceivable how a body could
be rendered _heavier_ by _losing_ something which had previously been
combined with it.
Nevertheless, the belief in this visionary phlogiston had continued for
nearly half a century; and it was only in the year 1775 that more correct
views had been propounded concerning the process.
At the time of young Davy’s commencing the study of this subject,
Lavoisier’s new theory of combustion had been in existence but a few
years, and the boy having obtained from Mr. Tonkin the loan of the
treatise in which the more correct views were originally propounded,
had eagerly perused the volume, being not a little delighted with the
precision of reasoning and the boldness of speculation contained in it.
Still Humphry was not satisfied with merely reading and acquiring the
ideas of others. He criticised the theoretical speculations of the great
French philosopher, doubted, and rejected, and advanced speculations of
his own, while speculation led him to experiment.[38]
Humphry began the investigation of the phenomena of combustion by an
experiment, to prove that the air in which combustibles are suffered to
burn till they are extinguished undergoes a very remarkable change.
For this purpose the lad put a little water in a soup-plate, and on
it he placed a small piece of candle, so that it might swim on the
surface. Having lighted the wick he covered it over with a large tumbler,
and found that the candle then burnt only for a short time, whilst
immediately the flame was extinguished the water rose in the tumbler
considerably above its level in the soup-plate.
Hence it was evident, that the portion of the air which was necessary for
combustion had been removed by the burning candle from the atmosphere
confined within the tumbler, and that, therefore, it was no longer
capable of sustaining the flame.
“But maybe,” thought Humphry, “the candle, in burning, gives off some
gas, which is prejudicial to combustion.”
So, to satisfy himself whether such were the case or not, the boy burnt
some charcoal in an old iron saucepan, that he had previously drilled
full of holes, in order to admit the air. Then, having fitted a tin tube
into the lid of this, he, by means of the chimney so formed, conducted
the gas evolved by the burning charcoal into a wide-mouthed bottle,
that he had previously filled and placed with its mouth downwards, on
a perforated stand in a pail of cold water; so that as the combustion
went on the gas produced kept bubbling up in the pail from the end of
the tube, and displacing the water as it rose into the inverted bottle
that stood immediately above it. The arrangement, however, will be more
readily comprehended by reference to the subjoined engraving:
[Illustration]
As soon as sufficient gas had been collected Humphry removed the tube
from the pail, and corked the bottle under water; then having set the
bottle of gas on a table, he attached a piece of candle to the crooked
end of a long wire, and lowering this, while alight, into the gas, found,
to his astonishment, that the flame was immediately extinguished.
“So then,” cried the delighted boy, “here is a kind of air that I can
neither see, nor feel, nor smell, and yet it extinguishes burning bodies
like water.”
But Humphry was too eager to examine the properties of the gas he had
collected to wait to reflect upon the curious results it afforded him.
Accordingly he procured a tall glass jar, and having placed a piece of
burning candle at the bottom of this, he proceeded to empty the gas from
the bottle into the jar, when to his surprise he discovered that he could
pour out the heavy air that had come from the burning charcoal as though
it had been a liquid, while, immediately it fell upon the lighted candle
at the bottom of the jar, the flame disappeared as suddenly as if so much
water had been showered upon it.
After this the boy amused himself by decanting the gas backwards and
forwards from one vessel to the other, and ultimately found that it was
instantaneously fatal to animals, destroying sentient life as rapidly as
it extinguished burning substances.
Humphry’s next step was to discover what substance was capable of readily
absorbing this gas, and after many trials he found that lime-water did so
with great facility.
Accordingly he added about an ounce of quick-lime to a quart of water in
a glass bottle, and corking it up closely he shook it several times, so
as to dissolve as much of the lime as possible; after which he allowed it
to settle, and then decanted off the transparent and colourless liquid
into a clean bottle with a glass stopper. This transparent solution of
lime in water he then poured into the glass jar containing the gas from
the burning charcoal, and having corked the vessel tightly up, he shook
it about, and immediately perceived that the lime-water was rendered
turbid by the gas, being no longer clear and transparent as before, but
changed to an opaque milky white; then having filtered the turbid water,
and so separated from it all the white particles that had rendered the
solution opaque, he dried and weighed the sediment, and found that the
quantity of lime which had been dissolved by the water had become nearly
doubled in weight by the gas which it had absorbed.
The youth had now learned how to remove the products of combustion, and
he was consequently in a position to determine whether the air, after a
substance had been burned in it, really had or had not been deprived of
anything during the process.
Humphry therefore placed a small quantity of lime-water at the bottom
of a wide-mouthed bottle, and through the cork of this he passed one
end of a long wire, while to the other end of it he attached a small
piece of wax taper. This he lighted, and then lowered down into the air
that stood above the lime-water in the vessel. The cork was now forced
tightly into the mouth of the bottle, and in a minute or two the taper
was extinguished. After this the jar was shaken well up, when the youth
beheld, to his great delight, the lime-water rendered turbid by the gas
evolved during the burning of the taper.
The next step was to discover whether the air which remained in the
bottle (and from which the products of the burning taper had been removed
by the lime-water) was still capable of sustaining combustion.
Accordingly another lighted taper was lowered into it, but this was as
rapidly extinguished as the one had been by the gas from the burning
charcoal itself. It was afterwards found, too, that that part of the air
which remained after combustion was as destructive of animal life as even
the charcoal gas had been discovered to be.
“How wonderful!” exclaimed the boy, “that the atmosphere round about
us should be made up of two different kinds of air—one that enables
combustibles to burn and animals to live in it, while the other
immediately extinguishes flame and destroys sentient life! How can I
collect that portion of the air which supports combustion and maintains
life, _apart_ from that which puts an end to it? I should like to see
what it would do by itself, and whether substances would burn brighter
in it alone; for surely such must be the case, since in the atmosphere it
is mixed with another kind of air that extinguishes flame and destroys
living creatures, so that the one must constantly be counteracting the
effects of the other.”
Humphry racked his brains for a long time for the means whereby to
separate the two kinds of air from each other. At last he remembered one
of Lavoisier’s experiments in connexion with the subject, and immediately
set to work to repeat it.
With this view the lad obtained some “calcined mercury,” for this
substance he knew to have been produced merely by burning metallic
mercury for a long time in a tube exposed to the air, so that the
portion of the atmosphere which supported combustion (instead of being
evolved in a gaseous form, as in the case of the burning charcoal) had
become _fixed, or rendered solid_, in the “_calx_” which resulted from
the process. The boy was therefore anxious to see whether it were not
possible, by burning the calcined product once more, to drive off that
portion of air which had been taken up by it during the previous burning,
and so to discover what are the peculiar and distinctive properties of
the air which had been absorbed. Consequently, he submitted some of this
calcined mercury to a red heat in a retort, and collected the gas that
was given off from it in a wide-mouthed bottle from which he had cut off
the bottom. This, having corked, he filled with water, and stood on a
perforated ledge in a pail—the gas being collected as before described.
When the water had all been displaced from the bottle, and it was
consequently full of gas, Humphry slid it, while under the water, off the
ledge into a soup-plate, and then, removing it to a table, proceeded to
investigate its properties.
Here, then, he had a jar-ful of the gas (named _oxygen_ by chemists) that
maintained the combustion of bodies in the open air, and separate, too,
from the other gas, which tended rather to retard their burning in the
atmosphere.
Humphry’s first experiment was to introduce into the gas thus obtained
a lighted taper, placed at the end of the wire as before, and the boy
was enraptured as he beheld the flame immediately enlarge (instead of
diminishing, as when confined in a jar of mere atmospheric air), and
become intensely bright, while the combustion proceeded at so rapid a
rate that the piece of taper itself was soon consumed. Then another
piece of taper was used, but this was blown out immediately after being
lighted, so that the wick was merely glowing on its introduction into the
gas. On being plunged into the jar, however, it was instantly rekindled,
and burst into the same vivid flame as before.
Next, the combustion of _sulphur_ was tried in the gas. This substance
burns in the open air, as is well known, with a small blue flame. On
placing a small piece of lighted sulphur, however, in a copper capsule
attached to the end of a long wire, it was no sooner lowered into the jar
than it began to burn with a beautiful purple or lilac-coloured light,
the flame becoming suddenly enlarged, and the sulphur itself appearing to
dissolve in the gas. At the conclusion of the experiment the water in the
soup-plate, in which the jar stood, was set carefully on one side, for
after-examination.
After this the lad tried the combustion of _phosphorus_ in another jar of
gas, in the same manner. Humphry knew the combustion of this to be very
vivid, even when inflamed in the atmosphere; so, to prevent accidents,
he used in the jar a piece not larger than a pea: but even this, when
lowered alight into the gas, produced so intensely white a flame that he
could scarcely bear to look at it, while clouds of white flaky matter
were evolved from it like smoke; the heat, too, was so great, that he
was afraid the jar would crack: and so it would have done, had he not,
luckily, employed a very large one.
The young experimentalist was overjoyed at the splendour of the
combustion of these substances, and longed to see whether it were
possible to burn the _metals_ by such means. So, having made another
jar-ful of the same gas, and placed it over some water in a soup-plate,
he took a piece of watch-spring, and when he had affixed the sulphur tip
of a match to the end of this, he lighted the match and plunged the whole
into the gas. He was soon well repaid for his pains; for in a short while
the metal burst into vivid combustion, throwing off a shower of the most
brilliant sparks, which played around it like a fountain of fire, whilst
goutes of the white-hot metal fell hissing through the water, and lay
beneath it for some time, red hot upon the plate, the glaze of which was
afterwards found to have been even fused at the points where the molten
iron had fallen upon it.
But the boy’s rapture on beholding the wonder of combustible iron was not
altogether unmingled with fear; for the heat produced by the burning of
the metal was so intense, that he grew nervous lest the glass jar should
break during the experiment. He was wise enough, however, to hold it in
his hand, so as to allow a little of the gas to escape, as well as to
prevent the jarring of the glass on the plate beneath.
Humphry was now nearly exhausted with his labours, and it was time to
reflect upon all that had occurred.
In the first place, then, it was certain that a considerable quantity of
the gas used in these experiments had disappeared during the combustion,
for the water had each time risen in the jar above the level of that
in the soup-plate. “What, then, had become of the lost gas?” he asked
himself. There was but one answer—_It had combined with the burning body,
and formed a new substance with it_. In the case of the burning sulphur,
the water that had been in the soup-plate below the jar was found, on
examination, to be sour to the taste, and to redden vegetable-blue
colours; so that here the gas had combined with the combustible and
produced an _acid_ that was soluble in water. Again, with the burning
phosphorus, the white flakes that had been evolved during the combustion
had been ultimately dissolved by the water, which likewise tasted sour,
while it stained vegetable colours in the same manner as the sulphur
product did; whereas, in the case of the burning iron, the metal appeared
to have been rusted, for the particles remaining at the bottom of the
soup-plate were found, after the experiment, to have lost their metallic
nature, and to have assumed all the character of a “_calx_,” or _rust_.
“Well, then,” said the lad, “it seems that during combustion one part
of the air combines with the burning bodies, and so either _rusts_ or
_acidifies_ them.” In confirmation of this view, he recollected “that the
gas evolved from the burning charcoal also gave a slightly sour taste to
the water it passed through.”
“Still,” mused Humphry, “if a part of the air really _does_ combine with
the combustible burning in it, the result, of course, should be, that the
combustible, after being burnt, should be _heavier_ than before—even as
the lime with which I absorbed the gas from the burning charcoal became
greatly increased in weight by it.”
The youth was not long in putting this part of the matter to a practical
test. Having accurately weighed a small quantity of calcined mercury
(which, as we said before, he knew to be a rust of the metal), he set to
work again to make it red hot, and to drive off the air with which it had
previously been made to combine while burning. This gas he collected in
a small glass jar, open at the bottom, and having a stop-cock at the top
of it. Then the boy took a thin hollow ball of glass, which had also a
stop-cock fitted to it. Having screwed this on to the metal plate of his
air-pump, he exhausted the glass ball as entirely of air as he could,
and then closing the cock he detached it from the pump, and proceeded to
ascertain the weight of the ball now that it was divested of air. This
done, Humphry screwed the stop-cock of the glass ball on to that of the
glass jar in which he had collected the gas from the calcined mercury;
then, turning on both the cocks, the gas rose from the jar into the
ball, and when the jar itself was full of liquid, and all the gas had
consequently been removed from it, he closed the stop-cock once more,
and, unscrewing the glass ball from the jar, proceeded to ascertain how
much the ball had gained in weight, now that it contained the whole of
the gas evolved from the calcined mercury.
The next step was to weigh the mercury itself. This, however, was no
longer the red powder that it was before the gas had been driven off
from it, but had now become “reduced” into so much bright liquid metal;
and on being put into the scales it was found to have lost just as many
grains in weight as the gas, which had been collected from it, required
to balance it in the scales.
But this was not enough to satisfy the cautious young experimentalist,
for he still desired to see whether, if the same quantity of metallic
mercury were burned in the same quantity of gas, the resulting compound
of the metal and the air would weigh exactly as much as the air and the
metal did separately.
Accordingly, Humphry proceeded now to burn the metallic mercury in the
gas, and so to cause them to combine once more. By keeping the metal at a
red heat with the gas above it, the combination was at length effected,
and then, on weighing the red “calx,” or rust, that resulted from the
process, it was ascertained to be precisely as heavy as the metal and the
gas had weighed when separate.
Here, then, it was manifest that substances by _burning were increased in
weight, and that they were just as much heavier after combustion as the
weight of the quantity of air which had been absorbed by them during the
process._
“Is it true, therefore,” mused the boy, “that the candle and the coals,
which appear to us to be destroyed by combustion, become positively
increased in weight by it?”
The experiment which Humphry had already performed in collecting the gas
from burning charcoal assured him that such was positively the case, for
he knew that this gas, though invisible, had an absolute weight, being
so much heavier than the atmosphere that it admitted of being poured,
like water, from one vessel to another. The experiment with the calcined
mercury, moreover, told him, that if he had weighed the charcoal before
it was burnt, as well as the quantity of air which it had consumed while
burning, he would have found the whole of the gas which resulted from the
combustion would have been precisely as heavy as the air and the charcoal
added together.
For the same reason, if the gases evolved from a burning candle were
to be collected, they, likewise, would be found to be heavier than the
candle itself; and just as much heavier, too, as the quantity of air
which had disappeared during the combustion.
_Combustion, therefore, was merely the rapid combination of a portion of
the air with a combustible body, accompanied with the evolution of heat
and light._
* * * * *
Still Humphry could not quit the subject without examining the conditions
which were necessary to produce such a combination. _The principal
requisite was manifestly elevation of temperature._
Some substances, however, inflame at ordinary temperatures—immediately on
entering the atmosphere—as, for instance, the gas called “_phosphuretted
hydrogen_.” This was a new discovery in young Humphry’s time, and the boy
delighted to produce the gas by heating a small quantity of phosphorus in
a retort completely filled with a moderately strong solution of caustic
potash—the heat being carefully applied until the solution boiled, while
the beak of the retort was kept under the shelf of a water-bath. Upon
coming into contact with the air, Humphry saw the bubbles of gas, as they
left the surface of the water, suddenly inflame, with a slight explosion;
and as the atmosphere was still, each bubble, on bursting, produced a
beautiful expanding ring of white smoke.
It is this gas which gives rise to the production of those
lights in the air which are known by the names of “_ignes
fatui_” (“_will-o’-the-wisps_,” or “_Jack-o’-lanterns_,”) and
“_corpse-candles_”—the former appearing over marshes, and the latter
being seen to rise from recent graves—but both alike proceeding from the
decomposition of organic matter.[39]
Again, _phosphorus dissolved in sulphuret of carbon_ produces a
spontaneously inflammable solution; so that if a small quantity of the
liquid be poured on a piece of paper it evaporates rapidly, and leaves
the phosphorus behind, which immediately bursts into flame.
The same phenomenon of spontaneous combustion also occurs with the
substance called “_pyrophorus_.” This is generally formed of _powdered
alum_ heated with an equal weight of brown sugar or honey. After the
materials have been melted and well mixed in an iron ladle, they are made
red hot in a phial coated with clay, and placed in a crucible of sand—the
heating process being continued until a blue flame appears at the mouth
of the bottle; this is allowed to burn for about five minutes, when the
phial is well stopped and removed from the fire.
The compound, on being cooled and exposed to the air, is spontaneously
combustible.
_Sulphate of potassa_, likewise, when heated to redness with half its
weight of lamp-black, forms a compound, which takes fire immediately on
exposure to air.
Again, _tartrate of lead_, heated to a dull red in a glass tube, forms,
when cool, a very perfect pyrophorus, which immediately inflames on being
shaken out into the atmosphere. Further, when _iron is in a state of
extreme mechanical division_—such as very fine powder—its affinity for
the oxygen of the atmosphere is such that it heats, and even ignites,
on exposure to the air. This is the case with the finely-divided metal
as obtained by the action of hydrogen gas upon red-hot iron-rust, so
that, when suffered to cool in this gas, the iron is as spontaneously
oxidizable as even _potassium itself_.
Moreover, if a small piece of _spongy platinum_ be held in a jet of
hydrogen, issuing from a small tube into the atmosphere, the platinum
immediately becomes red hot, while the gas itself bursts into flame.
_Platinum wire_, or _foil_, if the surface be perfectly clean, acts so
rapidly at common temperatures on a mixture of oxygen and hydrogen gases
(mixed in the proportion of 1 to 2), that it often becomes red hot on
being introduced into a vessel containing them, and kindles the mixture.
Handling the platinum, however, wiping it with a towel, or exposing it to
the atmosphere for a few days, suffices to soil the surface of the metal,
and so to prevent its action.
Finally, a piece of the metal called _potassium_ (procured from _potash_)
has so strong an affinity for oxygen, that when thrown upon water, at
ordinary temperatures, the metal decomposes it the instant it touches the
liquid, and so much heat is disengaged that the potassium is inflamed,
and burns vividly while swimming on the surface. The same spontaneous
combustion ensues, indeed, with _ice_—so that the cold body appears to
heat the metal even to inflammation.
But a still more curious instance of spontaneous inflammation is to be
found in the sudden explosion of a mixture of _Chlorine and Hydrogen
gases when exposed to sunshine_; for though the two gases, when mixed
together in equal volumes, may be preserved without change in a dark
place for any length of time, nevertheless, immediately they are
submitted to the direct solar rays, the whole mixture becomes suddenly
inflamed, and a violent explosion ensues.
Next to those substances which are spontaneously combustible comes
_phosphorus_, which inflames, when perfectly dry, at the low temperature
of 60°. Indeed, such is its tendency to combine with the air, that, if
free from all moisture, it takes fire by the heat of the hand alone.
Slight friction, as when rubbed upon a piece of coarse paper, also
produces the same result. It is very difficult, however, to light a
piece of paper by the flame of phosphorus, for the paper becomes coated
with a crust of the solid _phosphorous acid_, which is produced by the
combustion, and serves to protect it from the flame.
There is, likewise, a gas (for the knowledge of which we are indebted to
the after-discoveries of Davy himself), called _protoxide of chlorine_,
which requires so slight an elevation of temperature to decompose it,
that even the heat of the hand is sufficient to cause it to explode with
the evolution of heat and light. This gas is produced by the action
of hydrochloric acid on chlorate of potash and water, and it is so
explosive that it frequently detonates violently in being transferred
from one vessel to another. It should, therefore, be dealt with by none
but experienced chemists. A small piece of phosphorus let up into it
instantly takes fire, and burns with much brilliancy. Sulphur likewise
decomposes it with violent detonation, and even a piece of blotting-paper
introduced into the gas is sufficient to cause it to be suddenly resolved
into its elements.
The gas termed _Binoxide of Chlorine_ (called also the _Peroxide_) is
even more explosive than the Protoxide. It detonates violently when
heated to 212°, emits a strong light, and undergoes a greater expansion
than the simple oxide above described.
Again, the _Binoxide of Hydrogen_ (or _Peroxide_, as it is sometimes
denominated), when heated to 212°, gives off oxygen so rapidly as to
cause an explosion, while the rusts (oxides) of some of the metals
act upon it with such energy, that, when dropped into it, a violent
detonation immediately ensues, and the glass tube on which the experiment
is conducted becomes red hot.
Further, the gas called _Binoxide of Nitrogen_, when combined with
_sulphurous acid_ gas, produced a compound called _Nitro-sulphuric
Acid_, which is so prone to decomposition that it cannot be collected
in a separate state, and the salts of which are held together with such
slight affinity that even a little charcoal powder, or spongy platinum,
is sufficient to cause a violent evolution of gas, while at a temperature
only a few degrees above that of boiling water, an explosion ensues.
Moreover, the _Bisulphuret_ (called, also, the _Persulphuret_) _of
Hydrogen_, which is a yellow oil-like liquid, has its elements so feebly
united, that at a heat short of 212° it is instantaneously resolved into
_sulphur_, and the simple _Sulphuretted Hydrogen_ which is evolved in the
form of gas, with almost explosive violence. The same effect is produced
by the mere contact of most substances—especially the metals, flint,
and even the earths in powder—while the oxides of gold and silver are
“reduced” by it with such energy that they are rendered instantaneously
red hot.
These binary compounds of oxygen or sulphur have most of them been
discovered since Davy’s time. They are, however, remarkable in possessing
kindred affinities, and being severally decomposible at a temperature of
212°.
After these, in the order of ready decomposibility, come the compounds of
_Nitrogen_.
The peculiar black powder, called by chemists _iodide of nitrogen_, which
is produced by pouring some strong ammonia upon a very small quantity of
iodine, is so explosive, that it detonates violently as soon as it is
dried; and the slightest pressure, even when moist, produces a similar
effect. If put into pure ammonia, it explodes when lightly pressed in
that liquid. Heat and light are emitted during the detonation, which
is merely a species of instantaneous combustion. So dangerous is this
compound, that the most experienced chemists seldom operate on more than
a few grains of iodine at once.
The yellow oil-like liquid, called _chloride of nitrogen_, which results
from the action of chlorine gas upon sal ammoniac, also enters into
instantaneous combustion at very low temperatures; so that, when it is
heated to a little above 200°, it detonates with tremendous violence,
a vivid flash of light being produced at the same time, while the
vessel—which, to prevent accidents, is covered with a wire cage—is
broken to atoms. This compound is so dangerous, being one of the most
explosive substances yet known, that in dealing with it the face is
always protected by a mask, and only a small globule of it, no larger
than a mustard seed, experimented upon. Dulong, the French chemist who
discovered it, lost an eye and the use of a finger whilst operating
with it; and Davy himself, in after-life, was wounded in the face by
the effects of its detonation. The mere contact of this substance with
certain combustibles causes it to explode violently, even under water, at
ordinary temperatures. If touched with phosphorus, India rubber, common
oil, turpentine, caustic potash, or even soap, it detonates so violently
as to break to pieces the vessel containing it, and to scatter the water
in which it is immersed in a shower all around.
_Bromide of nitrogen_, again, is said to be even more easily decomposed
than the chloride. It is a dark-red oily liquid, having a fœtid odour,
and giving off a vapour that is very irritating to the eyes. This
compound, when touched with phosphorus, or even a small piece of arsenic,
detonates with tremendous violence.
Next to the above remarkable compounds of Nitrogen, the _fulminates of
the precious metals_ (into the composition of which, however, Nitrogen
also enters) must be ranked in the order of ready combustibility; for
these likewise explode at very low temperatures, with the production of
heat and light.
First come the _Nitrurets of Mercury and Silver_—that is to say,
compounds of those metals with Nitrogen. These are formed by the action
of Ammonia on the oxide of Mercury or Silver. The Nitruret of Silver
explodes with tremendous violence when gently rubbed or heated, and the
Nitruret of Mercury when struck with a hammer, or acted upon by strong
oil of vitriol.
_Fulminating Gold_, when suddenly heated to about 290°, detonates with
great force and a vivid flash; and if exploded upon platinum foil, the
metal is torn at the point of contact. Friction with hard bodies, or
an electric shock, also explodes it. The more it is washed and dried,
the more explosive this compound becomes; and if long retained at
the temperature of boiling water, so as to become perfectly dry, the
slightest friction causes it to detonate immediately and violently. If it
be moist, however, it does not explode on the application of heat till
dried, and those portions which first become dry explode the soonest; so
that, in such a case, a succession of detonations is produced.
_Fulminating Mercury_ requires a temperature of 300° to cause it to
explode, which it then does with a bright flame. It also detonates by
friction, so that the greatest caution is required in preparing and
dealing with it. This compound has even been known to explode in a moist
state, and in the most careful and skilful hands it cannot be touched
without considerable danger.[40] This is the substance used in the
percussion caps; it is introduced into the caps moistened with a little
tincture of benzoin, so as to be dropped into them, and then carefully
dried. Howard, the discoverer of the compound, endeavoured to substitute
it for gunpowder, but the explosion was found to be so sudden that it
burst the gun without expelling the shot.
_Fulminating Silver_ likewise explodes, with the evolution of light
and heat, at nearly the same low temperature. A grain, or merely half
a grain of this substance, detonates with great violence, when heated
or when touched with any hard body. On being placed upon a piece of
rock crystal, and rubbed in the slightest manner by another crystal, it
explodes with great force. It has sometimes exploded upon the contact of
a glass rod, even under water; so that merely the feather of a common
quill is generally used to collect it. It is dangerous to keep it in
a cork-stoppered phial, for serious accidents have arisen from its
unexpected explosion in a confined state. In short, persons cannot be too
careful in meddling with it, and its use for detonating balls and other
purposes of amusement is highly perilous and reprehensible.
_Fulminating Platinum_, on the other hand, explodes at a temperature of
420° with a loud report.
There is likewise a _fulminating powder_, composed of a mixture of 3
parts _nitre_ with 1 of _sulphur_, and 2 of dry _carbonate of potash_.
This substance explodes with much violence at the low temperature of
330°; so that if a little of the compound be heated up to that point upon
a metallic plate, it blackens, fuses, and detonates with great force.
Again, a mixture of 3 parts _chlorate of potash_ and 1 of _sulphur_
detonates loudly when struck upon an anvil with a hammer, and even
sometimes explodes spontaneously. If 2 or 3 grains of chlorate of potash
be reduced to powder in a mortar, and some very fine flour of brimstone
be then added to it, the two substances, when rubbed together, will
detonate with a smart noise, like the cracking of a whip. A mixture of
_chlorate of potash_ and _sulphuret of antimony_ takes fire by gentle
trituration, and deflagrates with a bright puff of flame and smoke.
Chlorate of potash was proposed by Berthollet (the French chemist) as a
substitute for nitre in gunpowder. The attempt was made at Essone, in
1778; but no sooner was the mixture of the chlorate with the sulphur and
charcoal submitted to trituration than it exploded with violence, and
proved fatal to several persons.
With _phosphorus_ and _chlorate of potash_ the explosion is dangerously
violent: 1 grain of phosphorus with two of the chlorate, if placed in
a small piece of paper and struck with a hammer upon an anvil, will
immediately explode, and the phosphorus be thrown about in an inflamed
state. Gunpowder, again, if mixed with powdered glass and struck with a
heavy hammer upon an anvil, almost always explodes.
Moreover, a mixture of _oxygen_ and _hydrogen gases_, suddenly submitted
to violent mechanical compression, unite with a vivid flash of light and
produce water.
Next to Phosphorus and the Fulminates, _Sulphur_ is the most easily
kindled. This body enters into combustion at about 500°. It is the
comparatively low temperature at which sulphur bursts into flame, that
makes it so important an ingredient in gunpowder, matches, &c. The easy
combustibility of sulphur may be well illustrated by propelling a small
quantity of it in powder into the current of hot air issuing from the
glass chimney of a gas lamp, when it will be seen to take fire at a
considerable height above the flame.
_Wood_, _cotton_, _paper_, &c. require, on the other hand, their
temperatures to be raised much higher than that required for the
inflaming of sulphur, in order to be made to enter into combustion.[41]
Paper merely becomes brown or scorched at the heat of 440°, nor can it be
lighted at a red heat, though the temperature of this is 1000°. Cotton
or tow, however, when greased with oil, occasionally absorbs air so
rapidly, and produces so much heat during the process, that spontaneous
combustion frequently occurs from this cause. The same effect sometimes
arises from _hay_ being stacked before being perfectly dry; the moisture
sets up a kind of fermentation in the interior of the stack, and this
evolves so much heat that the temperature becomes raised to the point
required for the material to enter into rapid combination with the
atmosphere, so that spontaneous combustion is the result.
Again, there are certain mixtures of gases which are explosible at a
red heat, while others cannot be made to enter into combustion at that
temperature, but require the presence of flame in order to fire them.
_Carburetted hydrogen (coal gas)_, _sulphuretted hydrogen_, and _carbonic
oxide_, can be made to inflame in air by red-hot iron or charcoal; and a
mixture of _oxygen_ and _hydrogen_ gases can be exploded by a red heat
visible in daylight, whereas a dull red heat only causes the two gases
to combine silently without detonation. _Fire-damp_, however, (which is
light carburetted-hydrogen gas, and the same as that which rises from
stagnant pools on disturbing the mud at the bottom), cannot be inflamed
by the strongest red heat; so that a fire made of charcoal, that will
burn without flame, may be blown up to whiteness without exploding a
mixture of this gas with air. A piece of iron also, at the highest degree
of red heat, and even at an ordinary white heat, does not inflame an
explosive mixture of _fire-damp_, but when brought to its _highest point
of white_ heat, iron immediately causes the fire-damp to combine with the
air with a violent detonation.
The knowledge of this fact, which we owe to the researches of Davy
himself, was of immense importance in the construction of the
safety-lamp, and, once discovered, it was treasured in the brain for
after use, for the preservation of life and the mitigation of suffering.
Again, there are some substances which are so readily inflammable that
they take light even at the approach of flame. These bodies consist
of highly vaporizable liquids, such as _alcohol_, _ether_, _naphtha_,
_sulphuret of carbon_, _oil of turpentine_, &c. From the tendency of
these combustible liquids to pass into vapour, the surrounding atmosphere
(immediately on opening any vessels containing them) becomes charged with
their fumes, so that an almost explosive mixture is formed with the air;
and as this extends to some distance from the liquid itself, the approach
of a lighted body instantaneously causes the whole volume of vapour to
pass into one sheet of flame—an effect which is occasionally attended
with the most disastrous results.
On the other hand, some combustible liquids require their temperatures
to be highly raised before they can be inflamed; such is the case with
the common fixed oils—as lamp-oil, and others. For this purpose a cotton
wick is usually employed in burning them, so that, by the capillary
attraction of the fibres, a small portion of the liquid may be raised
above the level of the rest, and the oil thus be brought into connexion
with the flame by minute quantities at a time; the consequence being
that, as each small portion of the liquid is heated, it is converted
into vapour, or gas—and this, by the high temperature maintained by the
burning wick, is made to enter into combustion with the surrounding air,
and so to be continually inflamed above it. Any porous substance which is
a bad conductor of heat (such as Bath-brick or sandstone), will, if cut
to a fine edge, answer all the purposes of the ordinary cotton lamp-wick;
for if a light be applied to this, so as to raise its temperature
sufficient to convert into gas the film of oil at the summit, the fluid
will be readily inflamed, and continue burning until such an incrustation
of charcoal ensues at the tip as to prevent the oil being heated any
longer by the flame.
Further, some substances cannot be made to burn at ordinary temperatures
in the open air, though, on being confined in a vessel of oxygen gas,
they readily enter into combustion when their temperature is raised. This
is the case, as we have seen, with iron, and some of the other metals.
* * * * *
The next problem to be resolved was, Whence come the light and heat that
are emitted during the process of combustion?
“All cases of combination with oxygen,” mused the youth, “such as the
rotting of wood, and the gradual rusting of metals in the open air, are,
probably, attended with the evolution of heat; but in such instances the
process is so slow, that the heat evolved is unobserved, and dissipated
without accumulation. When the combination, however, takes place in
a shorter time, as in the production of vinegar, the heat becomes
proportionately sensible; and when the combination with the oxygen is
so rapid that the whole of the heat is evolved in a much more limited
period, as during combustion, the increase of temperature is rendered
considerably more intense. A pound of charcoal, for instance, combining
with oxygen in the process of respiration, gives off the same amount
of heat as it does when in a state of ignition, and takes up precisely
the same quantity of the gas. In the one case, however, the combination
is spread over 30 hours, whereas in the other it occupies but as many
minutes.”
Humphry reflected for a long time as to the origin of the light and heat
evolved during the burning of substances.
“Are the light and heat,” said he to himself, “originally imprisoned, as
it were, in the combustible, and set free during the burning of it? or
are they merely the result of the rapid and energetic combination of the
oxygen of the air with the burning substance?”
The former assumption the boy knew to be Lavoisier’s theory of the
subject, but such an explanation appeared to him to be inconsistent with
the facts.
The _products of combustion_ are not always the same. In some cases they
consist of _gases_, as in the burning of Charcoal and Sulphur, &c.;
in others, _liquids_ are produced, as by the combustion of oxygen and
hydrogen gases, the product in that case being merely water; while in
others, again, a _solid_ product is the result—as when phosphorus is
burnt a white solid, called “phosphorous acid,” being then formed: and so
with zinc, the combustion of which produces a solid white rust, or oxide.
“Now, if the heat evolved during combustion,” thought Humphry, “proceeds
from the liberation of the _latent_ caloric, or that which previously
existed in the substances in an _insensible_ state, it would follow
that such heat should be given off only when the combustibles pass from
a _rarer_ to a _denser_ form; as, for instance, when water is produced
by the burning of its two constituent gases, or when the oxygen of the
atmosphere becomes fixed in some solid product, as in the oxides of zinc
or mercury.
“But by combustion,” the boy went on, “many solid bodies are converted
into gases; and in such cases, according to Lavoisier’s theory, instead
of heat being evolved, it should be _absorbed_, and positive _cold_
produced by the process of burning.
“This, however, is not the case,” added Humphry. “The explosion of
_Gunpowder_, for example, is attended by immense heat, and yet the
ingredients composing it, in passing from the solid to the gaseous state,
expand some hundred-fold, having their volume increased, it is said, no
less than 250 times. So, again, the gas called _Protoxide of Chlorine_,
at the instant of decomposition evolves light and heat with explosive
violence, and yet it is known to become one-fifth greater in volume
afterwards. The oily substance, too, called _Chloride of Nitrogen_, on
being made to enter into combustion, is resolved into its elements with
tremendous force of inflammation, expanding into more than 600 times its
bulk: so that, according to Lavoisier’s theory, a prodigious degree of
cold ought to be produced by such an expansion, whereas light and heat
are evolved by it.”
That the heat of combustion is due rather to intense chemical action
going on at such times, Humphry made many experiments to prove.
First, he generated some _Chlorine_, or green gas.[42] This he did by
mixing in a retort some common salt with a little black oxide manganese,
and some sulphuric acid and water. The gas which came over was of a
greenish-yellow colour, and had a pungent, disagreeable smell, exciting
cough and great irritation in the lungs when inhaled. Having collected
some of this gas over warm water in a receiver, with a stop-cock at the
top, the boy took a retort which had another stop-cock fitted to the
end of it, and having introduced into this some copper-leaf, he screwed
the retort on to the plate of his air-pump, and proceeded to exhaust it
of air as perfectly as he could. This done, he screwed the stop-cock of
the retort on to that of the receiver in which he had collected the gas;
then, turning on both cocks, the chlorine rushed up into the retort, and
the metal immediately became spontaneously ignited by it, and burnt in
the gas with considerable energy.
The experiment was then repeated with a little powdered antimony, and the
same vivid combustion ensued.
Now these Humphry knew to be cases of mere _chemical affinity_. The
chlorine gas had a strong tendency to combine with the metals employed,
and as these were used in the best form for promoting the combination,
the union of the gas with the copper and antimony was so rapid and
energetic, that combustion was the consequence.[43]
Next, the lad took some _Sulphur_, and in this he heated some shavings
of iron in a close vessel, when the metal, in a short time, was seen to
become intensely ignited, and to burn, as it were, in the vapour of the
sulphur. This was most curious, for sulphur he had never thought to be a
supporter of combustion.
Further, Humphry heated some platinum and tin-foil, and at the moment
when the two metals fused into one mass they became, to his astonishment,
vividly ignited.
Nor was this all: the boy slacked some recently-burnt lime in a dark
place, and found that, when the water was thrown upon it, the heat rose
to upwards of 500°, while a faint light was emitted.
Another substance, discovered since the date of the above experiments,
affords a striking illustration of the heat produced by energetic
chemical action. This is a peculiar liquid called _Peroxide of Hydrogen_
(see p. 264), consisting merely of water combined with an extra quantity
of oxygen. So readily decomposible is this fluid, that at the heat of
boiling water evolutions of gas are produced with such violence as to
cause an explosion; and almost all the metals, when in a state of minute
division, resolve it rapidly into its elements. The _peroxides_ of lead,
mercury, gold, platinum, &c., act on the liquid with surprising energy,
the decomposition of it being complete and instantaneous upon dropping
those substances into the liquid; for oxygen gas is then given off with
such force as to produce a detonation, while the temperature becomes so
intense that the glass tube in which the experiment is conducted grows
suddenly red hot.
Subsequently, Humphry amused himself by mixing a little _Chlorate of
Potash_, about the size of a pea, with the same quantity of _loaf sugar_,
having previously reduced each to powder. Then he placed the mixture on a
piece of tile, and, dipping a glass rod into a bottle of strong _Oil of
Vitriol_, let the acid drop from the rod upon the powdered Chlorate and
sugar, when they were instantly kindled, and burnt with a red and blue
flame. For the Vitriol immediately decomposed the Chlorate of Potash,
and so produced heat enough to ignite the materials; while the oxygen
given out from the Chlorate maintained the sugar in a state of vivid
combustion. A little _Camphor_, mixed with _Chlorate of Potash_, and
touched with a drop of _Oil of Vitriol_, may be made to inflame in the
same manner. A like effect is also produced when the _Chlorate_ is mixed
with _Spirits of Wine_.
After this, the lad placed a small piece of _Phosphorus_, and a few
grains of _Chlorate of Potash_, at the bottom of a thin glass vessel, and
then poured gently upon them some _hot_ water. The heat of the water was
sufficient to inflame the Phosphorus, while the oxygen evolved from the
Chlorate of Potash in connexion with it tended to keep up the combustion;
so that the two burnt with a vivid and pleasing light under the water.
Now Humphry knew that the reason of these effects was, that the
Chlorate of Potash contains a large quantity of oxygen, which, having
a strong tendency to combine with the combustibles, enters rapidly and
energetically into union with them immediately their temperatures are
elevated, and so gives rise to the phenomena of heat and light which are
evolved during the combustion.
The same result is produced by _Nitre_ in ordinary gunpowder, for this
also contains a large quantity of oxygen gas, which serves to inflame
the small particles of charcoal that are mixed with it, while the
sulphur—which forms the other ingredient—being inflammable at a low
temperature, renders the gunpowder capable of being united by a mere
spark.
_Oxygen_ thus appeared to Humphry to be the main “supporter of
combustion,” while the body burnt seemed to be the “combustible.”
But Humphry had seen that bodies burnt in _Chlorine_ gas as well as in
oxygen: this, therefore, was another “supporter of combustion.” The same
effect, again, he had found to be produced in the vapour of _sulphur_.
Sulphur, however, was a “_combustible_ body;” so that, in this instance,
the same substance was both a combustible and a supporter of combustion.
The division of all bodies, therefore, into these two classes, as
propounded by the French chemists, appeared to have no foundation in
nature.
Nevertheless, Humphry was determined to put the matter to the test
of experiment, and to see whether the light and heat evolved during
combustion proceeded from the combustible itself, or from the combination
of it with the air during the burning.
Accordingly, the lad inflamed a jet of hydrogen gas in a vessel of
oxygen, when the light and heat certainly appeared to proceed from
the jet of _hydrogen_, while the oxygen seemed to act merely as the
_supporter_ of the combustion.
On reversing the experiment, however, and causing the oxygen gas to issue
from the jet, Humphry found that he could inflame it in a vessel of
hydrogen, and that then the light and heat seemed to be evolved from the
burning _oxygen_, while the hydrogen appeared only to keep it inflamed.
It was evident, therefore, from the last experiment, that even oxygen
itself might be ranked among the _combustibles_, and hydrogen be
considered as a _supporter of combustion_; but Humphry now saw that the
real truth of the matter was, that the heat and light evolved during
combustion came from _neither one substance nor the other_, but arose
simply from the rapid and energetic chemical union of the _two_: for even
two metals, at the moment of their union, evolve heat and light, as shown
in the experiment of the fusion of platinum with tin-foil.
Combustion, therefore, is simply the consequence of the _rapid_ chemical
action of one body upon another; and the reason why it is necessary to
elevate the temperatures of certain bodies before they can be inflamed,
is merely because _heat_ promotes the chemical action of substances upon
each other.
CHAPTER XI.
HUMPHRY AND HIS “WONDERFUL LAMP.”
Humphry found it impossible to quit the subject of Combustion without
some inquiry as to the nature of flame.
“What,” said he to himself, “are those brilliant sheets of light that
dart from burning substances? And why is it that certain bodies burn
with the evolution of flame, and that others are merely capable of being
rendered incandescent at the highest heat?”
Now the boy had before reflected upon the production of _heat_ by the
_slow combination_ of oxygen with certain combustible substances, so he
immediately passed to the consideration of the production of _light_ by
the same means.
He knew well that a stick of phosphorus always appears luminous in the
dark, and that lines of fire can be written with it on a wall: the
reason of this being simply that the air combines with it _slowly_ at
ordinary temperatures. Again, the lad was aware that decayed wood emits
a faint light, which is visible by night, the light being due to the
same cause, viz. the _slow_ combustion which is continually going on in
it; for chemists have discovered that the rotting vegetable substance is
constantly absorbing oxygen from the atmosphere, and evolving carbonic
acid gas—the same as if it were really burning—though at a _less rapid_
rate. Moreover, the decay of many animal substances, Humphry had read, is
attended with like phenomena—the flesh of many fresh and saltwater fish
becoming luminous previous to putrefaction.
But, in all such cases, the light emitted is more like a halo, or feeble
_glow_, than the character of _flame_; or, rather, it seems as if the
substance had become _incandescent in the cold_, and acquired the
property of ignition at ordinary temperatures, so that it was capable of
giving out light without being sensibly heated.[44]
Humphry had noticed, too, that a green wax taper—the colour of which he
knew to be produced by “verdigris” (acetate of copper)—on being lighted
and blown out shortly afterwards, will continue glowing in the wick, or
burning without flame, until the whole of the taper be consumed.
Now this, the boy was well aware, was another case of combustion, going
on at a temperature _below_ that of flame, though it was a state of
continuous incandescence, requiring a higher heat for its production than
the phosphorescence of decaying vegetable and animal matter.
Having thought the matter well over for some little time, the boy-Chemist
eventually conceived that it might be possible, by some such means, to
produce a lamp which would burn continuously, and give light, without
flame; and this he imagined might be found of great service in the
coal-mines, since the fire-damp was not explosible at the highest _red
heat_, and required positive _flame_ to cause it to enter into combustion.
In the case of the glowing wick of a green taper, the youth saw that
it was necessary for the wick itself to be made red hot before the
ignition could be maintained; and he concluded that it was merely the
incandescence of the wick which caused the gases given off from the taper
to combine with the air, and so to keep it continually red hot. It struck
him, therefore, that if he were to use some combustible liquid that was
highly vaporizable, like ether, and to suspend above this a spiral coil
of fine platinum wire, which had been previously heated to redness, the
same continuous state of ignition might be made to go on, and that in a
much simpler manner.
Accordingly, Humphry procured a tall ale-glass, and having poured into it
a tea-spoonful of ether, he suspended within it a coil of platinum-wire,
which had been made red hot in the flame of a spirit-lamp.
The arrangement of the little apparatus is represented in the annexed
engraving.
[Illustration]
Immediately the red-hot coil was placed in the vapour from the liquid,
the excited boy was overjoyed to see it glow with a bright red heat. He
stood watching it for a long time, and was rejoiced to find that there
was not the least diminution of the incandescence till the whole of the
liquid had disappeared, and the vapour been made by the heat of the wire
to combine with the surrounding air.
[Illustration: HUMPHRY’S FIRST ATTEMPT AT THE SAFETY LAMP.—Page 291.]
Still it struck Humphry that a more convenient mode of attaining the same
end might be devised; so he took an ordinary spirit-lamp (for spirits of
wine he was aware had the same tendency to give off vapour as the liquid
he had previously employed), and wound a coil of fine platinum wire round
the wick—thus. Then, having lighted the lamp, he suffered it to burn
a few seconds, after which he put an extinguisher over the flame, and
instantaneously removed it. The consequence was, that the coil retained
heat enough to carry on the _slow combustion_ of the vapour from the
spirit, so that the ignition was kept up, and the wire continued glowing,
till the whole of the spirit had evaporated from the lamp.
[Illustration]
Overjoyed at the discovery he had made, Humphry hastened to exhibit his
lamp without a flame to Mr. Borlase and Mrs. Foxell, and his old friend
Mr. Tonkin: and as he did so, the boy descanted fervently upon the
superiority of such a means of obtaining light in coal-mines over the
“_steel-mills_” which were then used for the purpose.
Mrs. Foxell was as pleased as the boy himself at what she considered the
successful termination of his labours; and when Humphry ran over to her
the long train of investigation he had pursued in order to arrive at the
object, she was warm in her praises of his perseverance and genius, and
assured him that, by such patient inquiry, he must ultimately gain the
honours and the universal esteem which he so much desired.
Mr. Borlase was surprised at the talent of his young pupil, and went
with him to Mr. Tonkin to talk over the matter; and when Humphry’s
foster-father saw what he had achieved, and was informed of the
benevolent spirit which had stirred him to the discovery, the old
gentleman hugged the lad to him, and told him he was well repaid for all
the care and affection he had bestowed upon him.
* * * * *
When, however, the first impressions had subsided, and Humphry’s
exultation at the discovery he had made had been toned down by
continually reflecting upon the subject, he began to think his little
safety-lamp might be much improved if he could only increase the light
from it. In its present form he thought it would be available merely in
such cases as the steel-mill was used, viz. to explore those mines which
were _known_ to be charged with fire-damp. The miners, nevertheless,
needed some light at their work, and if candles or lanterns were
employed, the flame would be sure to ignite any fire-damp that might
accidentally become mixed with the atmosphere, and so to cause the whole
to explode.
What was required, therefore, was a light which would be as bright as
those the miners employed at their work, but which would, at the same
time, be incapable of inflaming the explosive gases evolved from the
coals.
Such a light Humphry knew could be obtained only from flame itself, for
he was now satisfied that it was impossible for any one to see to work by
the rays from a red-hot wire. Still the difficulty was, that the presence
of flame would be sure to cause the fire-damp to explode whenever it
became mixed with the air in the mines, and it was impossible to make a
lamp burn without air.
For a time the difficulties involved in the construction of such a lamp
appeared to the lad to be insurmountable. At length, however, nothing
daunted, he set to work to discover the nature of flame itself, and
so to find out what conditions were necessary for the production and
maintenance of it.
“_Flame_,” said Humphry, while reflecting upon the subject, “may be
regarded merely as a _sheet, or film, of gas, in a high state of
ignition_. The temperature of flame is always very intense, since it will
ignite substances that cannot be lighted, even with the highest heat of
incandescence. The _light_, however, emitted by different flames is by no
means equally vivid, since there are gases which burn with the production
of a flame so feeble as to be scarcely visible in broad daylight.”
Accordingly it struck the boy, that the best way to proceed would be to
produce a feeble light first, and then to ascertain by what means he
could increase the brilliance of it. So Humphry made a large bladder full
of hydrogen gas, and then another bladder of oxygen, and proceeded to
burn the gases together by means of two blow-pipes.
The flame produced was of a very faint blue colour, and though the boy
closed the shutters, he could barely see to read by the light, and yet
the heat of it was almost as intense as any that could be artificially
generated;[45] for, on holding a platinum wire in the flame, it
immediately became white hot, and the light was considerably augmented.
“So, then,” cried Humphry, “the light emitted by a flame seems to be
increased by solid substances introduced into it, and ignited by it.”
This set the boy wondering what would be the effect if he were to cause
some fine dust to pass continually through the flame, and whether the
brightness of the light would be as much increased then as it was when
the platinum wire was introduced into it.
The idea had no sooner struck him than he ran down to the shop, and
procuring a little magnesia in fine powder, proceeded to sift this into
the flame produced by the oxygen and hydrogen gases.
The result was almost magical.
No sooner did the fine white particles fall into the flame, than the
light was changed from a faint blue into a brilliant white; while the
little solid specks shone like so many gem-points in the sunshine,
producing a glare that it pained the eyes to look at, and that instantly
lighted up the little dark chamber almost with the vividness of daylight.
“It _is_ as I expected!” shouted Humphry, as he gazed with admiration at
the beautiful white light; “the luminosity of a flame depends chiefly
upon the particles of _solid_ matter diffused through it, and rendered
incandescent by the burning gases. The flame of a candle or oil-lamp,”
the boy continued, “owes its brightness merely to the same cause; for
there the oil, or grease, is converted into gas by the heat of the wick,
and this gas consists chiefly of carburetted hydrogen—that is to say, of
charcoal and hydrogen combined together: so that in burning, the hydrogen
unites with the oxygen of the air and forms water, while the charcoal
is set free, and passes up through the flame in the form of very minute
particles, which being burnt there, and so rendered nearly white hot,
cause the light to shine with great vividness.”
To prove that the light of an ordinary candle, or oil-lamp, is due merely
to the brilliancy of the incandescent particles of charcoal continually
passing through the flame, the lad was not satisfied merely with holding
a burning candle against the ceiling, and so causing the unburnt charcoal
to be deposited upon it in the form of “lamp-black,” as it is called;
but he passed some hydrogen gas through a small reservoir of naphtha,
and found that the brilliancy of the light was greatly increased by it:
for, the naphtha being merely a liquid form of charcoal and hydrogen in
combination, the gas, on traversing the fluid, took up a certain portion
of the naphtha in vapour, and this being very rich in charcoal, was the
cause of the increased illuminating power given to the flame.
The same principle has been applied since Humphry’s time to the
production of some of the most vivid of our artificial lights. The
“Drummond light,” for instance—which consists merely of a jet of oxygen
and hydrogen gases projected against a cylinder of lime—owes its intense
brilliancy solely to the particles of lime that are rendered white hot by
the flame of the burning gases; and that the lime passes off in vapour is
proved by the fact, that, during the combustion, the roof of the lantern
becomes covered with the sublimed particles.
Again, the brilliancy of the electric light itself is known to arise
from the particles of white-hot charcoal that pass over in vapour
from one pole of the battery to the other; for it is found that the
charcoal point in connexion with the _positive pole_ becomes decreased
and burnt into a cavity, while that in connexion with the _negative
pole_ is proportionately increased, having a small knob or protuberance
of charcoal deposited upon it: so that, strange as it may appear, the
brilliancy of the flame of a common tallow-candle depends upon the same
cause as that of the electric light itself. In the one case, however,
the combustion being imperfect, the points of charcoal are less vividly
ignited; whereas in the other, the heat being the most intense that can
be produced by artificial means, the incandescence is proportionately
higher, and the brilliancy of the light, therefore, increased to the
greatest point. In a word, the particles of charcoal in the flame of a
candle are only at an _orange_ heat, whereas those in the voltaic flame
are at the highest _white_ heat that art can generate.
Accordingly, _artificial light_ would appear to be due solely to
_artificial heat_, and the illuminating power of even flame itself to
depend upon the incandescence of the particles of solid matter diffused
through the burning gases.
* * * * *
Humphry’s next object was to discover whether flame really consisted, as
he before said, of a sheet or film of gas in a state of combustion.
“The flame of a common candle,” he mused, “appears to be a solid cone of
luminous matter. _Is_ it, however, really alight in the middle?” the boy
mentally inquired; “or is it burning at the outside only, so that the
thin film of white-hot vapour incloses a portion of combustible matter
within it, which cannot burn for want of air? How can I ascertain this?”
Humphry thought for a while, and then it struck him, that if he were
to place a piece of thin glass upon the flame itself, so as to press
it down as it were, he could then see whether or not it were alight in
the middle; for if it were alight round the edge only, he would behold
through the glass a bright ring of flame, with a dark spot in the centre,
indicating the portion where the combustion was not going on.
[Illustration]
The experiment was soon tried, and the result proved as Humphry had
anticipated. A luminous circle was seen through the glass, at the point
where the flame touched the surface, and in the centre of this there was
a dark spot, like a black wafer, showing where the decomposed charcoal
gas existed in the interior of the flame in an unburnt state. The
appearance, however, is represented in the subjoined illustration. Indeed
Humphry perceived, that as air was necessary for combustion, and it
was impossible for the atmosphere to get to the interior of the flame,
the outside only of the cone of combustible gases, evolved from the
decomposed tallow, could be alight; while the vapour in the centre could
be burnt merely as it passed off into the air at the upper part of the
flame.
The next step was to see whether the interior dark part of the flame
really consisted of the same inflammable gases as were in a state of
combustion at the exterior, or surface of it.
Accordingly, Humphry placed a small glass tube within the centre of the
flame from an ordinary candle, and found that the unburnt gas from the
interior readily made its way up the tube, and escaped at the top of it,
and that on applying a light to this it was immediately ignited; so that
two flames were thus produced from one candle, as here shown.
[Illustration]
Another proof that the gases in the interior of the flame were not in a
state of combustion was afforded by lighting some spirits of wine in a
large spoon; for Humphry then found that he could introduce some grains
of gunpowder, and even some pieces of phosphorus, by means of a tube,
into the middle of the flame, without igniting them.
Humphry, moreover, could now see that the reason why the blow-pipe so
much increased the heat of an ordinary flame is simply because a current
of air is made by such means to traverse the interior part of it, so that
the combustion being rendered more perfect, the temperature, of course,
becomes proportionately higher.
Again, the wick of the “Argand Lamp” is constructed circular, so that
a current of air may be supplied to the interior of the flame; and
hence the superior brilliancy of this kind of burner. Table-lamps are
now generally made upon the same principle, and so, indeed, are the
gas-burners in shops. Sometimes, however, a jet is used, with the holes
so arranged as to spread the gas into a thin wide sheet, and thus allow
the air to act upon a large surface of combustible matter at each side.
These are called “_bat’s wing_” and “_fish-tail_” burners, &c., according
to the shape of the flat broad flames produced by them.
The next point in the inquiry was to discover what would be the effect of
cooling down a flame, since it was evident that it required a _very high
temperature for its existence_.
Humphry was well aware that, by projecting a current of cold air
upon substances in a state of combustion, the flames were generally
extinguished. Still he wished to see whether it was not possible to lower
the temperature of a flame by some other means.
Accordingly he procured a piece of wax-candle, and proceeded to pull
out all the wick, except one thread, so that when it was lighted the
flame from it should be as small as possible. Then the boy made a little
ring of iron wire, about the eighth of an inch in diameter, and this
he affixed to a wooden handle. Passing the wire ring over the lighted
wick he found, that on bringing it a little below the flame the light
was immediately extinguished, for the metal, being a good conductor
of caloric, served to carry off the heat which was necessary for the
maintenance of the flame.
It then struck Humphry that the pins which housewives stuck in rushlights
that they wished to be extinguished at a certain time, acted merely upon
the same principle—the metal carrying off the heat, and so lowering the
temperature requisite for the existence of the light.
The youth thought for a long time as to whether he could avail himself of
this principle in any way for the construction of his safety-lamp, for,
as he had now discovered that it was possible to extinguish flame merely
by reducing its temperature—and that a piece of wire, placed in connexion
with it, did this most effectually—he fancied he might take advantage
of some such means for preventing the passage of flame to fire-damp.
But though he racked his brain long he could hit upon no plan for
accomplishing his object. So he contented himself with merely making a
memorandum of the fact in his note-book, saying to himself, that he knew
it would prove serviceable some day—and then passed on to another part of
his inquiry.
Fire-damp, he was well aware, is explosible only when mixed with
the atmosphere; for the air is necessary for the combustion of all
substances, and explosion, as before observed, is merely the result of
_instantaneous combustion_. Still, certain liquids, Humphry knew, are
attracted by the sides of fine tubes, so that the fluid in which they are
immersed is maintained within them above the level of that without. He
knew, too, that by the same principle a certain quantity of water might
be retained in a fine sieve without running through the holes; and that a
vessel full of small holes might, for the like reason, be immersed to a
certain depth in water without sinking, or any of the liquid entering it.
So the lad was anxious to see what would be the effect of fine tubes upon
gases, and whether, upon passing an explosive mixture of carburetted
hydrogen and air along a narrow glass pipe, the flame would pass down it,
or be arrested in its progress.
Accordingly, having prepared some carburetted hydrogen gas, and mixed it
with eight parts of air—for these proportions he had found to be the most
explosive—he passed the mixture along a glass tube, the bore of which was
only ⅛th of an inch in diameter, and discovered, on lighting the gas that
issued from the end of it, that the flame, instead of travelling down
the narrow channel, and being instantly communicated to the whole of the
explosive mixture, remained stationary at the orifice of the tube, where
the gases burnt as slowly and quietly as with an ordinary candle.
Humphry was half-bewildered with joy at the discovery he had made.
Fire-damp he had now ascertained was not explosible within narrow tubes,
and instantly a hundred and one plans flashed across his mind for
rendering the fact serviceable to those engaged in the coal-mines.
Still there was much to work out. It was necessary to know through how
large a tube flame _was_ capable of being transmitted; so he procured
another glass pipe, the bore of which was a quarter of an inch in
diameter, or double that of the one he had first employed, and passing
the same explosive mixture along it, he found that, on lighting the
gases at the end of this, the flame no longer remained stationary at the
orifice, but travelled slowly down the channel, taking more than a second
before it reached the other end, though the tube itself was only a foot
long.
Then pursuing the same course of experiments, Humphry subsequently
discovered that by diminishing the diameter of the tube he could shorten
the length of it without any danger of the flame travelling along it, and
so causing the gases without to explode. Moreover, the lad ascertained
that when fine metal tubes were substituted for glass ones, the security
was even more perfect.
To insure greater safety, however, by making the channel through which
the explosive gases passed as fine as possible, he ultimately inserted
some short pieces of metal tubing, one within the other; so that the
bores being of different diameters, they formed a series of broad
concentric rings, each enclosing another, and having a _fine channel_
between them—thus.
[Illustration]
These Humphry proved to be perfectly secure against all explosion, and
then proceeded to fix one set at the bottom of a lamp, and another at the
top of a glass chimney that was made to fit air-tight round the flame.
The concentric tubes at the bottom were for the supply of air to the
light, and those at the top for the issue of the smoke; so that thus, if
any fire-damp were present in the mine, and entered with the atmosphere
through the air-tubes at the bottom, the flame would not be communicated
to the explosive gases outside the lamp—and thus the safety of the miner
would be thoroughly ensured.
For rendering the arrangement adopted more intelligible, a design of
Davy’s first Safety-Lamp is here given.
[Illustration]
The principle upon which this lamp depended for its security had been
so carefully worked out that it scarcely required testing. To assure
himself, however, of its efficacy, the boy immersed the lamp in the most
explosive mixture of carburetted hydrogen and air that he could possibly
form, and was well repaid for all his labours by seeing that the flame
was incapable of being transmitted to the combustible gases without; so
that, upon their entering the lamp, the light itself was extinguished.
In a word, he found that his lamp was “absolutely safe,” and that the
fire-damp had been disarmed by him of its terrors.
Humphry’s friends were as delighted as the boy himself at the successful
termination of his labours. Mrs. Foxell, who had, in the first instance,
encouraged him to proceed, was, perhaps, the most gratified of all;
nor did she refrain from lecturing her brother, Mr. Borlase, upon his
previous want of faith in the accomplishment of the result, telling him
that it was cruel, where so many lives were at stake, to damp the ardour
of any one who could believe in the possibility of rendering them secure
for the future.
Mr. Borlase was sufficiently generous to confess his error, and frankly
acknowledged that he never thought Humphry would be able to accomplish
half as much as he had.
The boy’s old friend, Mr. Tonkin, too, felt prouder than ever of his
foster-son, when he saw the lad test the powers of the lamp in his
presence; and the old gentleman made Humphry’s heart swell again with
the praises he heaped upon him, and the tears start to his eyes with the
confession of the long love he had borne the boy.
At length, however, Mr. Tonkin said that he thought one improvement still
was required, though how it was to be effected he must leave Humphry
to find out, for it was more than he could manage. On the admission of
the fire-damp into the lamp the light was extinguished. “This,” the old
gentleman observed, “appeared to be a great defect, for the extinction
of the flame was a sure indication that danger surrounded those who
carried the lamp; and to leave the miner in darkness at a time when
immediate flight was necessary, struck him as being a serious drawback to
its utility. If Humphry, therefore,” he said, “could, by some means or
other, arrange it so that the light should continue burning, even in the
presence of the fire-damp itself, the ingenuity of the apparatus would
not only be much greater, but its value be considerably enhanced.”
Humphry saw the force of Mr. Tonkin’s objection, and it struck him that
the defect was almost irremediable. He was, therefore, not a little
vexed to find that an instrument which he had fancied perfect, was not
altogether faultless.
The lad pondered over the matter for many days, and tried a number of
experiments to overcome the difficulty which had been raised. It proved,
however, beyond his powers; so, exhausted by his long study, and vexed
at the idea of his comparative failure, Humphry, at length, dismissed
the subject from his mind, and locked the safety-lamp in his cupboard,
determined that it never should be made public until it was perfect in
all its arrangements.
It was in vain that Mrs. Foxell urged him to make known the invention,
even in an unfinished state; but the boy was firm in his resolve. “_Some
day_,” said he, “it shall be completed, and then the world may have it,
and welcome; but as it is now, it is hardly worth either the giving or
accepting, and no honours can come of it. When I know more,” he added,
“I can do better: nor do I intend, because I have been baffled in this
my first project, to give over studying; for I find such delight in
the perception of the new truths which are daily unfolded to me, that
I would rather forego any source of pleasure than that which comes from
scientific discovery.”
* * * * *
It was many years before the Safety-Lamp was perfected; and when it was,
the alteration was so slight, that it has been well observed—“The history
of this elaborate inquiry affords a striking proof of the inability
of the human mind to apprehend simplicities without a long process of
previous complication.”[46]
Humphry Davy, as we have narrated, had already discovered, while
experimenting upon the inflaming of explosive gases in narrow tubes,
that, provided he diminished the bore, he could shorten the length of
them in a corresponding ratio. Now a sheet of wire-gauze, it is obvious,
consists merely of a series of fine tubes, of very short lengths. It
appears strange, therefore, that our hero was some time before he
conceived the happy idea of constructing his lamp entirely of wire-gauze,
instead of feeding it with air through narrow channels, or “safety
canals,” as he called them; for, as flame cannot be transmitted along
small tubes, it is manifest that, on surrounding a lamp with a wire-gauze
cylinder, or cage, the fire-damp, as it enters with the air, will burn
within the cage itself, and the flame be incapable of passing through
the small apertures in the gauze, and thus exploding the gas outside of
it.
To render the action of wire-gauze in this respect more intelligible, a
few of Humphry Davy’s after-experiments may be cited in connexion with
this part of the subject.
If a small piece of wire-gauze (say about 9 inches square, and having
about 30 meshes to the square inch) be gradually brought down upon the
flame of a candle, the flame itself will be cut off where it touches the
gauze, and merely a dark spot be there observed, encircled by a ring
of light, while the combustible matter of the flame will issue through
the small apertures in the form of smoke; for during the passage of the
burning gases through the metallic meshes they will be so far cooled
down that the flame will be extinguished, and thus rendered incapable of
traversing the gauze itself.
If, however, a lighted taper be held above the wire-gauze, the
inflammable gas or smoke from the candle will be immediately rekindled,
and the flame continue to burn then, both on the upper and under side of
the tissue.
But the impermeability of metallic gauze to flame may be rendered
still more evident, by placing a piece of fine wire web above a jet of
gas previous to lighting it, when it will be found that, if a burning
taper be held over the wire web, the gas will be kindled above it, and
continue to burn upon the gauze itself, while the combustible matter
beneath will remain unignited; for the metallic threads, being good
conductors of heat, will so cool the flame as to prevent it passing
through to the gas on the lower side.
[Illustration]
Again, if a small piece of camphor be laid in the centre of a sheet of
wire-gauze, and a light held beneath it, the vapour from the camphor,
which is very inflammable, will be made to burn with a bright flame
_downwards_—instead of upwards, as flames usually do—while the camphor
itself will lie upon the upper side of the tissue in an uninflamed state.
[Illustration]
The power of metallic webbing thus to intercept or extinguish flame,
depends simply _upon the cooling effect_ it produces; so that as flame
requires a high temperature for its existence, it is plain that it no
sooner becomes cooled, by being passed through a good conductor of heat,
than it is extinguished.
If, however, the meshes of the wire-gauze be not sufficiently small, or
if the wire of the gauze itself becomes intensely heated, the flame will
traverse it in either instance; because the cooling power is reduced, in
the one case, by the largeness of the apertures, and in the other by the
high temperature of the wire.
For the knowledge of these facts, we repeat, we are indebted to the
after-investigations of Davy himself.
Now it is evident from the above experiments, that if a lamp be covered
with a cylindrical cage of wire-gauze, no flame will be able to pass
from the interior of the lamp to the exterior of it, in consequence of
the cooling power of the metallic web encompassing it; so that, if the
surrounding air be charged with an explosive gas, it will enter the
lamp through the meshes of the web and burn within the cage, while that
without will remain unkindled.
Such, then, is the principle of the “Safe-Lamp” as perfected by Davy, and
which is here shown:
[Illustration]
The safety of this lamp may be exhibited by immersing it in a large
jar, at the bottom of which is a little Ether; for the vapour from this
liquid, on mingling with the air, forms a highly inflammable atmosphere.
On introducing the lamp into this, the flame first becomes enlarged and
is then extinguished, while the whole of the cage remains filled with a
lambent blue light. On withdrawing the lamp, however, and bringing it
into the open air, the wick is suddenly rekindled, and the flame returns
to its natural size and colour.
For perfect safety, it is necessary that the wire-gauze of these lamps
should contain about 30 wires, or 900 apertures in every square inch, and
that the heat of the wire itself should never rise above redness; for
“if,” as Davy said, “the iron wire become white hot, the lamp will be no
longer safe. This, however,” he adds, “need never happen in a colliery;
for if a workman finds the temperature of the wire increasing rapidly
in an explosive atmosphere, he can easily diminish the heat by turning
his back upon the current, and keeping it from playing upon the gauze by
means of his clothes.”
Of this wonderful lamp it has been well said, “that it is a present from
Philosophy to the Arts, and to the class of men farthest removed from
the interests of science. We know of no discovery in which the admirer
of Science and the lover of mankind have greater reason to congratulate
one another. The discovery,” adds the late Professor Playfair, “is in
no degree the effect of accident; and chance, which comes in for so
large a share in the credit of human inventions, has no claims on this,
which is altogether the result of patient and enlightened research. The
great use of an immediate and constant appeal to experiment cannot be
better evinced than in this example. The result is as wonderful as it is
important. An invisible and impalpable barrier, made effectual against a
force the most violent and irresistible in its operations, and a power
that, in its tremendous effects, seemed to emulate the lightning and the
earthquake, confined within a narrow space, _and shut up in a net of
the most slender texture_; these are facts which must excite a degree
of wonder and astonishment, from which neither ignorance nor wisdom can
defend the beholder. When to this we add the beneficial consequences,
and the saving of the lives of men, and consider that the effects are to
remain as long as coal continues to be dug from the bowels of the earth,
it may fairly be said that there is hardly, in the whole compass of art
or science, a single invention of which one would rather wish to be the
author.”[47]
It should be borne in mind, moreover, that Davy in the accomplishment
of his noble task, not only did not seek, but positively rejected all
pecuniary reward; so that it becomes difficult which to admire the
most—the benevolence which prompted him to undertake the long train of
laborious investigations, the genius which carried him to so successful
an issue, or the noble disinterestedness which bade him refuse to traffic
in an invention that was destined to mitigate the sufferings of some of
the poorest and least educated of his fellow-creatures.[48]
CHAPTER XII.
HUMPHRY PRACTISES AS A SURGEON—ON HIMSELF.
Humphry by this time desired some little recreation. He had been at work
for many weeks uninterruptedly—his days given to his profession, and his
leisure, in the evenings, devoted to the prosecution of his scientific
discoveries; and what with the exhaustion of continuous thought, and the
vexation at his inability to perfect the lamp, from which he had such
high hopes of honour, the lad felt incompetent to resume his labours
until his mind, by diversion, had recovered somewhat of its ordinary
elasticity.
Accordingly, he determined upon enjoying a day or two’s fishing with his
uncle Millett—for this was Humphry’s favourite pastime throughout life;
and the sport, his biographer tells us, “was alike his relief in toil and
his solace in sorrow.”[49]
It was now spring, moreover, when the small streams, the youth knew, were
in the best state for angling—turbid, in a slight degree, from the mild
rains common in April and May; so Humphry, having arranged his tackle
on the over-night, sallied forth the next morning to join his uncle at
Marazion.
The lad’s fishing costume was singular enough to claim some notice. It
consisted of an entire suit of green—that colour being considered by
him the most likely to elude the observation of the fish. The coat was
half covered with lappets of the many pockets for holding the necessary
tackle; and his hat (which was a round felt one, with a broad brim like a
wagoner’s), had been dyed of the same colour as his clothes by a pigment
of his own composition; while round the hemispherical crown were coiled
a series of fine lines, each terminating in some peculiarly-coloured
artificial fly: so that the hat appeared somewhat like a clod of turf
upon which so many bees and moths had settled. His legs, again, were
encased in a huge pair of jack-boots, which, for the convenience of
wading through the water, reached above his knees; indeed, as Macbeth
says of the witches, “He looked not like an inhabitant o’ the earth, and
yet was on’t.”[50]
[Illustration: HUMPHRY EQUIPPED FOR A FISHING EXCURSION.—Page 319.]
Thus equipped, Humphry, as we said, sallied forth, with the joints of his
rod strapped together—like a small bundle of fagots—and resting on his
shoulder, while at his back projected the wicker fish-basket, as if it
were a huge cartouche-box. Over the other shoulder were slung the heavy
jack-boots, and at his side ambled his favourite water-spaniel “Chloe,”
her long tail wagging as she stopped now and then to look up in her
master’s face, for she seemed to be as delighted as the boy himself at
the anticipation of the sport that she knew was about to ensue.
The youth paused occasionally to fondle the knowing creature, for he had
her since a pup—having begged the gift of her when she was taken from her
mother, and about to be drowned with the rest of the litter as soon as
born, and it was only by great care that he had been able to rear her,
so that the two were as attached to each other as any human friends
could be; nor did Humphry treat her as a dumb animal, but spoke to her as
though she understood every word he said: and, perhaps, in his heart, the
boy (with his half-poetic and half-metaphysic theories) _believed_ she
did.[51]
* * * * *
It was late that evening before the couple returned from the day’s sport,
and then Chloe carried in her mouth a small basket containing a portion
of the spoil, for Humphry’s wicker knapsack was not capacious enough for
the whole, as he and his uncle Millett—so runs the record—had caught
no less than “seven dozen trout in the rivulet and mill-pond near the
residence of the Rev. Mr. Giddy, in the parish of St. Earth.”
Humphry’s success at his sport had made him too light-hearted to feel
the fatigues of the day, and although he was somewhat foot-sore from the
long use of the heavy boots he now carried over his shoulder, the boy
and Chloe _jogged_ merrily past the tanneries at the extreme west of the
town; for the dog knew as well as her master, by the leathery smell that
filled the air at that quarter, that they were not far from home _then_;
and Humphry, as he patted the fond animal, promised her a good supper of
fish for all that she had done that day.
As they passed down Market-Jew Street the oil-lamps and candles were
being lighted in some of the little shops, and Humphry saw, as he looked
towards the Town-Hall at the end of the street, that the sun had long
since set, for the sky was grey with the thickening dusk, and the stars
were beginning to peep out of the haze, one after another, through the
darkening firmament.
At length the Town-Hall itself was reached, and the youth was telling
Chloe that she should soon have her supper now, when suddenly a loud cry
was heard. As the lad turned round towards the street that led to Madern
Church to ascertain the cause of the noise, he beheld to his horror a
huge dog, at full speed, hurrying in that direction, white with foam
at the mouth, and followed by a mob of affrighted people, hooting and
hallooing at its heels. Some of the men were armed with pikes, and others
carried muskets, intent on the destruction of the rabid animal.
Humphry, with Chloe still by his side, was within a few paces of the
furious creature. The boy saw in an instant that flight was impossible,
and dreading lest his favourite dog should be attacked, he shouted
“Back! back!” to her in his most commanding tone. The order, however,
was too late, for Chloe, being a little in advance of her master, had
already attracted the notice of the infuriated brute, and Humphry saw
her danger at a glance. In another moment his own dog would be seized by
the rabid one, and the slightest graze from its teeth he knew would be
sufficient to render Chloe’s immediate destruction a matter of duty. It
was no time for reflection, so the excited boy, eager to save the life of
his favourite spaniel, rushed past her with his heavy fishing-rod raised
high in the air, and ready to fell the dangerous brute to the earth.
Ere he could aim a blow, however, the hunted dog had fastened on
Humphry’s leg, and fixing his fangs in the flesh, inflicted a wound that
made the lad shriek again with the suddenness of the pain.
The cry of her master brought Chloe instantly to the rescue, and dropping
her basket of fish she sprang, yelping, towards the savage brute. Humphry
knew the peril of the encounter, and finding the animal about to relax
its hold of his flesh, he seized it by the neck and held its head firmly
to the ground, while the townsfolk rushed immediately to the spot, and
with their weapons soon put an end to all the danger.
“Tha bee’st bitten, Master Humphry,” shouted Malachy Carteret, as he drew
his adze from the head of the animal; “run tha to Dr. Borlase directly,
and ha’ the bite looked to, or tha life, poor boy, a’n’t worth a dried
pilchard.”
Then came Jan Penberthy the miller—as white as plaster-cast in his
working dress—and he, with a cluster of others behind, were anxious in
their inquiries as to what was the nature of the wound, while each had
some novel and different remedy to recommend.
Humphry, however, knew sufficient of his profession to be aware that it
was no time for hesitation; so, while the eager throng crowded around
him, he raised the leg of his trousers, and observing the marks of the
animal’s teeth in his flesh, he deliberately drew his knife from his
pocket, and there, upon the spot, cut out the lacerated part without a
wince.
This striking instance of the boy’s intrepidity was hailed with wonder by
the people about him, and one and all were loud in their praises of his
courage and decision. Many who had known him from a child rushed up and
shook him warmly by the hand, while others, who had been the companions
of his father, declared he was Robert Davy’s own son every inch of
him—indeed all there had some encouraging word to say or some kindness to
proffer.
Humphry, however, was too sick and faint from loss of blood to be able
to listen to the remarks of those about him; so, having tied his
handkerchief round the wound, he begged Malachy Carteret to help him home
to Dr. Borlase’s. And as the little carpenter curled the boy’s arm about
his neck Humphry limped along with Chloe at his side, who kept looking up
sadly in his face, as if she was aware of all that had happened, while
the boy exclaimed as he went, “Thank God I have saved your life, poor
Chloe, even though it be at the expense of my own.”
Following in the wake of Humphry and Malachy walked many of the crowd—one
carrying the boy’s fishing-rod, another his jack-boots, and another the
basket of trout that Chloe had dropped in the road; and as they went
along, they wondered among themselves whether Master Humphry would get
the better of the bite; and some told curious country tales as to how the
poison had remained in the blood for years afterwards, so that a person’s
life was never safe from it.
On reaching the surgery, Humphry found that Mr. Borlase had been called
to visit a patient in the country that evening; so, being left to his
own resources, he proceeded forthwith to apply some lunar caustic to
the wound himself, and having done this, he begged Mrs. Foxell (who was
frightened to tears at the dangerous accident he had met with) to make
his excuses to her brother, the doctor, when he returned, for the poor
boy told her he was anxious to reach his mother’s house before the news
of his having been bitten by a mad dog was carried home, so that he might
lighten her alarms on his account.
Humphry, however, had barely taken leave of Mrs. Foxell before Mrs. Davy
herself rushed into the surgery, half frantic with fear at what she had
heard; and she no sooner caught sight of him than she fell upon his neck,
and wept and laughed by turns, hysteric with the intensity of her emotion.
The boy endeavoured to assuage her, assuring her that from the promptness
and vigour of the remedies he had applied there was little cause for
alarm. But to no avail: the poor woman was satisfied her darling boy was
doomed to the most frightful of all deaths sooner or later, saying, “that
if she was deprived of him her cup of bitterness would be full indeed.”
Then the heavy privations she had already suffered in life rose again
to her mind, and in the agony of her despair at the calamity which now
threatened her she wrung her hands, and cried aloud to God to have mercy
upon her.
Nor could she in any way be soothed until the lad told her it was
necessary for him at such a time to remain in perfect quietude, and that
the least excitement might develope the very symptoms which she dreaded.
This had the desired effect. Such was her love and care of the boy,
that not another tear did she afterwards shed in his presence; but,
dismissing her own trials, she talked to him only of the subjects she
knew he delighted in; and, when she had him removed to her own house, she
sat by his bed, day after day and night after night, reading to him from
works on the different sciences till his eyes were closed in sleep, and
then the poor widow would fall upon her knees, and with her pent-up tears
streaming in secret from her eyes, and her voice choked with her sobs,
pray the Great Ruler of All to spare the only protector left her.
[Illustration: HUMPHRY’S MOTHER READING TO HIM DURING HIS ILLNESS.—Page
326.]
CHAPTER XIII.
THE FIRST SUN-PICTURES.
Humphry remained confined to his bed for many weeks, and every day his
mother dreaded seeing some development of the symptoms which generally
ensue from the bite of so rabid an animal; but the boy assured her, again
and again, that, owing to the promptitude of the remedies he had applied,
there was no cause for alarm.
While the youth was still a prisoner in his room, a gentleman came to
lodge at the house, and Humphry, to his great delight, learnt, in a
few days afterwards, that the new lodger was no less a person than Mr.
Gregory Watt, son of the celebrated James Watt, the inventor of the
present steam-engine.
It did not take long before the two became acquainted, and then Humphry
ascertained that the young Mr. Watt had but recently quitted the
University of Glasgow, and had been recommended by his physicians, owing
to his declining state of health, to reside for some time in the West of
England; hence the cause of his visit to Penzance.
Nor was Humphry long in discovering that the mind of his new friend
was enriched beyond his age with science and literature, and that
he possessed, like himself, a spirit devoted to the acquisition of
knowledge, and soaring far above the little vanities and distinctions
of the world. The two kindred spirits, therefore, soon contracted
an intimacy of the warmest nature; and this ultimately ripened into
a friendship which continued to the period of Mr. Watt’s premature
dissolution.
Mr. Gregory Watt felt not a little astonished, on being introduced to
the son of his landlady, to find him shortly afterwards speaking upon
subjects of metaphysics and poetry; for when Mr. Watt spoke to him of
the courage he had displayed at the time of the accident in excising the
wounded parts of his leg, Humphry confessed “that he had no belief in
the existence of pain, whenever the energies of the mind were directed
to counteract it.”[52] “For,” went on the boy, “in states of profound
attention, all perception of external things fades from the mind; the
clock in the room ticks, and we hear it not; persons enter the apartment,
and their presence is unheeded by us. And if by the intensity of the
intellectual operations the senses can cease performing their functions
in the one case, why not in the other? Martyrs at the stake have, while
in deep prayer, held their hand unmoved in the flames, and who can say
that the very fervour of their heavenly aspirations did not deprive them
of all sense of pain for the time being?”
Mr. Gregory Watt, however, had but little taste for metaphysical
discussions; and, smiling at Humphry’s stoicism, sought to divert the
conversation into a more congenial and practical channel.
The steam-engine that had been recently set up at the Wherry Mine by
Mr. Watt’s father became the theme of their converse; this soon led to
comments on the theory upon which its powers depended, and then Humphry’s
companion was surprised to find that a youth, who had been brought up in
an obscure town in Cornwall, was as well acquainted with the doctrine of
“latent heat”—and, indeed, the whole science of caloric—as he himself,
who had been reared, as it were, in a factory, where the workings of it
were every day visible.
The new laws of combustion naturally followed as the next subject
of discussion, when Humphry observed, “that he would undertake to
demolish the French theory in half-an-hour;” and so saying, he rapidly
ran over the experiments he had performed, to prove the falsity of
Lavoisier’s notions respecting the origin of heat during the burning of
substances.[53]
The lad had now touched the true chord, and the interest of Mr. Watt
becoming more excited, he conversed with young Davy upon his chemical
pursuits, and was at once astonished and delighted at his sagacity;
so that the couple—congenial in taste—already began to feel a growing
friendship for each other.
* * * * *
Humphry had quitted his chamber, but was still confined to the house,
from his inability to walk, when Mr. Watt—who had now known the youth
long enough to be proud of his acquaintance—returned from his morning’s
stroll along the sea-shore, in company with two friends whom he had met
in the town.
The gentlemen proved to be Mr. Josiah Wedgwood, the eminent potter of
Staffordshire, and his brother Thomas, who was alike distinguished for
his scientific abilities.
The learned potter was not long in Humphry’s company before he discovered
the high merits of the lad; nor was he a little pleased when he found
that the young Cornish apothecary knew all about the pyrometer he had
invented for measuring high degrees of heat by the contraction of
a ball of clay; and the old gentleman found considerable delight in
explaining to the boy the various processes concerned in the manufacture
of earthenware and porcelain, telling him anecdotes as to how Bernard
Palissy—who was the first to discover the means of giving a glaze to the
baked clay—had been reduced to such poverty by his experiments, that he
was forced to burn the doors, and even the boards, of the house in which
he lived, in order to get a supply of fuel for his furnaces; and how he
afterwards amassed an immense fortune by the invention, and ultimately
died in the French Bastille, a martyr to the Protestant creed, which he
had espoused. Mr. Wedgwood added that, “When we eat our food we little
think of the labour and privations that have been endured in order to
give a glassy surface to the plates and dishes upon which it is served;
for but few are aware that, previous to this invention, the ordinary
earthenware articles were more like tiles than our present crockery.”
Then Mr. Wedgwood talked with the youth about the rocks, inquiring
whether he had ever noticed any of the finer species of clay in those
parts, and was surprised to find how closely the boy had marked the
changes in the soil; for Humphry told him “he had observed that the
felspar in the granite decomposed long before either the mica or the
quartz, and that it was chiefly by the action of the atmosphere upon this
same felspar that the huge granite rocks became disintegrated, or broken
up; and that, as the felspar consisted principally of clay in the purest
form, he fancied that some advantage might be taken of this in producing
a finer species of porcelain than had yet been manufactured in this
country.”
The old gentleman thanked Humphry for his suggestion, and warmly praised
him for his observation and sagacity; whereupon the youth promised,
immediately that his leg would permit of his accompanying him, to point
out to Mr. Wedgwood the places where he had noticed the finest deposits
of felspar to occur.
The conversation then changed to a subject that Mr. Josiah Wedgwood said
“he had no doubt would be highly interesting to one of Humphry’s turn
of mind.” The old gentleman told the boy how his brother Thomas had
discovered a means of copying pictures upon glass, and even of fixing
the images of the _camera obscura_, by the action of light; “so that,”
he said, “the sun itself could be made to turn artist, and to produce a
representation of an object that no human hand could possibly rival.”
Humphry was enraptured with the new wonder, and more eager than ever to
learn all about it; so he begged Mr. Thomas Wedgwood to explain to him
the whole process.
“I should tell you, then,” said the potter’s brother, “that it has been
long known to chemists that a solution of _nitrate of silver_ (called
_lunar caustic_ in the shops), when washed over a sheet of paper—although
it does not undergo any change while kept in a dark place—will speedily
change colour on being exposed to daylight, and that then it passes
through different shades of grey and brown, and ultimately becomes nearly
black. These alterations in colour,” continued Mr. Thomas Wedgwood, “take
place more rapidly according as the light is more intense. In the direct
beams of the sun, two or three minutes are sufficient to cause it to
darken; whereas in the shade, several hours are required to produce the
full effect. The light, too, when transmitted through different-coloured
glasses, acts upon the nitrate of silver with different degrees of
intensity. It is found, for instance, that the sunbeams, when passed
through red or yellow glass (so that only red or yellow light shall fall
upon the paper), have very little action upon the lunar caustic; green
glass, however, is more efficacious, while blue and violet produce the
most decided and rapid changes.
“Now to make you clearly comprehend,” went on the gentleman, “the reason
of these changes, I should tell you that nitrate of silver consists
of nitric acid (aquafortis) and silver,[54] and that if a vessel of
pure and colourless nitric acid be exposed to the sun’s rays, it will
become decomposed, so that red fumes of nitrous acid will be evolved,
and these mixing partly with the liquid itself, will shortly turn it
to a reddish-brown tint. Well, the same change takes place when the
nitric acid is combined with the silver, and so made to form nitrate of
silver. The consequence is, that as the nitrous acid which is evolved is
unable to combine directly with the silver, the decomposed aquafortis is
dissipated in the form of vapour, while the silver itself remains behind
in the paper; and in such an extreme state of minute division, that the
particles, instead of being white, like ordinary silver, appear black to
our eyes.”
Humphry expressed himself delighted with the explanation, and said he
could now see how it was possible to produce sun-pictures by such means.
Still he begged Mr. Wedgwood to proceed.
That gentleman then told Humphry that his first attempt concerning the
production of sun-pictures, was to fix the evanescent images formed by
the camera obscura, but though he was able to impress these upon paper
in a bright sunlight, he found that he could not produce them in any
moderate time in ordinary daylight; so that, from the length of time
required before the impression was taken, the effects of light and shade
had materially altered. “With paintings on glass, however,” he added, “I
have been more successful; in order to copy these I apply the solution of
nitrate of silver to leather, for this I find to be more readily acted
upon than paper—probably owing to the tanning in the material. When the
surface of leather is thus prepared, I place it behind a painting upon
glass, and expose it to the solar light, when the rays, being transmitted
through the different parts of the picture, produce distinct gradations
of black and white, according to the lights and shades in the original;
for where the light passes freely through the glass, the colour of the
nitrate of silver, of course, becomes the deepest. By this means, then,
you will perceive that the lights and shades of my picture are entirely
reversed: all the black parts in the original being left white in the
copy, since the light, being unable to pass through these, cannot act
upon the solution; while all the white parts of the original, on the
other hand, become the blackest in the copy, owing to the rays passing
freely through the glass there, and so producing the strongest effect.
Accordingly, I am obliged to have the original pictures painted, in the
first instance, with their lights and shades reversed, or else I cover
another glass with a thin coating of isinglass, and apply the solution
of nitrate of silver to this; so that, when I have transferred the
original picture by this means to a second plate of glass, in which the
lights and shades are the direct opposite to what they are in nature, I
proceed to take a second copy of it—but this I do upon leather, as before
explained—and so obtain a perfect reproduction of the original, with all
the lights and shades in their proper places.
“But the same method of copying,” proceeded Mr. Wedgwood, “may be applied
to other purposes. It may be rendered subservient, for instance, for
making delineations of all such objects as are partly opaque and partly
transparent, such as leaves and the wings of insects. For this purpose
it is only necessary to put the objects to be copied between a plate of
glass and the prepared leather itself, when the sunlight, being more
or less intercepted by their forms, will leave the figures accurately
impressed upon the leather, so that they will appear as beautiful white
pictures upon a black ground. There is, however,” added Mr. Thomas
Wedgwood, “one great defect connected with the production of sun-pictures
by the means I have described, and this consists in the impossibility
of fixing them so that they shall be no longer susceptible of being
darkened when exposed to the light. I have already tried several methods
of obviating this difficulty. I have covered the sun-pictures with a
thin coating of varnish, but to no purpose, for they darken almost as
rapidly with the varnish over them as others do without it. Again, I have
submitted the pictures to frequent washings, in the hopes of dissolving
out of the paper or leather all the undecomposed nitrate of silver; yet,
even after this, a certain portion of the active matter still adheres to
the white parts of the sun-picture, and so causes them to blacken all
over on being exposed to the light. The consequence is, that the pictures
produced by the action of the sun must, in order to be preserved, be
examined always in the dark, and be kept continually in some place where
no light can penetrate.”
Humphry no sooner heard this than he suggested a number of expedients
by which he fancied the difficulty might be overcome; and as the lad
explained his reasons for the various methods he proposed, both Mr.
Wedgwood and his brother were as astonished at the extent of the boy’s
knowledge, as they were delighted with the acuteness of his sagacity.
The evening was passed in examining a portfolio of the sun-pictures that
Mr. Thomas Wedgwood had brought with him, and Humphry grew so charmed
with the then entirely novel process of “_photography_” that he declared
he would not rest until he had investigated the matter himself, and
ascertained experimentally whether any means could be found of rendering
the pictures permanent.
CHAPTER XIV.
THE WONDERS OF THE REFRACTION OF LIGHT.
Our young hero was still too weak to leave the house, for the wound in
his leg was of so dangerous a description that Mr. Borlase had strictly
enjoined him to take as little exercise as possible; consequently, while
Humphry’s evenings were passed in conversation with young Mr. Watt, and
occasionally the Wedgwoods, his mornings were spent in his chamber alone,
so that he was glad to resort to study as a means of enlivening his
solitude.
Still the lad wanted some incentive to stir him to enter upon a fresh
branch of science. This, however, he had now found in the wonderful
photographic impressions Mr. Wedgwood had shown him, and he was eager
to make himself acquainted with the laws of the mysterious principle by
which they had been obtained.
The contemplation of the sun-pictures naturally led Humphry to think of
the more transient pictures formed in the _camera obscura_ by the light;
and he passed from the fixing of the images to the consideration of the
conditions by which the images themselves are produced.
“How comes it,” he mentally inquired, “that a piece of glass, merely
because it is rounded on one or both sides, is able to copy the forms
and colours of external objects? And why do several of such glasses, in
combination, bring the images of even the most distant things so close to
us, that we are enabled to make out their minutest parts?”
Humphry knew that the effect could arise only from some alteration in the
direction of the rays of light as they passed through the glass itself;
and, accordingly, he determined to set to work and discover the precise
change that a luminous ray undergoes on traversing different substances.
In the first place, however, it was necessary to determine what was the
_natural_ direction of a ray of light emitted by a luminous body.
For this purpose Humphry—who was still unable to undergo the exertion
of arranging his own experiments—had to avail himself of the assistance
of his sister Kitty; and so pleased was the girl with the office, that
she readily gave up to her brother every moment she could spare from her
household duties.
Kitty Davy was some two years younger than Humphry himself, so that they
had been infant playmates together, sharing the same toys and taking
parts in the same childish gambols. It was to Kitty, too, that the boy
had first recounted the fairy stories he loved to invent; and when, in
his after-youth, Humphry compounded his celebrated “thunder-powder,”
Kitty invariably aided him in the manufacture of the composition: so that
each year had served to increase the love which, from the remembrance of
the pleasures they had enjoyed together, had been begotten almost with
the dawn of memory itself.
Kitty was now budding into womanhood. She had grown so tall within the
last year, that her figure was spare, and not particularly comely; while
the curls, which once fell, with childish beauty, in tortuous profusion
about her neck, had now been displaced, and her hair twisted into the
more womanly, but less gainly, protuberance at the back. This style of
head-dress had long been a point of ambition with the young lady, and now
that she had risen to the dignity of wearing her hair like her mother,
Miss Kitty had grown to fancy that she was no longer a child.
Nor was she. The increasing strength of her affection for her brother
showed that her woman’s nature was developing, and she seemed to cling to
Humphry and her mother with a new tenderness, as if she needed some one
to heap her strengthened love upon. The doll upon which she had, until
the last year or so, bestowed her caresses, had been given to one of her
younger sisters; and now she appeared to take almost a mother’s pride in
tending little Johnny, her brother, instead.
Humphry during his illness had been constantly “nursed” by her, and
now that she was allowed to officiate in the kitchen, the girl loved
to surprise him each day with some new posset or jelly which she had
prepared for his gratification. She had read to him, too, so long by his
bed-side from the scientific books which her brother loved to listen
to, that, aided by his explanations of the more difficult parts of the
subject, she herself had acquired a slight knowledge of the phenomena of
the universe: so that, while assisting him in his experiments, she felt
almost the same taste for the work as Humphry did, and was not a little
delighted when the hour came for her to take her accustomed post in her
brother’s sick chamber.
* * * * *
Humphry, as we said, was intent upon discovering the natural direction
of a ray of light proceeding from a luminous body. With this view he got
Kitty to close the shutters so as to completely darken the room, and then
to pierce a fine hole through them. This being done, Humphry pointed
out to the girl that the beam was in a perfectly straight line—for the
course of it was rendered plain by the little particles of dust that
floated in the atmosphere, flashing, as they danced in the ray, like so
many tiny fire-flies. Then as Kitty wheeled her brother’s chair so that
the light fell directly upon his eye, the boy could see the sun itself
shining through the hole; thus proving that the beam proceeded in a
direct straight line from the orb of light to him.
The next step was to procure a flexible tube, and with this held
_straight_ before the eye, Humphry could still distinguish the sun
through the hole in the shutter, though when the tube was _curved_ the
effect was totally different, for then no light at all could be perceived.
Kitty was not a little delighted with the demonstration that a ray of
light proceeds in a straight line; and Humphry, to make her understand,
as well as to prove to himself experimentally, that all luminous
bodies _projected an infinity of such straight rays in every possible
direction_, bade the willing girl fetch him the rushlight and shade from
below.
While the shutters were still closed the candle was lighted, and then
Humphry pointed out to his sister how the little luminous circles that
were spattered over the wall all round the room, as the light passed
through the holes in the shade, showed that the rays proceeded from it in
every direction; and that they travelled in a straight course was easily
proved, by covering with the finger any one of the holes in the shade,
when the luminous circle on the wall, which was in a direct line with the
hole, and the flame became immediately obscured.
[Illustration]
It was plain, then, that a ray of light travelled naturally in a straight
line. Still, as a further proof of this phenomenon, Humphry threw the
shadow of an opaque body, of a certain size, upon a white screen, and
there measured its dimensions. Having cut a piece of millboard, exactly a
foot square, he placed it 1 foot distant from the flame of a candle, and
then arranging the screen at double the distance, or two feet from the
light, he found upon measuring the shadow of the millboard that it was
exactly (2 × 2) 4 times larger than the millboard. When the screen was
three feet distant from the candle, or 3 times as far from the light as
the millboard, the shadow was ascertained to be (3 × 3) 9 times larger
than the surface from which it was projected; while at the distance of
4 feet, the dark space upon the screen was discovered to be (4 × 4) 16
times greater than the millboard itself.
[Illustration]
[Illustration]
This effect could arise solely from the rays of the candle proceeding
in a straight direction, as will be rendered evident by the preceding
diagrams; where it will be seen that the opaque body, interposed between
the light and the screen, prevents the rays which fall upon it reaching
the screen itself, so that a dark space appears upon the latter, as
many times larger than the opaque body, as the distance of the screen
from the candle is greater than the body projecting the shadow; for
it is manifest, that if a series of right lines be drawn from the
luminous point to the edges of the opaque body, and thence to the screen
itself, they will exactly circumscribe a space whose dimensions will be
proportional to the distance of the one to the other.
“Well,” said Humphry to his sister, “we now see that luminous bodies
_emit_ rays of light _in all directions_, and that each ray from them
proceeds in a _straight_ line, while those substances which are called
opaque prevent, when placed before the light, the rays from reaching
other substances behind them—the rays, in such cases, being _stopped_
or intercepted. Some bodies, however,” added the boy, “are capable of
_transmitting_ the rays of light: that is to say, they allow the beams to
pass through them, and these are, therefore, termed transparent; since,
unlike opaque bodies, they project no shadows when placed between the
light and other bodies. Let us now see what occurs when a ray of light
passes through a transparent body.”
Accordingly, Humphry procured a small open vessel, in one of the sides of
which there was a hole near the top, large enough to admit the light from
a candle. The lad then proceeded to ascertain the exact place where the
ray of light from the flame fell at the bottom of the vessel; and found
that, when the vessel was empty and the candle placed at a short distance
from the hole, there was a small circle of light formed at the bottom,
which was, of course, in a direct line with the hole and the flame, as
here shown.
[Illustration]
Having then set a mark at A, where the circle of light appeared, he
directed Kitty to pour water into the vessel until it was half full; and
when she had done so, he noticed that the ray of light from the candle
no longer fell upon the same spot as it did when the vessel was empty,
but at a little distance nearer the candle, so that it was plain that the
ray, instead of proceeding in a straight line as before, had, in passing
through the water, _been bent down out of its visual course_, in the
manner indicated at B.
[Illustration]
“You see, then,” remarked Humphry, “that a ray of light, when it falls
in a _slanting_ direction upon a transparent body, _no longer travels on
in a straight line, but is refracted_,” as it is called, “_or bent out
of its previous course, at a certain angle_. Consequently, if the spot B
was a fish lying at the bottom of a river, it would be seen by a person
on the shore in the direction of the point A; and thus it would appear
out of its true place, and, in order to strike it with a spear, we should
have to direct the weapon at a spot _nearer_ to us than where the fish
_seemed_ to be lying.”
Then her brother told her that it was for the same reason that a straight
stick appeared to be crooked when half immersed in a pool of water, and
a crooked stick a straight one under the same circumstances; “for,” said
he, “if instead of the straight ray of light we imagine a straight stick
to be passed through the hole, so that the point of it may be at the spot
A, it is plain that in the water the end of the stick will appear at
B, since the part of it which is immersed will seem to be bent in that
direction; whereas if the stick itself be bent, so that the end of it is
at B, it will, for the same reason, appear when in the water at A, and,
consequently, seem to be perfectly straight.”
The next step was to try and measure the _degree_ of refraction, or, in
other words, to find out how much a ray of light was bent out of its
usual course on passing through different transparent substances.
Accordingly, Humphry procured his old school-slate, and having managed,
with Kitty’s assistance, to mount this on a heavy pedestal, he described
upon the slate a circle with two diameters, each perpendicular to the
other; then having bored a hole in the centre, he fitted into it a large
cork; this had a straight tube afterwards let into it, so that the tube,
by means of the cork in the middle, could be moved freely round the
circle, turning, as it did, upon the centre of it. The whole apparatus
was then inserted in a vessel of water, so that the fluid reached exactly
to the level of the horizontal diameter—thus, without touching the end of
the tube.
[Illustration]
The youth then found, that when the tube A was directly perpendicular to
the surface of the water, a ray of light, on passing down it, suffered
no change at all in its direction; and that on placing a sixpence in the
water, exactly in a line with the perpendicular diameter, B, it could
be seen distinctly through the tube, so that the rays from the coin, on
quitting the water, proceeded in the same straight course as they had
pursued while passing through the fluid.
Hence it was evident that a ray of light, on _entering or quitting a
refracting surface in a PERPENDICULAR LINE, is not refracted or bent out
of its course_.
[Illustration]
It was different, however, when the tube was _slanted_, instead of being
placed _straight_ above the surface of the water, for when a ray of light
passed down it in that direction, the ray was found to be refracted, or
bent out of a straight course, as shown in the above diagram.
Now the precise direction of the refracted ray having been marked upon
the slate, the apparatus was removed from the water, and the distance
of the tube A from B, the vertical diameter, measured, as well as the
distance of the refracted ray, a, from the same perpendicular line,
_b_, and it was then found that the refracted ray, _a_, was as near as
possible 3 inches from the diameter, _b_, while the ray A, which passed
down the tube, was as much as 4 inches distant from B, the same line.
The tube A was then set at about 1⅓ inches from the diameter B, and the
apparatus replaced in the water, when it was discovered that a ray of
light, _a_, on passing down it, fell exactly at 1 inch from _b_, the
perpendicular line.
Several other positions of the tube were afterwards tried, and invariably
with the same result: let the tube be slanted as it might, the distance A
... B of the ray which passed down it, when compared with the distance
_a ... b_ of the refracted ray from the perpendicular line, was _always
ascertained to be in the same proportion_—viz. very nearly as 1⅓ to 1;
or, more correctly speaking, when A ... B, the sine of the angle at which
the ray entered the water, was 1⅓, then _a ... b_, the sine of the angle
formed by the refracted ray, was exactly 1.
On referring to his books, Humphry found that the number 1³³⁶⁄₁₀₀₀
constituted what was termed the _index of refraction_ for water.
On performing the same experiment with oil of turpentine the lad
discovered, that when the ray which passed down the tube was almost as
much as 1½ inch distant from the perpendicular, the refracted ray was 1
inch distant from the same line; whereas with sulphuret of carbon (though
with this the experiment was performed on a smaller scale), when the ray
passing down the tube was 1⅔ inch removed from the perpendicular—the
refracted ray was still only 1 inch away from the same line.
Humphry then consulted a table of the refractive power of different
bodies, and learned that hydrogen gas is the least refractive of all
known substances (that is to say, a ray of light passing through this gas
is bent down by it out of its previous course, less than by any other
known body), and that the diamond has very nearly the greatest refractive
power of all, while the refraction of the air in its ordinary state is
only 294 millionths greater than that of a _vacuum_.
This, however, is the refractive power of the atmosphere at its average
density near the earth’s surface. But we learn from the barometer that
the density of the air diminishes as we mount above the earth, and it
has been found by experiment that the refractive power of the atmosphere
decreases in proportion as it becomes more and more rarified; so that the
atmospheric refraction is greatest at the earth’s surface, and gradually
diminishes upwards, till the air becomes so rare as to be able to produce
scarcely any effect at all upon the rays of light.
[Illustration]
“In order to understand this,” said Humphry to his sister, “we must
consider the earth to be encased in a series of shells, as it were,
of atmosphere, each of a less density than the one below it, and,
consequently, of a less refractive power—thus.
“Let us suppose,” the lad continued, “the round ball in the centre here
to represent the earth, and the ring of atmosphere immediately next to it
to have a refractive power nearly 300 millionths greater than a vacuum;
and the refractive power of the ring, or shell of atmosphere immediately
above this, to be equal to only 200 millionths compared with the same
standard, while the power of the third ring decreases to 100 millionths,
and that of the outer one to 50 millionths, whereas beyond this no
refraction whatever exists, so that the rays moving through free space
will continue in the same line as that in which they are emitted from
the sun. What, then, will be the effect of such a series of atmospheric
shells upon a ray of light passing through them?”
To illustrate this Humphry drew the lines here shown through the
following diagram.
[Illustration]
“The orb S, outside the earth,” continued Humphry, “represents the sun
below the horizon, emitting, let us suppose, his rays in all directions.
These, passing through free space, proceed onwards in a perfectly
straight line; and one of them is here made to fall upon the outer ring
of the earth’s atmosphere, where it is slightly refracted, or bent down
out of its former course, so that, instead of continuing in the direction
of the dotted line _a_, it proceeds through the upper portion of our
atmosphere in the direction of the unbroken line, until it reaches a
part of the atmosphere of greater density—as in the second ring; when,
instead of going on in the direction of the second dotted line _b_, it
is again refracted, or bent down, in a greater degree than before. Then
travelling onwards, it reaches the third ring, where the atmosphere,
being of a still greater density, and, consequently, having a greater
refractive power, it is once more bent out of its course _c_, and that to
a still greater extent than before. In this manner it ultimately arrives
at a stratum of atmosphere which immediately envelopes the surface of the
earth, and which, having the greatest density of all, has the greatest
refractive power; so that the ray, instead of continuing in the direction
_d_, is here bent down more than ever, and finally reaches the eye in
the direction _e_, which is in a direct line with the orb _s_. The
consequence is, that as _every object is seen in the direction that the
ray has at the instant of arriving at the eye_, the sun itself appears to
be above the horizon when it is positively _below_ it, as at S; so that,
by the refraction of the atmosphere, the sun is seen by us before he
rises in the morning, and for a short time after he sets in the evening.”
Kitty was so astonished at the above conclusion, that, though she
understood the explanation, she told her brother that she could hardly
help doubting the fact; saying, “it was almost the same as asserting that
we could see a thing that was out of sight.”
Humphry undertook to prove to his sister, by experiment, that such a
result was quite possible by refraction.
Accordingly, he bade Kitty place the wash-hand basin upon the table,
and then having deposited a shilling at the bottom of it, he told the
girl herself to recede from the table until the edge of the basin
obscured the shilling from her sight; and when Kitty assured him that
she could no longer see the coin, he poured some water into the vessel,
and immediately the girl exclaimed—“Dear, dear, how odd! I can see the
shilling quite plainly now.”
“You perceive, then, Miss Kitty,” cried the boy, triumphantly, “it is
quite possible by refraction to see things that are out of sight, for the
ray from the shilling, A, on passing out of the water into the air, is
bent out of its course, and you behold it in the direction of the line in
which it enters your eye—thus, at _a_.
[Illustration]
“But far more wonderful things than this have been brought to pass by the
same means, Kitty,” said her brother, delighted to impart the knowledge
he had obtained from his books on this subject, “and these are what
are called _mirages_, or optical illusions, produced by extraordinary
refractions in the atmosphere. For instance, the cliffs on the French
coast are 50 miles distant from Hastings, on the coast of Sussex, and
they are actually hidden from the eye by the convexity of the earth; that
is to say, a straight line drawn from Hastings to Calais or Boulogne
would pass through the sea. A year or two ago, however, Mr. Latham, a
Fellow of the Royal Society, who was residing at Hastings, was surprised
to see a crowd of people running to the sea-side. Upon inquiry into the
cause of this, he was informed that the coast of France could be seen
by the naked eye. He immediately went down to the shore to witness so
singular a sight, and there discovered distinctly the French cliffs
extending for some leagues along the horizon, and so vividly that they
appeared to be only a few miles off. The sailors and fishermen, with whom
Mr. Latham walked along the water’s edge, could hardly, at first, be
persuaded of the reality of the appearance; but as the cliffs gradually
became more elevated, they were so convinced that they pointed out to
Mr. Latham the different places they had been accustomed to visit: such
as the bay and the windmill at Boulogne, St. Vallery, and other places
on the coast of Picardy, even as far as Dieppe, all the French shores
appearing to the English sailors as if they were sailing at a short
distance from them towards the harbours. With the aid of a telescope
the French fishing-boats were plainly seen at anchor, and the different
colours of the land upon the heights, together with the buildings, were
perfectly discernible. The day when this occurred is said to have been
extremely hot, without a breath of wind stirring, and the phenomenon
continued visible in the highest splendour until past 8 o’clock in the
evening, having been seen for three hours continuously.”
Some few years after the date of the above, a no-less-marvellous optical
illusion was seen by Professor Vince of Cambridge, in company with
another gentleman, at Ramsgate. Between this town and Dover there is a
hill, on the farther side of which stands Dover Castle, the summits of
whose four turrets can, in ordinary states of the atmosphere, be just
seen projecting above the brow, while the body of the castle itself is
usually hidden from view by the rising earth between it and Ramsgate. On
the evening of the 6th of August, 1806, however, when the air was very
still, and a little hazy, not only were the tops of the four towers of
the castle visible above the brow of the hill in the distance, but the
_whole_ of the castle itself appeared transferred to the side of the
hill next Ramsgate, as if it had been really built there, instead of
on the other side of the eminence. This phenomenon was so singular and
unexpected that Dr. Vince, at first sight, thought it an illusion. On
continuing his observations, however, he became satisfied that what he
saw was a real image of the castle. To assure himself that it was no
deception, he gave the telescope to a gentleman who was with him at the
time, and who also saw the same clear image of the entire castle, situate
on the near side of the hill, as the Doctor himself had witnessed. The
view of the castle was very strong, and well defined; and though the
rays from the farther side of the hill must, undoubtedly, have reached
the eye at the same time, still the strength of the image of the castle
itself so far obscured the background that it made no sensible impression
on the spectators. Dr. Vince continued to observe the image for about
20 minutes, during which time the appearance remained precisely the
same, but rain then came on, and he was prevented making any further
observations.
* * * * *
Humphry now began to study how, by means of extraordinary refraction,
inverted images of objects might be seen in the atmosphere.
[Illustration]
With this view he drew the subjoined diagram. “There, Kitty,” said the
lad, as he laid down his pencil and compasses, “the drawing represents
a ship below the horizon, and concealed from the eye of an observer by
the convexity of the earth. Well, if we suppose the refractive power of
the air at a little above the earth’s surface to be less than it is at
the surface itself, then the rays which proceed upward from the ship, and
which never could, in the ordinary state of the atmosphere, reach the eye
in the position here shown, will be refracted into curved lines, so that
they will cross one another; while the ray which came from the masthead,
instead of being uppermost, will change places with that coming from the
hull, and becoming the undermost of the two, will enter the eye in that
position: consequently, as every object is seen, as I said before, _in
the direction of the rays at the moment of their arriving at the eye,
without reference to their previous course_, an inverted image of the
ship will be perceived in the air, in the direction of the dotted lines,
and thus appear elevated above the horizon.”
Kitty said she could hardly follow the explanation, and wished to know
whether her brother could not devise some experiment in proof of it.
After a few moments’ consideration, Humphry requested his sister to
fetch him a square phial, and to make him a little clear syrup with some
lump-sugar and water. When this was prepared, the boy poured a small
quantity of the syrup into the phial, and upon this again he poured
very carefully an equal quantity of pure water, so that it might float
upon the syrup. Now the syrup, being a fluid of greater density than the
water, had a proportionately greater refractive power, and as the two
combined with each other they formed strata, having different refractive
powers, the same as those which had been supposed to exist in the
atmosphere at certain times, at a little distance above the surface of
the earth. Then having printed the word SYRUP upon a card, Humphry held
this behind the bottle, and the letters were seen _erect_ through the
stratum of syrup at the bottom, but _upside down_ at the part where the
syrup was mixing with the water, and _erect_ again through the layer of
water itself at the top.
After this the lad poured the same quantity of spirits of wine carefully
over the water itself, so that the spirit being lighter than this might
float above this again; and then having printed the word SPIRIT on the
upper part of the same card, the letters of this were seen _erect_
through the layer of water, but _topsy-turvy_ at the part where the
spirit and water were mingling, and in their _proper form_ through the
uppermost stratum of spirit itself.
Humphry afterwards produced the same effect by holding a heated iron
above a tumbler of water, so that the upper surface of it became warmed
while the lower remained cold, and the portion in the middle became
tepid. The heat, therefore, expanded, and so rendered rarer the upper
portions of the liquid; and as it forced its way downwards, produced
strata of different densities, and, consequently, of different refractive
powers. The result was, that on looking through the glass vessel three
images were seen as before; the upper and the lower ones—which arose from
the rays passing through the colder and the warmer strata—being erect,
and the middle one, or that which proceeded from the rays passing through
the portion in the middle, being inverted—as previously observed. The
same effect may be produced by looking along the side of a red-hot poker,
at an object 10 or 12 feet off, when an inverted image will be seen at
the distance of about ⅜ths of an inch from the line of the poker, and an
erect image within and without this.
The youth, having now demonstrated to his sister how it was possible to
produce three distinct images, and one of these inverted, from the same
object, when seen through strata of different densities, proceeded to
recount to Kitty stories of similar phenomena observed at sea. He told
her how Dr. Vince had seen at Ramsgate a ship whose top-masts only were
visible above the horizon, while over this, in the air, two images of the
complete ship were observed, the uppermost being _erect_, and the under
one _inverted_, with the pennant from the masthead of the inverted image
nearly touching that from the real ship, seen peeping above the horizon.
This was distinctly visible through the telescope; the sea appearing
between the two ships in the air, as here represented:
[Illustration]
“As the ship rose to the horizon,” said Humphry, “the upper image
gradually disappeared, and while this was going on the lower and inverted
image as gradually descended; but the mastheads of the real and the
spectral inverted ship never exactly touched. On the real ship becoming
entirely visible, the aërial images were found to have been perfect
representations of it, even though the whole of the vessel at the time
must have been concealed below the horizon.”
There is, however, it may here be added, a still more marvellous story
in connexion with this part of the subject, though it occurred at a more
recent date than that recounted by Humphry. During a voyage to the coast
of Greenland in the year 1822, Captain Scoresby, having seen an image
of an inverted ship in the air, directed his telescope to it, and was
able to discover that it was _his father’s vessel, which was at the time
below the horizon, and cruising in a neighbouring inlet_. “The image,”
says the captain, “was so well defined that I could distinguish by a
telescope every sail, the general ‘rig of the ship,’ and its particular
character, insomuch that I confidently pronounced it to be my father’s
ship the ‘FAME,’ which it afterwards proved to be; though, on comparing
notes with my father, I found that our relative position at the time gave
our distance from one another 30 miles, which is about 17 miles beyond
the horizon, and some leagues beyond the limit of direct vision. I was so
much struck by the peculiarity of the circumstance,” adds the captain,
“that I mentioned it to the officer of the watch, stating my full
conviction that the ‘Fame’ was then cruising in the neighbouring inlet.”
The same officer, while navigating the Greenland sea in 1820, saw the
images of several ships in the air. Some of these were double, and
inverted, while along with them there appeared aërial images of the ice,
in two strata; the highest of which had an altitude of a quarter of a
degree.
The representation of ships in the air by unequal refraction has, no
doubt, given rise in early time to the superstitions of phantom-ships,
which are always said to sail in the eye of the wind, and to plough
their way through the sea when there is not a breath of wind to ruffle
its surface. The story of the “Flying Dutchman” had, probably, a similar
origin; and the legend of the wizard beacon-keeper of the Isle of France,
who saw in the air the vessels bound to the island long before they were
visible in the horizon, doubtlessly arose from the man’s observation of
some such phenomena.
CHAPTER XV.
THE WONDERS OF THE REFRACTION OF LIGHT (_continued_).
Young Humphry now sought to discover the circumstances upon which the
formation of images, or pictorial representations of objects, depends.
“In the first place,” said he to his sister, “you must bear in mind that
all objects throw off from them, in all directions, rays of light, which
are of the _same colour_ as the objects themselves. The soldier’s coat
appears red to us, because it sends _red rays to the eye_; the fields are
green, because they emit rays of _green light_; and the summer clouds are
white, because the light they reflect to us is of _that colour_. Indeed
every flower, whatever may be its tint, is seen by us coloured as it is
merely because the rays of light proceeding from it are of the _same hue
as the flower itself appears in our eyes_.”
Kitty told Humphry that she could hardly comprehend this; saying, “that
the pattern of the paper on the wall was green and yellow, and yet, let
her look at it in whatever way she might, _she_ could see no green and
yellow rays coming from it.”
Her brother, however, assured her that, if _no rays_ from the paper
entered her pupil, she would not be able to see it at all; that is to
say, the wall would appear absolutely _black_ in her eyes; whereas, if
the rays it reflected were _colourless_, it would seem perfectly _white_
to her.
“In ancient times,” continued Humphry, “it was believed that the
eye itself had some peculiar power of emitting light, and thus of
distinguishing objects by its own agency; but now we know that no such
power resides in the organ of sight, the eye being almost _passive_
during vision, and seeing only those objects _which emit or reflect rays
of light to it_: for it is merely by such rays of light entering the
pupil, and forming a picture of the object at the back of the eye, that
we are enabled to distinguish the forms, as well as the colours, of the
things around us. So you must bear in mind, Kitty,” he added, “that the
figures and tints which you see _come to your eye, instead of your eye
sending out anything to them_; for, were it otherwise, you would be able
to see without any light at all.”
[Illustration: THE WONDERS OF THE REFRACTION OF LIGHT.—Page 371.]
Humphry then applied himself to prove, experimentally, that all objects
send off rays of light of the same colour as themselves.
Accordingly, he took an empty cigar-box, and having drilled a fine
pin-hole at one end of it, he bored another small hole in the lid—the
latter being for the purpose of looking through. Then, inside the box,
at the end opposite the pin-hole, he pasted a piece of white paper, and
placed a rose-tree at some short distance in front of the box itself, so
that the rays of light from the plant might pass through the pin-hole,
and be projected upon the white paper at the farther end of the dark box.
The arrangement being complete, he bade Kitty apply her eye to the hole
in the lid, and tell him what she saw.
“Dear, dear!” cried the astonished girl, “I declare if there isn’t a
little tiny picture of the rose-tree painted on the paper inside the box.
It isn’t very plain though, Humphry; but I can just see patches of red
for the roses, and patches of green for the leaves.”
“Yes,” said her brother, “and how could the colours come there, unless
the plant itself was giving off different tinted rays from its leaves and
flowers?”
“But, Humphry,” the girl exclaimed, as she continued gazing through the
hole, “I _do_ believe it’s upside down; for the patches of red that I
see are below the green, and in the rose-tree itself the flowers are up
above, and the leaves underneath. How very strange!”
The lad having had a peep himself at the image, proceeded to explain to
Kitty the reason of the picture appearing inverted.
With this view he drew the annexed diagram:
[Illustration]
“The rose-bush,” said he, “is sending off rays of light in all
directions. Well, let us suppose two of these rays to pass through the
pin-hole in front of the dark box, one coming from the top, and another
from the bottom of the plant. Now the consequence would be, that the two
rays, on passing through the pin-hole, would cross each other; so that
the one which was uppermost would be transferred to the lower part, and
that which was originally the bottom ray take the place of the top one.
Hence it is plain that the image, or picture, of the object must appear
upside down.”
Kitty was perfectly satisfied with the explanation, but, wishing to see
the picture of the rose more plainly upon the paper, she asked Humphry
whether he could not admit more light into the box.
The brother smiled at the simplicity of the request; but, to let the girl
see the result of enlarging the light-hole, he set to work to make a
greater aperture in front of the box. This done, he told Kitty once more
to peep through the hole in the lid.
“Why, what’s the matter with it, Humphry?” cried the sister; “I don’t see
anything at all now.”
Humphry smiled at his sister’s wonder, and proceeded to recount to her
the reason why the picture had become obliterated. He told her that when
the hole was a very small one, no two rays from different parts of the
object fell upon the same place; but that, now the hole was enlarged,
the rays that were being sent off in all directions from every part of
the rose-tree became confused with one another, so that those from the
green leaves fell upon the same part of the paper as those from the red
flowers, and the consequence was that the one colour obliterated the
other.
For the easier comprehension of this part of the subject, Humphry drew
the subjoined representation of the rays proceeding from one of the
flowers and one of the leaves; where it will be seen that, owing to the
enlargement of the aperture at the front of the box, the red ray from
the flower, and the green ray from the leaf, fall upon the same part of
the paper at the back; for as the leaf and the blossom each send off
rays in all directions, it is evident that—supposing only two of these,
for simplification’s sake, to pass through the aperture—one of the green
leaf-rays would fall upon the same spot with one of the red blossom-rays,
and one of the blossom-rays, on the other hand, become blended with one
of the leaf-rays.
[Illustration]
Kitty was not a little disappointed at the result which had followed the
enlargement of the light-hole; but Humphry, to console her, said that
it was possible, by means of a lens, to increase the light, and yet to
_prevent_ the rays from different points of the object falling upon the
same part of the paper at the end of the box.
For this purpose the lad placed a double convex lens, which he had
previously made out of two watch-glasses cemented together, into the
aperture at the front of the cigar-box, and then told his sister to look
once more through the hole in the lid.
Kitty no sooner applied her eye to the sight-hole than she cried aloud,
“O how beautiful! I declare it is much brighter than the first, and I can
now see every leaf and blossom perfectly made out. It’s the picture of a
little fairy rose—that it is. But tell me, Humphry,” said the girl, “how
could a little bit of rounded glass like that which you put into the box
produce so wonderful a change?”
“Well,” returned the brother, “you recollect I told you that every object
which we see is sending off rays of light from every part of it in all
directions. In the first case, when there was a mere pin-hole in front of
the box, the aperture was so small that only _one_ ray from each point
of the rose-tree passed through it, and, therefore, the image was so dim
you could scarcely make it out. With the convex lens, however, as many
more rays enter the box from every part of the plant, as the lens itself
is bigger than the small hole which we had in the box at first; and the
reason, again, why these rays are prevented from becoming confused one
with the other, and so obliterating the picture—as was the case when
we enlarged the light-hole in front of the box, without inserting any
lens in it—is because they are all duly refracted by the lens, so that
they severally fall in their proper places. But you will understand this
better by a drawing.” And, so saying, Humphry prepared the illustration
below given:
[Illustration]
“Here you see there are three rays,” continued the lad, “drawn from the
top, bottom, and centre of the object; three only are given for the sake
of simplicity, though every point of the plant is sending light from it
in the same manner as here indicated. Well, Kitty, the rays from the
flower at the top of the tree fall upon every part of the glass, and,
by the laws of refraction, are made to come together at a point on the
other side of it. Again, the rays from the leaves at the bottom of the
tree fall upon every part of the lens, and are so refracted that they all
meet at another point on the other side of the glass; while those from
the rosebud in the centre are likewise blended into a focus at the same
distance behind the lens. But you will perceive, that the rays which come
from the upper part of the object fall at the lower part of the image;
and those, on the other hand, which proceed from the top, fall at the
bottom. This is because the rays from these parts cross one another in
the centre of the lens, while those which are sent off from the rosebud
in the middle suffer no change of position, because _they_ proceed—as you
observe by the dotted lines in the drawing—directly through the glass,
rather than traversing it obliquely as the others do.”
“Oh, thank you, Humphry,” said Kitty; “I can make it out well now. The
image from the lens is so much brighter because it not only allows more
light to pass through the aperture, but prevents the rays from the
different parts of the object mingling one with the other. But, Humphry,”
ejaculated the girl, as a new thought struck her, “the image, as you call
it, is much smaller than the rose-tree itself: why is that?”
“To make you understand this, Kitty,” answered the boy, “I will place the
tree farther from the lens, and you shall tell me the effect.”
Humphry had no sooner removed the plant to a greater distance, than the
girl cried, “Oh, it’s much smaller than ever now!”
“And now that I bring it nearer the box, what do you see?” inquired the
youth.
“Why it seems to grow and grow, Humphry,” replied Kitty, as she continued
peeping through the hole in the lid, “so that I fancy it would get as big
as the tree itself. The picture, though, is not nearly so bright.”
“No,” returned her brother; “that is because it gets out of focus.
Now look you here, Kitty; I will do another drawing, to enable you to
comprehend how the size of the image depends upon the distance of the
object from the lens.
[Illustration]
“You must bear in mind,” proceeded Humphry, on the completion of the
diagram, “that _the rays which pass through the centre of a lens
never change their direction_. Well, I have drawn here, you see, one
ray from the tip of the arrow, and one from the bottom; and as these
rays necessarily form the extremes of the image, and so regulate its
size, you will readily comprehend that, when the object is, as here
represented, 4 yards, or feet—or, indeed, 4 measures of any kind—in front
of the lens, and the image falls, also, at 4 such measures behind it, as
at the arrow 4, the image itself must be exactly of the same size as the
object. If, however, the image fell at half the distance behind the lens
which the object was from the front of it, then the picture would be only
half the size of the body producing it—as here, at the arrow 2; whereas
if the image was at twice the distance of the object from the lens, as at
the arrow 8, then it would be exactly twice the size of it. Consequently,
the dimensions of the image produced by a lens bear always the same
proportion to the object _as the distance of the object from the lens
does to that of the image_: that is to say, if the object be 3 times as
far from the lens as the image is, then the image will be 3 times smaller
than the object itself, and _vice versâ_, if the object be 3 times nearer
the lens than the image, then the image will be 3 times larger than the
object.”
Kitty having informed her brother that she thoroughly understood the
matter now, Humphry went on to tell her that, in order to produce an
image, it was necessary that the picture should be received upon some
opaque or in-transparent substance, otherwise the rays of light would
pass _through_ the substance itself without being reflected from it or
sent back to the eye.
“The opaque body,” continued the youth, “upon which the image is thrown,
should be of a white colour, for this reflects the greatest amount of
light.”
To elucidate this part of the subject, Humphry removed the wooden end of
the cigar-box that he had previously employed, and substituted a piece of
ground glass in its stead; when Kitty, on placing her eye behind the box,
saw the picture of the rose-tree once more portrayed upon it.
“Now,” added her brother, “if I smear the ground glass over with any
grease, or even water, so as to increase its transparency, you will see
that the image immediately disappears.”
This done, Humphry explained to Kitty that the image might be received
upon smoke, or, indeed, any vapour that consisted of a number of opaque
white particles, and then he recounted to her the story of the “spectre
of the Brocken.”
“The Brocken,” said he, “is the name given to the loftiest of the Hartz
mountains, which is a picturesque chain of hills situate in the kingdom
of Hanover. The highest of these is elevated 3300 feet above the sea,
and commands the view of a plain upwards of 200 miles in extent. This
spot has been the seat of the marvellous from the earliest periods. One
of the accounts given of the ‘Spectre of the Brocken’ is that of M.
Haue. After having been on the summit of the mountain no less than thirty
times, he had, at last, the good fortune of witnessing the object of
his curiosity. The sun rose at about 4 o’clock in the morning through a
serene atmosphere. In the south-west, towards Achtermannshohe, a brisk
wind carried before it the transparent vapours which had not yet been
condensed into thick, heavy clouds. About a quarter past 4 M. Haue looked
round to see whether the atmosphere would afford him a free prospect
towards the south-west, when he observed, at a very great distance
towards Achtermannshohe, a human figure, of a monstrous size. At this
moment a violent gust of wind ensued, and M. Haue suddenly raised his
hand to his head, to prevent his hat being carried away, when, to his
great astonishment, he beheld the colossal figure in the distance do
the same. He immediately made another movement by bending his body, and
this action, too, was instantly repeated by the spectral figure. There
was now no doubt that what was termed the ‘Spectre of the Brocken’ was
an enormous image of the spectator himself seen in the distance. M. Haue
was desirous of making other experiments, but the figure disappeared. He
remained, however, in the same position, expecting its return; and in a
few minutes it again made its appearance on the Achtermannshohe, when it
once more mimicked his gestures as before. M. Haue then called another
person to him, and having both taken the same position which he himself
had previously occupied, they looked towards the Achtermannshohe, but saw
nothing. In a very short space of time, however, two colossal figures
were formed above the eminence, and, after bending their bodies, and
imitating the gestures of the spectators, they disappeared. M. Haue and
his companion, nevertheless, retained their position, and kept their eyes
still fixed upon the same spot, when the two gigantic spectres were again
beheld by them, but this time they were joined by a third, and, strange
to say, every movement they made was imitated by _all the three figures_.
The effect, however, varied in its intensity, being sometimes weak and
faint, and sometimes strong and well-defined.
[Illustration]
“These figures were merely shadows of the observers, projected on dense
vapour, or thin fleecy clouds, which have the power of reflecting much
light. They are seen most frequently at sunrise, because at that time the
vapours and clouds necessary for their production are usually generated;
and they can be perceived _only_ when the sun is throwing his beams
horizontally, because the shadow of the observer would be otherwise
projected up in the air, or down upon the ground. It is very probable
that the third figure observed by M. Haue was formed by a duplication of
one of the others, produced by unequal refraction; though M. Haue himself
does not state which of the two figures was doubled.”
It may here be added, that another story of the same kind is told by Sir
David Brewster. “A young lady had ascended to the top of the Mynydd,
a steep hill, about 500 feet above the valley of New Rednor, in South
Wales. The sun was bright and hot (it being about 2 o’clock in the day).
Having picked some flowers on the top of the hill, the girl descended a
little way, to a spot from which she could see the road and the carriage
with her companions whom she had left in it below. After waving the scarf
which she held in her hand to her friends, she suddenly perceived, upon
turning round, a figure standing a few yards from her upon a wet spot,
_from which a little thin mist was rising_. The figure stood exactly
facing her, and wavered a little; but she did not discover it to be her
own image, till she observed that, like herself, it held a scarf and
a bunch of flowers in one hand. The dress and flowers were precisely
similar to her own, and the colours so vivid that she could even trace
her own features in the image. The effect was the same as if she had been
before a looking-glass—when she moved her hand, the figure did the same.
The two friends in the carriage saw the image of the young lady, and
asked her, when she joined them, what companion she had had on the hill.
There can be no doubt,” adds Sir David Brewster, “that the figure was a
reflexion of the young lady, produced by the thin mist rising from the
damp ground; for it may be proved, by experiment, that when the particles
of vapour are sufficiently small, they reflect light as distinctly as a
surface of glass.”
* * * * *
From the production of images, Humphry passed to the consideration of the
circumstances by which lenses appear to _increase the size of objects_,
and so to make them seem as if _brought nearer to us_.
“When a shilling,” said Humphry, “is at the distance of 6 or 8 inches
from the eye, we can read the inscription round it with perfect
distinctness. At the distance of 3 yards, however, we can no longer
make out the inscription, but see only the king’s head upon it. Again,
at the distance of only 20 or 30 yards, we lose sight of the head, and
can then just distinguish that it is a round body; whilst, when placed
at about 100 yards from us, the coin is scarcely visible. The reason of
this is, that the shilling decreases in size the farther it is removed
from us, for we then see it _under a smaller angle_, as it is termed; and
it is found that the smallest angle under which an object can be seen,
is, upon an average for different sights, the 60th part of a degree, or
_one minute_ in space; so that when an object is removed from the eye
about 3000 times its own diameter, it will only just be distinguishable.
Consequently, the greatest distance at which we can behold an object like
a shilling, of an inch in diameter, is 3000 inches, or 250 feet.
[Illustration]
“Another drawing,” added Humphry, “will enable you, Kitty, readily to
comprehend how an object appears to diminish in size, according as it
becomes more and more distant from us, and so gets to be seen under a
smaller angle.”
“There,” continued the boy, “the first arrow is seen under an angle of
120°,[55] whereas the angle under which the second arrow is regarded is
only 60°. Consequently, though the objects are the same in size, the
one will appear only ½ the length of the other. The third arrow, again,
being seen under an angle 4 times smaller, will seem to be only ¼th the
size of the first; whilst the fourth arrow, for the same reason, will
look as if it were only ⅛th the height of the one next the eye; and the
farthest arrow of all but ¹⁄₁₂th as large as the nearest. Moreover, if
we suppose another arrow still to be so far removed that the angle under
which it is seen dwindles down to the 60th part of a degree, or 1′, as it
is called, this will then appear so reduced in size as to be only just
distinguishable to us.
“Well, Kitty,” the youth went on, “you now understand that an object
appears to diminish in size the _farther_ it is removed from us, merely
because it is seen under a _lesser_ angle; and, consequently, an object
must seem to us, on the other hand, to _increase_ in size when the image
of it is brought _nearer_ to the eye, and so gets to be viewed under
a _greater_ angle. This, then, is all that lenses _really_ do when
they appear to magnify objects: that is to say, they do not absolutely
_increase the dimensions_ of the bodies under view, but merely _bring
their images nearer_ to the eye, and so enable us to see them under a
_larger angle_. You remember I told you that, with a shilling, we can
just see the king’s head upon it at the distance of about 10 feet from
the eye. Now, when the coin is at that distance, if a convex lens, having
a focus 2½ feet long, be placed midway between the shilling and the eye,
the lens will, of course, be 5 feet from the eye and 5 from the shilling;
so that, in this case, it is plain, from what I before explained to you
about the size of images,[56] that the image of the shilling seen behind
the lens will be exactly of the same dimensions as the shilling itself in
front of it. The object, therefore, will not have been directly magnified
by the lens. The image, however, will be thus brought so near to the eye,
that the coin may be seen by us at the distance of 6 inches, instead of
10 feet; and, consequently, being viewed under a proportionately larger
angle, the shilling will seem to be magnified as many times as 10 feet is
greater than 6 inches; or, in other words, it will be made to appear 20
times larger in our eyes. Hence the shilling will have been, apparently,
magnified 20 times, merely by bringing the image of it 20 times nearer
the eye—thus. Whereupon the boy proceeded to delineate the following
diagram; in which the dotted lines from the object A represent the angle
that the shilling itself would be viewed under without the lens, C, by
the eye at B, while the dotted lines from the image D show the much
larger angle that such image would be seen under with the lens.”
[Illustration]
Humphry then prepared an experiment illustrative of the apparent
magnifying of objects by lenses, when their images are brought nearer
to the eye. For this purpose he got Kitty to bore a hole in the shutter
large enough to allow a lens to be inserted in it. Then fixing the glass
in the aperture, he bade his sister close the shutters, and place her eye
at about 2½ feet from the lens, for such he knew to be the length of its
focus.
“How beautiful!” cried the girl, as she gazed through the transparent
circle. “I see a tiny image of Madern Church; and so close, too, that I
could fancy it was in the room here.”
“Well,” said Humphry, “the church itself, you know, is about 1½ mile
distant, or, let us say, 7500 feet, and the focus of the lens is 2½
feet; consequently it follows, from what I have before told you, that
the image of the church you see is 3000 times smaller than the church
itself, for 7500 / 2½ = 3000. Nevertheless, if we could copy the image of
the church upon a piece of paper—or, what would be better still, fix it
upon a sheet of glass, we should find that, _on holding it just as far
from the eye as it is now from the lens_, the tiny image that you now
see would exactly cover every part of the distant object, and so appear
precisely of the same size as the church itself—in this manner.
[Illustration]
“Let us suppose the large dart, marked A here,” the lad continued, as
he drew the plan upon paper, “to be the height of the church, and the
smaller dart, _a_, on the other side of the lens, C, to be the size of
the image that we see. Well, if you were to place your eye where the lens
now is, and the image just as _far in front of the glass as it now stands
behind it_, it is plain, by the dotted eye and arrow at _a′_, that the
one would exactly cover the other.
“Hence it is evident,” added Humphry, “that the image which you see
behind the lens is really of the same size as the distant object
_appears_ to be, even though, as in the case of Madern Church, the
image is no less than 3000 times smaller than the object itself _really
is_. But when you look through a glass, Kitty, the image of the distant
object is only about 6 inches from your eyes; so that, though it is of
the same size as the object itself _appears_ to be, you are viewing it
at a shorter distance than the length of the focus of the lens; and,
therefore, owing to your regarding it under a greater angle, it seems to
be magnified. Now, as I told you, the focus of the lens we employed was
2½ feet, or 30 inches long; and, supposing your eye to have been where
the lens was, and the image transferred to the other side of the lens (as
indicated by the dotted eye and arrow marked _a′_), the image would have
seemed exactly the same size as the object itself, provided it had been
placed at a distance of 30 inches in front of you. If, however, the image
had been placed only 6 inches away from your eye, it is plain that you
would have been viewing it 5 times closer than 2½ feet, and this would
be the same as if the dotted arrow had been shifted from _a′_ to _x_;
consequently, it would then have looked to you 5 times larger than it
really was, because you were regarding it under an angle 5 times greater
than its own.
“The result which we come to is, therefore,” concluded the youth, “that
the _magnifying power of a lens is always equal to its focal length,
divided by the distance at which the eye regards the image_. The latter,
in your case, Kitty, was about 6 inches; so that the lens, having its
focus 30 inches off, the magnifying power of it is arrived at in this
manner: 30 / 6 = 5.”
Kitty asked whether it was possible to magnify an object any more than
that; when Humphry told her that, had the focus of the lens he employed
been longer, its magnifying power would have been greater; “as for
instance,” said he, “if the length of the focus had been 5 feet, instead
of 2½, it would have magnified 10 times instead of only 5, for 60 inches
/ 6 inches = 10. So, again, had the focus of the lens been 10 feet, its
magnifying power would have been doubled again, for 120 inches / 6 inches
= 20. But,” continued the boy, “the magnifying power might be increased
in another way—namely, by bringing the eye nearer to the image. As yet
we have estimated the distance at which the eye views the image produced
by the lens at 6 inches, because that is the length at which we see near
objects distinctly. Hold your finger before your eye, Kitty, and you will
see that when you bring it very close you can scarcely distinguish it.
With a lens, however, having a short focus, you would be able to see the
finger much nearer than naturally; and then, for the reason I have before
given you, it would appear to be as much magnified as the distance at
which you beheld it distinctly _with_ the lens was less than 6 inches,
which is the distance at which you beheld it distinctly _without_ the
lens. In my cupboard you will find a burning-glass, and that has a focus
of only 2 inches. Do you get it, Kitty, and look at your finger through
it.”
The girl did as she was bidden, and immediately cried, “Oh, Humphry! what
a horribly ugly, coarse, thick-looking thing it is! Why, I declare my
skin looks like an elephant’s hide through it; and I can see every line
in it, like the veins on a leaf!”
“Yes,” returned her brother, “that is because you are now looking at your
finger 3 times nearer than you could see it distinctly without the lens,
and, consequently, you behold it under a proportionately larger angle; so
that it appears to you to be 3 times magnified—for 6 inches / 2 inches =
3. Let us then apply the same principle to the image of Madern Church as
seen through the lens in the shutter, and which, you remember, appeared
to be magnified 5 times, because you saw it at 6 inches from your eye
instead of 30 inches, which was the focal length of the lens. But now, by
means of this burning-glass held near your eye, do you look at the image
once more, and tell me, Kitty, what you see.”
The shutters were accordingly closed again, and the girl proceeded to
take another peep at the distant church through the two lenses.
“Oh, Humphry!” she cried, “I see it much plainer than ever; and it is, as
you say, a great deal bigger, too.”
“Of course it is,” returned the brother, “for the length of the focus
of the burning-glass is, as I said, 2 inches; so that you now see the
image of the church at that distance from your eye, instead of 6 inches,
as before. The image, therefore, appears to be magnified 5 times by the
first lens, and 3 times by the second, or 5 × 3 = 15 times in all; and
the reason of its appearing to be that number of times larger to you,
is simply because you are looking at it at 15 times a shorter distance
than the focal length of the lens in the shutter, which is called the
‘object-glass,’ and so seeing it by means of the other lens, which is
called the ‘eye-glass,’ _under 15 times a greater angle than you behold
the object itself with your naked eye_.”
“I understand it now perfectly, Humphry, thank you,” said the sister,
pleased with the explanation. “And are the telescopes that the sailors
use made upon the same principle?”
“Precisely so, Kitty,” responded the brother. “And in order to find out
the magnifying power of any of these, we have merely to _divide the
length of the focus of the object-glass by that of the eye-glass, and
the quotient will tell us how many times the objects are enlarged by
them_; whilst in order to make a telescope for ourselves, we have merely
to procure a lens of a long focus—say 12 inches, and one of a short
focus—say 2 inches, and then to set these in a tube at the length of the
two foci, or 12 + 2 = 14 inches apart. This tube, however, should be a
sliding one, so as to admit of the distance between the two lenses being
increased according as the objects viewed are nearer at hand; for I told
you before, you remember, that the _nearer_ the object the _farther_ is
the image from the lens, and _vice versâ_, the more _distant_ the object,
the _shorter_ the focus of the glass becomes.”[57]
Now that Kitty understood the principle upon which telescopes were
constructed, she begged her brother to promise to construct one as soon
as he was well; and Humphry having consented, the two then passed on to
the consideration of the principle of the _microscope_.
“I have already told you,” said Humphry, on entering upon the subject,
“that the nearer an object comes to us, the larger it appears. But, as
you saw, when you held your finger close before your eye, it grew so
indistinct and confused, that the form of it was almost as obscure as if
it had been at a great distance from you. Now this effect is produced
by the greater divergence of the rays of light, whenever an object
is brought nearer to us; and when the divergence is very great, the
crystalline lens within the eye has not power to collect the rays into a
focus on the retina at the back of the eyeball. You will understand how
the rays come to diverge more and more the nearer an object approaches to
us, by the following illustration.
[Illustration]
“There, we will suppose the eye to be looking at some very minute object,
like a speck of the dust from a butterfly’s wing, at the distance of 6
inches, 4 inches, and 2 inches. Well, at 6 inches, the rays of light
given off by it, you perceive, diverge but slightly in comparison with
the angle at which they enter the eye at 2 inches. Consequently, the
image produced within the eye itself would, in the latter case, be so dim
that we should be almost unable to distinguish it. In order, however, to
look at a very small object, we must bring it as close as possible to the
eye; so that, to enable us to see it _distinctly_ at a short distance,
we must find out some means of _decreasing the divergence_ of the rays
of light from near objects—or, what would be better still, of making the
rays enter the pupil in _parallel_ lines.
“Now I showed you, a short while back, that a convex lens causes the rays
of light from objects placed in its focus to pass out on the other side
of the glass parallel to each other. Consequently you perceive that, by
means of a double convex glass, we can see objects distinctly when held
at ½ an inch—or even the ⅒th of an inch—from our eye, provided such be
the focal length of the lens employed; and thus we shall, for the reasons
before explained, obtain a _magnifying power which will be equal to the
distance at which the naked eye can see minute objects distinctly divided
by the focal length of the lens employed_. For example, the distance of
distinct vision for very minute objects may be taken at 5 inches, so
that if we make use of a lens having a focus of 1 inch, the magnifying
power will be equal to 5 inches divided by 1; that is to say, an object
viewed with such a glass will appear to have its length and breadth
increased five-fold; so that its _length_ being magnified 5 times, and
its _breadth_ 5 times also, its _entire surface_ will be increased as
much as 25 times, or 5 × 5. If, however, we employ a lens having a focus
of only ⅒th of an inch, the _linear_ magnifying power will be equal to
5 inches divided by ⅒ (or ⁵⁰⁄₁), that is to say, to 50-fold; while the
_superficial_ magnifying power will amount to 50 × 50, or 2500-fold; and
if, again,” went on the lad, “the lens employed have a focus of only
¹⁄₁₀₀th of an inch, then the _linear_ magnifying power will be equal to
5 inches divided by ¹⁄₁₀₀th (or ⁵⁰⁰⁄₁),—that is to say, to 500, and the
_superficial_ magnifying power to 500 × 500, or 250,000.
“A lens of a very short focus,” added Humphry, “constitutes what is
termed the _single microscope_. For this purpose the lens is usually
made spherical,—as a sphere, or round ball of glass, has its focus at
a distance from its centre equal to 1½ its own _radius_; so that if we
had a small glass ball, of 1 inch in diameter, the focus of such a lens
would fall at ¾ths of an inch from the centre of the ball itself; whereas
if the ball was ¼th of an inch in diameter, it would have the focus at
³⁄₁₆ths of an inch from its centre: so that you will readily comprehend,
Kitty, how tiny a sphere must be used in order to give great magnifying
power with a single microscope. To have a lens of ⅒th of an inch focus
that will, consequently, be able to magnify an object 50 times in length
and breadth, it would require the glass sphere to be only about ¹³⁄₁₀₀ths
of an inch in diameter. The perfect execution of such lenses requires
considerable skill in the grinding and polishing, therefore other means
of constructing them have been desired. One simple method of forming a
microscopic lens consists in drawing out (by means of a spirit-lamp) a
thin strip of window-glass into threads, and holding the end of one of
such threads in the flame until it runs into a globule. The globule is
then cut off and set in a small aperture, in such a manner that none
of the rays may pass through that part of the tiny ball where it was
originally united to the thread. Another process,” continued the youth,
“consists in taking up some fine-pounded glass on the wetted point of a
needle, and then melting it by a spirit-lamp into a globule, after which
the globule is removed, and once more taken up, by the wetted point of
the needle, on its round side, when it is again inserted in the flame,
until it becomes a perfect sphere. Moreover, drops of water, as well as
drops of oil or varnish, have been used for microscopic lenses. These are
placed on a small piece of plate glass, and have considerable magnifying
powers. Further, the lenses from the eyes of fish have been used for the
same purpose; but, in this case, it is necessary to look through the lens
in the direction of its axis—or, in other words, in the same manner as
the fish did.[58]
“A good _extempore_ microscope may be formed out of two test-tubes filled
with water, and placed one across the other, like the algebraic sign +.”
To please his sister, Humphry had his spirit-lamp lighted, and proceeded
to form some little globules of glass in the flame, in the manner before
explained; and then, having set these upon a plate of brass, he showed
the delighted girl how wonderfully objects were magnified by them; and
afterwards he went on to explain to her how it was possible to increase
the microscopic power of lenses, even without diminishing their size.
“Suppose,” said he, “that we have a lens of ½ an inch focus, and which
would, therefore, magnify the diameter of objects 10 times; and then
suppose that, instead of looking directly at the image, we place another
lens of a short focus—say 1 inch—between it and our eye, and so view the
image through the second lens. Well, this second lens would, for the
reasons before given, magnify the object 5 times more; so that it would
thus be made to appear 50 times bigger in all, the image being rendered
ten times greater by the first lens, and that image, again, 5 times
greater by the second. This constitutes what is termed the _compound
microscope_; and, by means of this instrument, objects may be magnified
to almost any extent.”
Humphry having now thoroughly made out to himself, as well as his sister,
the principle upon which the power of the _microscope_ and _telescope_
depends, concluded the subject by drawing the following diagrams,
illustrative of the opposite action of the two instruments:
[Illustration: _Telescopic arrangement._]
[Illustration: _Microscopic arrangement._]
“There,” said the boy, “in the one case, as in the preceding diagram, the
_object_ is at a considerable _distance_ from the lens, and the _image
near_ it; while in the other, as in the above diagram, the _object_ is
_near_ the lens, and the image at a considerable _distance_ from it.
Now if we suppose, Kitty, the object to be 10,000 feet in front of the
first lens, and the image 10 feet behind it, it follows that the image,
in this case, would be 1000 times smaller than the object itself; and
if we suppose, on the other hand, the object to be ¹⁄₁₀₀th of an inch
in front of the second lens, and the image to be 10 inches behind it,
then the image, in that case, would be 1000 times larger than the object
itself. Let us now imagine another lens to be placed before each of the
images—as here shown—so that the eye may view them at a shorter distance
than it could see them distinctly without any such aid; and let us say,
again, that the focal length of this second lens is, in both cases, 1
inch. Well, then, we should be regarding the image in the upper diagram
at a distance of 1 inch instead of 10 feet, which is the focal length of
the object-glass, and so bringing it 100 times nearer to our eye; the
consequence would be, that it would appear to us to be 100 times larger
than it would at the distance of 10,000 feet, so that this would be the
magnifying power of the instrument, which, as I said before, is always
_equal to the focal length of the object-glass, divided by that of the
eye-glass_. Such, then, constitutes the arrangement of the astronomical
telescope. In the compound microscope, however,” added Humphry, “the
magnifying power is estimated by _multiplying_ instead of _dividing_
the power of the object-glass by that of the eye-glass; so that, as we
supposed the first lens in the lower diagram to magnify the object 1000
times, and the second lens now enables us to view the image distinctly
at 5 times nearer the eye than we could without it, the gross magnifying
power, therefore, must amount to no less than 1000 × 5, or 5000 times.”
[Illustration: _Astronomical Telescope._]
[Illustration: _Compound Microscope._]
The youth then went on to explain to his sister that the same relation
which exists between the telescope and the microscope, also holds
good between the _camera-obscura_ and the _magic-lantern_. In the
camera-obscura, for instance, the object, as in the telescope, is at a
considerable distance in front of the object-glass, and the image at a
short distance behind it; whereas in the magic-lantern, the object, as in
the microscope, is at a short distance in front of the object-glass, and
the image at a considerable distance behind it. In the camera, therefore,
the image is as much diminished as it is _nearer_ the lens than the
object; whilst in the magic-lantern the image is as much magnified as it
is _farther_ from the lens than the object.
The annexed drawing will illustrate the action of these two instruments
clearer than words can describe them:
[Illustration]
The reader has only to suppose the image produced by the camera—a
portrait, let us say—to be fixed upon glass (by the “collodion process”
of photography, an invention since Davy’s time), and this image to be
made to serve as the object (or, in plainer language, the slide) of
the magic-lantern, in order to comprehend how the object in the one
instrument may be made the image in the other, and _vice versâ_, the
image of the first the object of the second.
CHAPTER XVI.
THE WONDERS OF THE REFLEXION OF LIGHT.
Hitherto Humphry had considered only the laws which regulate the
transmission of light through transparent bodies. This constitutes the
branch of the subject called _dioptrics_ (from δια, through, and οπτομαι,
to see). The other branch, termed _catoptrics_ (from κατα, from, or
against, and οπτομαι, to see), deals with the laws of light when it is
reflected or thrown back from the surface of any body _against_ which it
falls. Accordingly the lad passed, in due order, from the _transmission_
to the _reflexion_ of the luminous rays.
To explain this part of the subject the youth first procured a piece of
an old looking-glass, and having got Kitty to close the shutters once
more, he placed the looking-glass upon the ground, so that the ray might
fall just in the middle of it; when, as the room was thoroughly darkened,
it was easy to observe the inclination, or angle, at which the light fell
on the reflector, as well as to perceive the course it took afterwards.
“Why, I declare,” cried Kitty, as she looked at the bright streak, “it
goes down and then up again; and I can see the beam slanting away from
the glass on each side, for all the world like a big letter V!”
“Yes,” returned Humphry, “you see the course of the beam is stopped by
the looking-glass, and instead of going through it, the thread of light
that streams _down_ from the hole in the shutter no sooner falls on the
mirror than it is driven up from it, precisely in the same manner as if
the luminous particles were a series of hard balls projected against the
glass, and so made to bound off from its surface.”
The youth then called for his arc, and proceeded to measure the angle at
which the light fell upon the glass, and also the angle at which it was
reflected from it—thus:
[Illustration]
“Do you see, Kitty,” he cried, as the eager girl stooped down beside her
brother, “the ray that slants down from the shutter falls upon the glass
at an angle of 45°, and this is what is called the angle of _incidence_;
while the ray which slants upwards from it is reflected from the glass
at 45° also, and this is what is called the angle of _reflexion_: so
that, you perceive, _the one is exactly equal to the other, and this
constitutes what is termed the law of reflexion_. For, no matter what the
form of the mirror itself, or in what direction a ray of light falls upon
it, it is always reflected or driven back from the surface at precisely
the same angle as it strikes upon it. As you say, the two rays form a
kind of letter V, and one prong of the letter always slants just as much
as the other.”
“But suppose the surface of the glass, Humphry, was to be hollowed out
like a bowl, would it do so then?” inquired the girl.
“Certainly,” was the reply; “and if the rays falling upon it then were
parallel one to the other, you would find, upon drawing the figure on
paper, that they would all meet together at one point in front of the
glass, which would, consequently, be the focus—_the distance of such
focus being equal to half the radius, or semi-diameter, of the curvature
of the mirror itself_. Give me the compasses and open the shutters,
Kitty, and you shall soon see what I mean.” In a few minutes the
following diagram was described:
[Illustration: _Concave Mirror._
_Convex Mirror._]
“There!” cried Humphry, as he put the last touch to the drawing, “the
two curved lines represent the surfaces of a _convex_ and a _concave_
mirror, the curvatures of which form portions of a circle, having its
centre at the point where the unbroken lines meet. Now, these unbroken
lines, being drawn in each case from the centre to the surface of the
mirror itself, are exactly perpendicular to the points where the rays of
light fall; and if you measure with the arc the angle which the dotted
parallel lines form on one side of the unbroken ones, you will find that
they are, in every case, here equal to that which the dotted slanting
lines form on the other side of the same perpendicular. Consequently,
you perceive that, by a concave mirror, the rays are made to _converge_
to a focus in front of the mirror itself; whereas by a convex mirror
the rays are made to _diverge_, as if they came from a focus behind the
mirror itself. Now this, you remember, is precisely the same as what
takes place with concave and convex lenses; for a concave lens has its
focus in front of it, like a concave mirror, but, owing to the rays
passing _through_ the lens in the one case, and being driven back _from_
the mirror in the other, they are made by the lens to _diverge_ and by
the mirror to _converge_. So that, while the concave lens _diminishes_
the apparent size of objects, the concave mirror _magnifies_ them. The
same thing holds good,” continued the boy, “with a convex lens and a
convex mirror; they both have their focus behind them, but the rays, in
passing _through_ the lens, _converge_ to a point, whereas, being driven
_back_ from the mirror, they _diverge_; and so, while the convex lens
_magnifies_, the mirror _diminishes_ the apparent size of objects.”
Next, Humphry directed his sister to place herself alongside the
looking-glass over the mantel-piece, while he did the same facing her,
and in such a manner that neither could see their own figure reflected in
the mirror. It would then be found, he said, that they would each behold
the other, and at exactly the same distance behind the glass as they were
in front of it. In this manner:
[Illustration]
“Now the reason why I see you,” said the lad, “and you see me in another
place than we really occupy, is, because the rays reflected by the glass
enter our eyes in that direction; and, as I told you before, _an object
is always seen by us in the direction which the ray has at the moment
of reaching the eye, without regard to what may have been its course
previously_. Your image, of course, Kitty, is _on_ the surface of the
glass itself, and not _behind_ it, as it appears to you to be; and what,
I dare say, will sound stranger to you, is, that the image itself upon
the glass is exactly half the size that it seems to be behind it: for
since, when you look at yourself in the glass, your image appears to be
just as far at the back as you are standing in front of the mirror, it
is evident that the mirror itself must be half-way between you and your
apparent image; so that it will cut in half the cone of rays which enter
your eye from the surface of the looking-glass.
[Illustration]
“There is a picture,” continued Humphry, as he put the drawing before his
sister, “of a person looking at himself in the glass; and you will see,
by the rays from his chin and forehead, which are reflected in a point to
the eye, that a vertical line A...B, at the surface of the glass, must be
exactly equal to half the length of the image, since the image and the
eye of the spectator are always at equal distances from the glass itself.
But the image, which _appears_ to be behind the glass, is seen under the
same angle as the image, which is _really_ on the surface of it; and so,
for the reasons I before gave you, when speaking of the apparent size of
objects in general, the one behind the glass appearing to be at twice the
distance of the other, naturally seems to be twice as large as the image
on the surface really is.
“I have already shown you, Kitty,” went on the youth, after a pause,
“that if two persons stand in front of a mirror, and each at opposite
sides to it, they will see one another, but not themselves; and this
constitutes the principle of what is termed the ‘magician’s mirror.’
[Illustration]
“Here is a plan,” said Humphry, “of the ordinary arrangement. The black
lines we will suppose to represent the walls of two adjoining apartments.
At the end of each of the rooms there is an aperture, made large enough
to place behind it a looking-glass that is capable of reflecting the
whole figure. In each of these apertures there is inserted a sheet of
plate-glass, which is surrounded with a gilt frame, so as to have the
appearance of an ordinary mirror; and behind this a real looking-glass is
placed, slanting at an angle of 45°, and so large, that a person looking
into the sheet of plate-glass cannot see the edges of the slanting mirror
behind it. With such an arrangement, it is plain that a person looking
into either of the mirrors will not see himself, but any one who may
chance to be looking at the same time into the mirror in the adjoining
room. Consequently, on looking into the mirror and believing that he
should see his own figure reflected in it as in an ordinary looking
glass, his astonishment will be great in beholding himself transformed
into another person, or, indeed, into some living animal that may be
placed in front of the neighbouring glass.”
Kitty observed to her brother, that she remembered having seen the same
kind of an apparatus in a booth in a fair; and by it persons were said to
be shown their future lovers.
Humphry told her that it was by the same means that people were made to
see, apparently, through paving-stones.
“For if,” said the boy, “by the arrangement I have explained to you,
it is possible to see a figure in another apartment—a brick wall
intervening—it is obvious that, by the same device, an object placed on
one side of a paving-stone could be readily seen on the other.
“But the concave mirror,” continued Humphry, “is capable of producing
far more wonderful effects, for the image from this appears suspended
in the air; so that if the mirror and the object are hidden from view,
the effect is almost, supernatural. This illustration represents the
arrangement usually employed in such cases—
[Illustration]
“Here you perceive, Kitty,” added the lad, “the two sides of a room,
at the end of which there is a square opening, with a picture-frame
surrounding it. Outside the room, in an adjoining apartment, is placed
a large concave mirror; and so arranged, that when an object is set a
little above the floor in front of it, a distinct image of it may be
formed in the centre of the aperture at the end of the room, where the
spectators are assembled. Now, if the opening be filled with smoke,
that is made to rise in clouds from a chafing-dish concealed outside,
the image of any object placed in the one focus of the mirror in the
adjoining apartment will appear in the other focus at the centre of
the frame, and seem to be depicted on the clouds of smoke there as a
background. It is a favourite experiment to place a skull, strongly
illuminated, in the outer apartment, and to reflect an image of it amid
the smoke, so as to be visible to the spectators in the inner room. The
trick of the mysterious dagger, too, is very popular. The ordinary way
of performing this is by placing a basket of fruit in the one focus of
the mirror, so that a distinct aërial image may be formed of it in the
frame. One of the spectators is then desired to take some fruit from
the basket; and as he approaches for that purpose, a person, properly
concealed, withdraws the real basket of fruit with one hand, and with
the other substitutes a dagger, the image of which seems to strike at
the body of the spectator, and the thrust of the bright polished steel
at his breast never fails to produce a powerful impression. Now, it can
scarcely be doubted that a concave mirror was the principal instrument
by which the heathen gods were made to appear in the ancient temples.
Jamblichus informs us, that the ancient magicians made the gods visible
to the people among clouds of incense. And in the middle ages the
Pontiff, Theodore Santabaren, who was celebrated for his power in working
miracles, exhibited to the Emperor Basil of Macedonia the image of his
lost son, magnificently dressed, and mounted on a superb charger. The
apparition of the youth seemed to rush towards his father; and, throwing
himself into his arms, vanished. This effect was doubtlessly produced by
reflecting the image of a picture of the emperor’s son on horseback; and
the picture being brought nearer to the mirror, the image, of course,
appeared to advance until it reached the emperor’s arms, where it
naturally eluded his grasp. The celebrated Benvenuto Cellini has left us
an account of a more modern necromancy, in which he himself took a part,
in the middle of the sixteenth century.
“‘It happens,’ says Cellini, proceeded Humphry, as he read the account to
his sister, ‘through a variety of odd accidents, that I made acquaintance
with a Sicilian priest, who was a man of genius, and well versed in
the Latin and Greek authors. Chancing one day to have some conversation
with him, when the subject turned upon the art of necromancy, I, who
had a great desire to know something of the matter, told him that I had
all my life felt a curiosity to be acquainted with the mysteries of the
art. The priest made answer, that the man must be of a resolute and
steady temper who enters upon that study. I replied, that I had fortitude
and resolution enough, if I could but find an opportunity. The priest
subjoined, ‘If you think you have the heart to venture, I will give you
all the satisfaction you can desire.’ Thus we agreed to enter upon a
plan of necromancy. The priest, one evening, prepared to satisfy me, and
desired me to look out for a companion or two. I invited one Vincenzio
Romoli, who was my intimate acquaintance; and he brought another with
him. We repaired to the Coliseum; and the priest, according to the custom
of necromancers, began to draw circles upon the ground, with the most
impressive ceremonies imaginable. He likewise brought hither asafœtida,
several precious perfumes and fire, with some compositions also, which
diffused noisome vapours. As soon as he was in readiness, he made an
opening to the circle; and having taken us by the hand, ordered the
other necromancer, his partner, to throw the perfumes into the fire at
a proper time, entrusting the care of the fire and the perfumes to the
rest; and thus he began his incantations. This ceremony lasted above an
hour and a half, when there appeared several legions of devils, insomuch
that the amphitheatre was quite filled with them. Cellini afterwards
tells us, ‘that the necromancer called by their names a multitude of
demons, who were the leaders of the several legions, and questioned
them by the power of the eternal, uncreated God, who lives for ever,
in the Hebrew language, and likewise in Latin and Greek, and then the
amphitheatre was almost in an instant filled with demons, more numerous
than at the former conjuration. The necromancer requested me to stand
resolutely by him, because the legions were now above a thousand more in
number than he had designed; and, besides, these were the most dangerous.
The boy who had accompanied us was in a terrible fright, saying that
there were in that place a million of fierce men, who threatened to
destroy us; and that, moreover, four armed giants of enormous stature
were endeavouring to break into our circle. Vincenzio Romoli quivered
like an aspen leaf. Though I was as much terrified as any, I did my
utmost to conceal the terror I felt; so that I greatly contributed to
inspire the rest with resolution. But the truth is, I inwardly gave
myself over as a dead man. The boy placed his head between his knees,
and said, ‘In this posture will I die, for we shall all surely perish.’
In this condition, concludes Benvenuto, we stayed till the bell rang for
morning prayers.’”
CHAPTER XVII.
THE WONDERS OF COLOUR AND PHOTOGRAPHY.
The young philosopher had now completed his investigations concerning the
refraction and reflexion of light. He had ascertained—
1. That all substances in nature are divisible into two classes, viz.
_luminous_ and _non-luminous_ bodies.
2. That _luminous_ bodies send off rays of light from them _in all
directions_, and that such rays proceed in _straight_ lines while
traversing the same medium.
3. That _non-luminous_ bodies are _transparent_, or _opaque_; that is
to say, they either allow the rays of light emitted by luminous bodies
to pass through them, or else they arrest their progress; sometimes, in
the latter case, driving them off from their surfaces, and sometimes
absorbing them.
4. That when a ray of light falls obliquely on a transparent body it is,
on entering it, refracted or bent, in a greater or less degree, out of
its previous straight course.
5. That when a ray of light is driven off or reflected from opaque (or
even transparent) bodies, the angle of reflexion is invariably equal to
the angle at which the ray falls upon their surfaces.
As yet, however, Humphry had dealt only with white or ordinary uncoloured
light, and he was now about to study the phenomena of colour itself—to
investigate the laws which regulate the production of the varied tints
on the earth, and to ascertain, if possible, the means by which the soil
is painted with a thousand hues, and how the colourless sunbeam becomes
broken up into countless dyes as it falls upon the flowers and the rocks,
and is driven back by them to the eye, arrayed in all the charm of
variegated lustre.
“How comes it,” said Humphry to himself, as he thought over the subject,
“that the earth by night is black and sombre, as if a pall were spread
over the dead globe—that the trees have then a dim, spectral look—that
the sky is dusky as a canopy of smoke, and that the buildings seem like
masses of dense shadow darkening the air, so that the world about us is
as colourless as a cavern, and the beauty of surrounding nature blotted
out with the universal gloom? And how is it, too,” mused the poetic
boy, “that the beams of the returning sun have power to dye the fields
and sky with the richest hues—to crimson the clouds with the glowing
tints of dawn, and to revive, as it were, in an instant, the infinite
colours of the flowers, so that the ground grows suddenly iridescent
with their various dyes? How comes it, again, that at the tropics, where
the sun steeps the earth in a flood of light, the plumage of the birds
and the blossoms of the plants are unrivalled for the gorgeousness of
their colours; and that as we proceed thence to colder climates we find
a regularly declining chromatic scale, the tints becoming less and less
vivid till we reach the Poles, where Nature is arrayed in one unvarying
robe of white?”
But Humphry was too anxious to experiment to continue dreaming over the
matter, and, accordingly, he got his sister to darken the room once
more, and then attaching a prism in front of the hole in the shutter,
he proceeded to throw the spectrum on the wall in the manner before
described.
Kitty was as delighted as the boy with the beauty of the image, nor could
she help wondering how it happened that a simple stick of white glass
could resolve the sunbeam into such exquisite tints.
Humphry told her that the image she saw on the wall was merely an oblong
picture of the sun itself, the orb being drawn out to that figure by the
refraction of the glass. The vividness of the colours depends upon the
smallness of the aperture through which the light is admitted, and the
distance of the screen upon which the spectrum is made to fall; so that
if the hole in the shutter were smaller, and the wall farther off, the
spectrum would be much brighter. The colours, he added, came from the
decomposition of the sunbeam into its elementary tints.
“You, doubtless, Kitty, think the light of the sun to be simple and
uncompounded, and little dream that every ray of white light which meets
your eye is made up of seven other beams, and each coloured with some
one of the tints you see in the spectrum here; and it is solely because
the sunbeam is a _compound_ rather than a simple thing, and that each
of the seven rays of which it is composed have different properties and
refrangibilities, that this glass prism has the power of separating
them one from the other, and so of resolving the compound beam into its
seven elementary rays. Look here,” he continued; “were it not for this
prism the beam which comes through the hole in the shutter would proceed
in its previous course, and strike upon the floor; but by means of
this instrument it is _refracted_, or bent out of its path, and as the
coloured beams of which it is composed are, as I said, all differently
refrangible, the red ray here, being the least refrangible of all, is
the least bent out of its course, and so made to appear at the bottom
of the spectrum; whereas the violet ray, which is the most refrangible,
undergoes the most deviation, and thus is found at the upper part of the
coloured image.”
Kitty acknowledged that it was beyond her power to comprehend how a
beam of white light, which appeared to her to be devoid of all colour
whatever, could be _really_ composed of every kind of colour. “How was it
possible,” she said, “for violet, and blue, and green, and yellow, and
red, when mixed together, to form white?”
Humphry smiled at his sister’s incredulity, and said he would show her
that it was quite possible to put the parts of the sunbeam together
again—for by the same means as he had decomposed the white beam into its
seven coloured rays, so would he compound those seven coloured rays again
into one colourless beam.
The girl was all eagerness to see the composite nature of light thus
practically demonstrated, and in obedience to her brother’s instructions
she proceeded to place a sheet of white pasteboard against the wall, so
that the spectrum might fall upon it, and then to bring it gradually
nearer the prism.
As Kitty did this, she noticed that the spectrum grew smaller and dimmer;
but though the colours began to mix and encroach upon one another as she
advanced towards the prism, she found that, even when the pasteboard
screen was brought close to the face of the glass she could still
recognise the separation of the light into its elementary coloured beams.
This done, Humphry proceeded to annex to the prism already employed
another, which was exactly similar in all respects—being made of the like
kind of glass, and having a like refracting angle—to the previous one.
The second prism, however, was placed in the opposite direction to the
first, so that while the base of the one was uppermost, that of the other
was underneath—as here shown:
[Illustration]
The reason of this arrangement was, as Humphry explained, that the second
prism might exactly undo what the first had previously done, so that
the rays being now refracted by the one in an opposite direction to the
other, they would be all brought together again, and made to strike upon
the same spot as they would have fallen upon had no such instruments been
interposed.
The apparatus being fixed, the ray from the hole in the window-shutter
no sooner passed through the two prisms than the coloured spectrum which
the beam had been previously resolved into vanished from the wall, and a
round white spot of light appeared upon the floor.
Kitty was so wonder-stricken at what she saw, that she looked at Humphry
with the same fixed stare as a child gazes at some parlour magician.
“You see, then, sister,” said the lad, “that seven coloured rays may
be compounded again into one white one, even as one white beam can be
decompounded into seven coloured ones. So that, incredible as it may seem
to you, it is impossible to avoid the conclusion that white light is a
_composite_ thing, and made up of a number of other kinds of light that
are widely different from it. But,” continued Humphry, “there is another
and simpler means of proving this point to you.”
For this purpose the girl had to procure from the colour-shop seven
different colours in powder, each of the same tint as one of the rays in
the spectrum. These were afterwards mixed in the same proportion as the
rays themselves bore to one another, and, to Kitty’s astonishment, the
result was a kind of _greyish white_, produced by the mingling of the
whole; and Humphry told her that, were it possible to obtain colours of
precisely the same tint as those in the spectrum, a perfect white would
be the consequence.
“Again,” the youth continued, “if we take a circle, and paint round it
the several prismatic tints in the same proportion as they exist in the
spectrum itself, and cause these to revolve so rapidly that the eye is
unable to see any _one of them_, but rather perceives the whole at once,
the paper will no longer appear coloured like the rainbow to us, but seem
really white as it flashes past the eye.”
It was necessary, however, before doing this, to measure the several
lengths of the coloured spaces in the spectrum itself, when it was found
that the various prismatic tints were in the proportions hereunder given:
[Illustration]
The next step was to colour a circular piece of paper, as nearly as
possible in the same manner as the spectrum, and this was done after the
following fashion—where it will be seen that the outer coloured circle
is nothing more than the prismatic spectrum bent round till its two ends
meet at the point between the violet and red—the entire circle itself
being supposed to be divided into 360 parts.
[Illustration]
The circular spectrum, when finished, was placed upon a humming-top, and
the top being made to spin as rapidly as possible, the prismatic disc,
as it whirled past the eye, appeared to be absolutely colourless; for
each tint as it revolved left its impression upon the retina but for an
instant, and this being immediately afterwards covered by the tint which
was next to it, the result naturally was, that the whole of the seven
colours fell upon precisely the same part of the retina itself, and so
produced a composite impression—the seven coloured rays being perceived
all at once, rather than one after another. Hence the circle seemed to
be devoid of any _one_ of the colours painted upon it, and to partake of
that white tint which naturally results from the blending of the whole.
Humphry himself was almost as delighted as his sister with the result
of his experiments. It was demonstrable that the light of the sun which
fills the air by day, and seems absolutely colourless to us, is not of
that simple homogeneous nature which we are naturally led to believe, but
really made up of _seven_ coloured rays, which the eye itself is unable
to separate, and from which proceed all the several hues with which the
earth is painted, for the composite white beam falling upon the different
objects around is broken up by them into its elementary tints, and some
one of these reflected by them to us, so that the object itself naturally
appears of the same colour as the beam it sends to the eye.
* * * * *
Humphry had now to investigate the several properties of the spectrum
itself.
It will be remembered that he before found the point of greatest _heat_
to exist at the very extremity of the red ray, and he now ascertained
by means of a _photometer_ (or an instrument for measuring the relative
intensity of different lights) that the point of _greatest light_
existed at the boundary of the orange and the yellow rays. Consequently,
as the red (or calorific) rays were less refrangible than the yellow
(or luminous) rays, there was but one conclusion to come to—_light was
itself more refrangible than heat_; that is to say, the light in passing
through the prism, and being there separated from the heat with which it
was previously associated, was bent farther out of its course than the
heat was, so that the two principles were differently acted upon by the
glass, and consequently possess different powers and susceptibilities.
“But if,” said Humphry, “the red rays are the calorific ones, and the
yellow rays the luminous ones, what peculiar properties belong to the
rays at the upper end of the spectrum, where the sunbeam is bent the
farthest of all out of its previous course? What special power appertains
to the violet and blue portion of the coloured image?”
The youth knew, from the books he had studied upon the subject, that
these constituted the chemical beams—that is to say, the violet extremity
of the spectrum had been found to possess the power of separating silver
from some of its compounds, and Humphry was now anxious to observe the
effect for himself.
Having brushed a paper over with a solution of _nitrate of silver_
(lunar caustic), he placed a strip of it a little way beyond the violet
extremity of the spectrum; another strip he deposited in the violet ray
itself; a third was left in the blue ray; while in each of the other
coloured portions a piece of the same paper was exposed, and the light
admitted to them all at the same time.
It was then found that the nitrate of silver darkened the _most_ rapidly
at that part of the spectrum a little _beyond the violet extremity_—that
the chemical effect was the greatest after this in the violet ray itself.
Next, the blue ray possessed a greater decomposing power than the green;
whilst in the yellow and red rays no such power was perceptible, for the
solution of silver remained undarkened there.
“So, then,” cried Humphry, “the wonderful sunbeams that stream every
day upon the earth contain not only all the colours of the rainbow, but
three distinct, subtle principles, locked up in them—heat, light, and
chemical influence; each of these being differently refrangible and
existing in a ray of a different colour; the heat inhering in the red
or lower portion of the spectrum, and the chemical power in the violet
or opposite extremity, whilst the light occupies, as it were, a middle
place, residing principally in the yellow portion.”
Humphry then delighted his sister by preparing different chemical
solutions, to be acted upon by the violet rays of the sun.
First, he made some _chloride of silver_ by steeping a paper in salt and
water and then brushing it over with a solution of _lunar caustic_. This
he found to darken even more rapidly than the nitrate of silver itself,
and he then set to work to ascertain the cause.
Now _chloride of silver_ he knew to consist of chlorine (a green coloured
gas) and silver, and he was anxious to see whether light would act upon
chlorine more powerfully than it did upon nitric acid, as Mr. Wedgwood
had told him.
Accordingly he filled a jar with equal portions of hydrogen and chlorine
gases, and submitted this to the action of the sun’s rays, when, to the
astonishment of himself and terror of Kitty, the jar was no sooner placed
in the sunshine than the two gases detonated with the noise of the report
of a pistol, and the jar itself was almost shivered to pieces in the
explosion.
Delighted with the result, and anxious to repeat the experiment in a less
dangerous form, he filled a tube, about half an inch in diameter and
twelve inches long, with the same gases, and while the end of the tube
was inserted in a vessel of water, the upper part of it was shaded with
an opaque cover, so that by removing this for an instant he could allow
the gases within the tube to be acted upon by the light for as short a
time as he pleased.
In this manner the ingenious youth found, that the moment the opaque
cover was removed and the tube exposed, even to the diffused light of
day, a cloudiness appeared within it, owing to the instantaneous, though
silent, combination of the two gases, while the water rose more or less
rapidly within it according to the intensity of the light. The effect
even of a passing cloud was thus distinctly seen to retard the rapidity
of the combination, while, when exposed to the full solar light, the
union of the two was so instantaneous that the gases suddenly disappeared
from the tube, and the water rushed violently up into it to fill the
vacuum.
Next Humphry found that the two gases, when exposed to the sun’s rays
in a tube of violet-coloured glass, combine rapidly, but, strange to
say, without explosion; whereas when they are submitted to the action of
sunlight in a tube of red glass, the gases scarcely act upon one another.
It was, moreover, ascertained, that when standing in a perfectly dark
place, the two gases do not enter into combination in any length of time.
The lad could now understand why the chloride of silver darkened so
rapidly in the sun’s rays. The chlorine with which the metal was combined
was attacked by the moisture in the atmosphere, and as this moisture
consisted of oxygen and hydrogen in the form of water, the hydrogen of
it was made by the chemical influence of the sunbeam to enter into rapid
combination with the chlorine, and thus the silver was left behind, but
in such minute particles that the metal, instead of appearing white as it
usually does, assumed the form of a black powder, which, being fixed in
the paper, naturally caused it to darken in those parts where the light
had fallen upon it.
Filled with the knowledge he had thus obtained, Humphry set to work to
produce some sun-pictures for his sister. Patterns of pieces of lace were
thus made to impress their forms in a few seconds upon paper that had
been prepared over-night with a coating of chloride of silver. Where the
light fell, the silver was separated from the chlorine, and precipitated
in minute black particles, so that the paper was darkened in those parts;
while in the places where the threads of the lace prevented the rays from
reaching the paper, the solution was undecomposed, so that a white line
exactly corresponding to the pattern of the lace itself became impressed
upon the black ground.
Kitty was overjoyed at the first picture she beheld her brother produce
by the light, and Humphry smiled as he saw her take it to the window to
examine it more minutely, for he knew that as she looked at it the light
would begin to act upon the parts that had been previously screened by
the lace itself, and where the solution still remained undecomposed in
the paper; and sure enough, in a few minutes, she gradually saw the
pattern vanish, and the whole ultimately become of one uniform dark-brown
tint.
“What a pity,” cried the girl, “that so beautiful a thing should be
so perishable! If you could only find out, Humphry, how to fix the
pictures, what a great thing it would be for you to do!”
The brother told her, that in order to accomplish this it was necessary
to discover some substance that would remove the undecomposed chloride
of silver, forming the white parts of the picture, and which would not
attack the decomposed silver itself, forming the dark parts of it.
To attain this end, the young chemist made an infinity of experiments,
but without avail; for though he tried a number of acids and alkaline
solutions, he could find no liquid that would remove the undecomposed
chloride from the paper, and after weeks of toil and disappointment he
was obliged to confess, unwillingly, that the difficulty was one he
lacked the power to master.
Many years after Davy’s time, however, it was discovered that the
chemical substance termed _hyposulphite of soda_ readily dissolves
chloride of silver, and has little or no action upon the precipitated
silver itself; and from this period may be dated the perfection of the
wonderful art of photography (or sun-painting) that Thomas Wedgwood was
the first to attempt, and at which Davy himself was one of the early but
unsuccessful experimenters.
Of this art there are now two distinct branches, viz. one in which the
pictures are produced upon metal, the other upon paper or glass. In the
metallic process, _iodide of silver_ is the chemical agent rather than
the _chloride_; this is formed by submitting a perfectly clean plate of
polished silver to the action of the vapour of iodine, and sometimes to
_bromine_ afterwards, in order to quicken the action. The plate thus
prepared is placed in the camera, so that an image of the object to be
copied may fall upon it; the consequence is, that in the “_lights_” of
the picture the _iodide of silver_ becomes decomposed, the iodine itself
going off in the form of _hydriodic acid_ gas, by combining with hydrogen
in the moisture of the air, and the pure silver being left behind;
whereas in the shades of the picture where no light reaches, the iodide
of silver remains undecomposed. The action usually takes place in some
few seconds, according to the intensity of the light and the nature of
the “quick” used. When the plate is removed from the camera no picture is
visible upon its surface, so that the _developing_ part of the process
has then to be performed. This consists in submitting the plate to the
fumes of mercury, which attach themselves to the parts where the pure
silver has been separated from the iodine with which it was combined.
These parts constitute, as we said before, the “lights” of the picture,
and there the mercurial vapour is condensed, and clings in the form of
minute globules; whilst to the parts which have been undecomposed the
mercury does not attach itself, having no affinity whatever with the
_iodide of silver_ that remains there. The consequence is, the globules
of mercury which cling to the portions where the rays have fallen,
reflect so much light to the eye that they form the “whites” of the
picture; whereas the undecomposed iodide of silver, sending no light
to the retina, constitutes the “blacks:” and thus the image, which was
latent on the plate, is developed, or brought out, with such marvellous
fidelity, that when examined with a microscope, characters that were
several miles distant in the original may be clearly read in the minute
sun-copy.
After this comes the fixing process, and that consists merely in
submitting the plate to the action of _hyposulphite of soda_, which
dissolves, and so removes all the undecomposed _iodide_ of silver from
it, and thus renders it incapable of being further acted upon by the
light.
The above constitutes what is now usually known as the “_Daguerreotype_
process.”
The production of photographic pictures upon paper, on the other hand,
forms what is termed the “_Talbotype_ process”—the names of the two types
being derived from, those of their inventors. In the latter method of
producing sun-pictures there are almost the same different stages to
be gone through. The paper itself has first to be iodised, or rendered
_sensitive_ to the action of light, by means of coating it with a surface
of iodide of silver. This is done by washing it over first with a
solution of _nitrate_ of silver, and when this is dry, with a solution of
_iodide of potassium_; the consequence is, the one solution decomposes
the other, so that nitrate of potash and iodide of silver are formed. The
nitrate of potash, being soluble, is then washed out of the paper, while
the insoluble iodide of silver remains fixed in it. Then follows the
“_quickening_” part of the process. This consists in washing the sheet
of iodised paper over with a solution of what is termed _gallo-nitrate_
of silver, which consists of a small proportion of gallic acid (the acid
from gall-nuts) dissolved in water, and added to a solution of lunar
caustic, having a little acetic acid, or pure vinegar, in it. The gallic
and acetic acids are used because it is found that the presence of any
vegetable or organic matter hastens the decomposition of nitrate of
silver when exposed to light. The paper is now ready for the camera, and
is so sensitive to the action of light that it is said to transcend the
ordinary iodised paper in this respect more than a hundred-fold, so that
even a second or two of time is sufficient to impress a latent image upon
it.
Then, as in the daguerreotype method, the _developing_ process has to be
resorted to in order to bring out the picture, which is imperceptible on
removing the paper from the camera, and the existence of which would not
be suspected by any one who had not been forewarned of it by previous
experiments. To render the picture visible, the paper is washed over
once more with the gallo-nitrate of silver before described, and then
warmed gently before the fire; whereupon that part of the paper upon
which the light has acted begins to darken, while the other part of
the paper retains its whiteness. After this, as in the “Daguerreotype”
method, the _fixing_ process has to be resorted to. This, for
“Talbotypes,” consists in washing the paper in _bromide of potassium_,
which dissolves out all the undecomposed chemicals, and so leaves an
indelible impression behind.
The picture thus produced, however, is what is termed a “negative”
one—that is to say, the lights in the original are represented by shades
in the photographic copy, and _vice versâ_, the shades in nature are
rendered as lights in the picture. The Talbotype, therefore, has to be
again copied, in order that the lights and shades may be accurately
represented. For this purpose, however, the paper need not be so highly
sensitive; so that the ordinary _quickening_ part of the process by
means of the gallo-nitrate may be dispensed with, or the paper may be
coated with chloride of silver instead of the iodide before described.
Again, the _developing_ process is no longer necessary, the picture being
produced _directly_ by the action of light, rather than indirectly by
means of some developing agent. The _fixing_ process in this stage is
usually performed by means of hyposulphite of soda, and by these means
the negative picture before produced is rendered positive, and the lights
and shades thus made an accurate representation of those in nature.
It will now be seen that the art of producing sun-pictures, whether by
the Daguerreotype or the Talbotype, comprises usually four distinct
processes, viz.:
1. The _preparatory_ process, which consists in preparing the plate or
paper—that is to say, in coating it with some solution of silver that is
capable of being decomposed by the action of light.
2. The _quickening_ process, which consists, again, in rendering the
plate or paper more highly sensitive to light by the addition of some
other chemical, which facilitates the decomposition of the compound of
silver, with which the surface has been previously coated.
3. The _developing_ process, which consists in rendering visible the
latent picture which has been impressed upon the plate or paper while
exposed to the action of the light in the camera.
4. The _fixing_ process, or the dissolving out of all the undecomposed
silver compound, and so preventing the light from having any further
action upon it.
Now it must not be supposed that the compounds of silver are those only
which are capable of being decomposed by the sun’s rays, for photographic
pictures have been produced by compounds of all the precious metals—such
as gold, platinum, mercury, &c., these substances having but slight
affinities, and so being easily separable from the elements with which
they are united. Again, iron has been used successfully for the same
purpose—for this body, also, is readily decomposed when combined with
certain substances. Further, the gum-resins and bitumens admit of being
employed in the same manner, and many vegetable juices have been used by
Sir John Herschel for a like purpose. Indeed it has been truly said, that
almost every substance in nature is affected, in some way or other, by
the solar rays, for we now know that no substance can be exposed to the
sun’s rays without undergoing chemical action.
The changes, therefore, that are continually occurring in the external
world are quickened by the rays, which at one time it was believed gave
only light and heat to the globe that we inhabit; and even the very
changes of the seasons, the growth of vegetation, the blossoming of the
flowers, and the ripening of the fruits, are all due, in a measure, to
the chemical influence of those elementary rays which lie concealed in
the compound sunbeam: for it has been proved that the sunshine itself is
necessary, even to the breathing of plants through their leaves, and
that in the shade they cease absorbing the carbon from the atmosphere
which is ultimately destined to form part of their woody structure. Thus
light becomes not only the source of beauty to the world, and the agent
upon which one of our most wondrous senses depends, namely, that by which
we are enabled to recognise the form and nature of objects at a distance
from us, but it is also the source of health and vigour to our frames,
by maturing the products of the earth upon which we live, as well as by
promoting in our own frames those subtle chemical changes, by which our
bodies are nourished and our faculties developed, since in darkness men
can no more thrive than plants.
Had Davy lived to see the development of the chemical influence of light
that has been opened up to us since his time, he would have been the
loudest in his praises of the marvels wrought by it, and, doubtless,
among the foremost to have extended our knowledge of its action. But
these are discoveries made since his time, and discoveries which he, with
his deep insight into Nature, was unable to foresee, or even to assist.
To such perfection, however, has the photographic art been carried since
the days when Davy vainly essayed to fix the images which took him some
quarter of an hour to produce, that not only can stationary objects have
their forms indelibly impressed upon paper by the very light itself
which renders those forms visible to us, but the passing shadows that
give beauty to the landscape can be made permanent, the very undulation
of the corn can be seized, the rustling of the leaves detained, and even
the rippling of the waters, the playing of the fountains, and the curling
of the smoke, whose particles never for two moments together remain in
the same place, can be arrested, and their evanescent forms painted by
themselves, as it were, upon the tablets; so that the effect of a mere
instant can, by its marvellous agency, be prolonged for years. Thus time,
which is known to us only by the changes which are continually occurring
without and within us, has all the fixity of space; and those historical
events which our forefathers were unable to convey to us, from the want
of some such art, can now be handed down, rendered with all the truth of
light itself, so that future generations gazing at them may behold the
same scenes that were impressed at the back of our eyes years before.
Indeed, the photographic art itself is but the process of individual and
transient vision made universal and permanent; for the eye itself is but
the camera through which we gaze at the world without, and the retina
at the back of the organ of sight no more than a photographic plate, as
it were, impressing the images that flit before our vision more or less
permanently upon our memories.
As an instance, however, of the perfection, we repeat, to which this
process of fixing the most transient images has been carried, we need
only mention the experiment performed by Mr. Talbot, in which a moving
body, that was made to revolve at an enormous speed, and that was
illuminated but for an _instant_ by the electric spark, was photographed
as a stationary object.
It is well known that a wheel revolving at a rapid rate is barely visible
to us, the spokes passing with such velocity before the eye that we are
unable to distinguish one from the other, so that the whole appears to
us almost as one entire disc; such a wheel, however, if made to rotate
in the dark, and then suddenly illuminated for that inappreciable
portion of time which the electric spark—the miniature lightning of the
laboratory—endures, then appears to us as if absolutely standing still;
for as we see it under such circumstances only in that place which it
occupies so long as the light lasts, and this being but for the least
conceivable term of duration, it has no time—however rapidly it may be
turning on its axis—to pass from one point of space to another, so that
it can but appear to us as if utterly stationary.
In the experiment we allude to, a wheel was thus made to revolve so
rapidly, that its revolutions were counted by the musical note produced
by the vibrations of a spring, that moved backwards and forwards once at
each turn. The revolutions were performed in the dark; and during this
the chamber and wheel were suddenly lighted by one spark drawn from a
powerful electric machine. At this moment a photographic apparatus was
presented to the wheel itself, and on developing the image thus produced
upon the paper which had been previously inserted in the camera, it was
found to be impressed with a perfect copy of the wheel itself, with all
its spokes distinctly visible, and precisely the same as if the image
had been taken from the wheel while in a state of rest. It was the same
stationary image, too, as the spectators themselves had beheld during
the instantaneous illumination of the object; and thus, by the aid of
the same fluid as the lightning itself, and with the assistance of music
to register the rate of revolution, that mysterious principle of motion
which has puzzled philosophers since philosophy began, was made to
appear like rest, and even the sensibility of the eye itself rivalled by
photographic agency, so that the dead paper was made to be impressed with
the very same figure as the living retina itself perceived.
CHAPTER XVIII.
CONCLUSION.
During the prosecution of his later experiments Humphry had formed
the acquaintance of Mr. Davies Giddy, a gentleman of high scientific
attainments, better known under the name of Davies Gilbert, and who was
then resident at Tredrea, near Penzance.
This gentleman, who ultimately became President of the Royal Society,
proved of great service to young Davy, for not only did he lend the boy
such apparatus as he required for the carrying out of his experiments,
but he delighted to converse with Humphry; and though he could not help
smiling occasionally at the strangeness of his theories, he grew to
have a lively sense of the ardour of the youth’s imagination, and the
originality of his mind.
Now it so happened that Davies Giddy was acquainted with Dr. Beddoes,
who had formerly been one of the Oxford Professors, but who had recently
opened a Pneumatic Institution at Bristol for the cure of diseases by
the inhalation of gases; and it was during one of Dr. Beddoes’ visits to
Davies Giddy that Humphry made the acquaintance of the Doctor, and so
favourable an impression did he make upon the gentleman, that not long
afterwards a letter was sent, offering Humphry the post of Assistant to
the Bristol Institution.
The lad was delighted at the prospect of removing from so remote a place
as his native town, and lost no time in talking the matter over with
his friends. Mr. Giddy told him of the Doctor’s influence, and how his
Institution was already the resort of some of the most eminent persons in
the country, and warmly advised him to avail himself of the offer.
Mr. Borlase, to whom Humphry repeated all that Mr. Giddy had said,
counselled the boy to take the same step, and added, that he had been so
pleased with his conduct while under his roof that he would in no way
impede his advancement, but would rather cancel his indentures, even
though he was just beginning to be of service to him.
Mrs. Davy, too, was anxious that her boy—whom she felt more and more
convinced was destined to take a high rank in the world—should be
transferred to a wider sphere, where his abilities would have greater
chance of being called into play, and she gladly accompanied Humphry to
their old friend, Mr. Tonkin, to break the matter to him, and hear what
he thought of the proposal.
The old gentleman, however, could not be made to listen to the project,
and did not hesitate to denounce Humphry’s desire for worldly honour as
the “wild-goose chase” which led many an ambitious simpleton astray,
saying, that if the boy would make up his mind to settle in his native
place, he might be assured of a comfortable independence, for he would
find but few able to compete with him there. Nevertheless, in a large
town—however striking his talents might appear in a small one—the circle
of his competitors would be so much increased, that he would sink into a
mere nobody, and end his days as one of the many fools who had struggled
after the world’s prizes, and found, when too late, that there was no
chance of obtaining them.
Mrs. Davy, however, mother-like, felt satisfied that Mr. Tonkin took an
erroneous view of her son’s powers, and she strove to assure her old
friend that he did not know what Humphry was capable of doing so well as
she did, and that if he did, he would have as little fear as herself of
his failure.
Mr. Tonkin, however, was not to be argued out of the notion he had taken
up, and ultimately grew so annoyed with what he fancied to be merely a
mother’s silly prejudice on Mrs. Davy’s part, that he ended the interview
by vowing that the boy should never quit Penzance with _his_ consent.
This, for a time, put a stop to the correspondence on the subject. At
length, however, Dr. Beddoes became so urgent that Humphry should join
him, that, despite the objections of Mr. Tonkin, who still would not
listen to the plan, his friends advised him to accept the offer; and it
was accordingly arranged that young Davy should leave Penzance as soon as
he conveniently could.
Accordingly, on the 2d of October, in the year 1798, Humphry, not then
twenty years of age, quitted his native town for the first time in his
life, and that to commence fighting his way in the world.
His mother parted from him as full of high hope as the boy himself; and
as the boy hugged the widow to his heart alone in her chamber, before
he left her, he said, with the sobs in his throat, “Mr. Tonkin does not
know me, mother, yet: but be you of good cheer, I will live to be an
honour and a glory to you still; and it shall be my proud lot to say some
day that I was the means of raising you and all that belong to me to a
position of comfort and eminence. Years ago now, mother, I told you I
_would_ do it, and the resolve is still _deep_ in my heart.”
Mrs. Davy assured him she had every confidence in his attaining the noble
object he had in view, and she parted from him, though with tears in her
eyes, with a smile of high hope upon her lip.
Mr. Tonkin, however, was resolute to the last, and at his leave-taking
denounced Humphry’s plans as visionary schemes; and when the boy had
left, and the old gentleman found his favourite plan of settling Humphry
in his native town as a surgeon had been thwarted, he altered his will,
and revoked the legacy of the house that he had previously bequeathed to
his foster-son.
On young Humphry’s journey to Bath he met his friend Mr. Davies Giddy at
Oakhampton, and while breakfasting there, the mail-coach from London drew
up at the door of the inn, covered with laurels and ribbons, and bringing
the first news of Nelson’s victory of the Nile.
“I have a greater fight than that to fight,” said Humphry to himself;
“and, please God, I will gain the victory, too.”
* * * * *
It was Mrs. Davy’s happy lot to witness the realisation of all the hopes
she had formed of her boy in his youth; for, during her life, he rose to
be elected President of the Royal Society, and to be created a Baronet,
for the many additions he had made to the stock of knowledge; to be
rewarded with the first prize instituted by the Emperor Napoleon for
the greatest scientific discovery of the time; and to be allowed a free
passage through France at a time when all other Englishmen, no matter how
high their rank or character, were denied admission into that country.
FOOTNOTES
[1] Extracted from Dr. Paris’s “Life of Davy.”
[2] Mr. Tonkin (says Dr. John Davy, in his Memoirs of the Life of his
brother) “will long be remembered in Penzance, both for excellences and
peculiarities. The latter marked him as a person of the gone-by time, and
attracted the notice even of the careless observer. He held in aversion
modern changes of fashion, and in his old age wore the dress of his
youth—the cocked hat, large powdered wig, hand-ruffles, upright collar;
in brief, the professional dress of the beginning of the last century—and
his manly form and countenance suited well with this venerable costume.”
Dr. Davy says, moreover, that Mr. Tonkin “held a distinguished place
among his fellow-townsmen, being looked up to for his sterling worth and
strength of judgment, and very dear to his friends for his benevolence,
kindness, and very generous and friendly disposition.” He was “of a quick
temper,” he adds, “but his anger was of short duration.”—See _Life of Sir
Humphry Davy_, vol. i. p. 109.
Sir Humphry himself, in his last letter to Mr. Tonkin, says, “If I was
nearer I would endeavour to be useful to you. I would endeavour to pay
some of the debts of gratitude I owe to you, _my first protector and
earliest friend_. As it is, I must look forward to a futurity that will
enable me to do this; but believe me, wherever I am, and whatever may be
my situation, I shall never lose the remembrance of obligations conferred
on me, or the sense of gratitude which ought to accompany them.”
[3] “When Mrs. Davy became a widow, she was in her thirty-fourth year,
with five children, all of whom were still to be educated, excepting
Humphry, her eldest son. Her income at this time was about £150 a year,
and it was encumbered with a debt of £1300.”—_Dr. Davy’s Life of Sir
Humphry_, vol. i. p. 7.
[4] “Mrs. Davy was the third and youngest daughter of Grace and Humphry
Millett.... Mr. Millett was engaged in business in the town of Penzance
as a mercer. He and his wife died young, _and in the same week_—he on
the 3d of June, 1757, and she on the 9th.... Mr. John Tonkin was their
friend, and supplied the place of a father to them (the orphan children),
and they retained through life a most grateful sense of his kindness,
and of the great obligations they owed to him. At the time of the death
of Mr. and Mrs. Millett, he (Mr. Tonkin) was residing in their house (I
suppose in lodgings), and there he continued to reside for some years,
the children being under the care of a Miss Peggy Adams, their cousin, in
whose name the mercer’s business was continued, by the profits of which
the family was chiefly supported.”—_Dr. John Davy_, p. 7, vol. i.
Dr. Paris, in his life of Sir Humphry Davy (p. 2), says, speaking of the
philosopher’s mother, “Her maiden name was Grace Millett, and she was
remarkable for the placidity of her temper and the amiable and benevolent
tendency of her disposition. She had been adopted and brought up with
her two sisters, under circumstances of affecting interest, by Mr. John
Tonkin.... To withhold a narrative of the circumstances which led Mr.
Tonkin to the adoption of these orphan children would be to deprive
the world of one of those bright examples of pure and disinterested
benevolence which cheer the heart and ornament our nature.... The
parents of these children having been attacked by a fatal fever expired
within a few hours of each other. The dying agonies of the surviving
mother were sharpened by her reflecting on the forlorn condition in
which her children would be left. For, although the Milletts were
originally aristocratic and wealthy, the property had undergone so many
sub-divisions as to have left but a very slender provision for the member
of the family to whom she had united herself.” On the decease of Mrs.
Millett (Dr. Paris tells us), Mr. Tonkin immediately took charge of her
three orphan daughters, and “continued their kind benefactor until each
in succession found a home by marriage.”
[5] “The state of society in the Mount’s Bay only half a century
ago,” says Dr. John Davy, “was peculiar and different from what it is
at present. Cornwall was then without great roads. The roads which
traversed the country were bridle-paths rather than carriage roads.
Carriages were almost unknown, and carts even very little used. I have
heard my mother relate that when she was a girl there was only one cart
in the town of Penzance, and if a carriage appeared in the streets it
attracted universal attention. Pack-horses were then in general use
for conveying merchandise, and the prevailing manner of travelling was
on horseback. In the same town, where the population was about 2000
persons, there was only one carpet; the floors of rooms were sprinkled
with sea sand, and there was not a silver fork. The only newspaper which
then circulated in the West of England was the ‘Sherborne Mercury,’ and
it was carried through the country, not by the post, but by a man on
horseback, specially employed in distributing it.... Visiting was then
conducted differently from what it is at present. Dinner-parties were
almost unknown, excepting at the annual feast time. Christmas, too, was
then a season of peculiar indulgence and conviviality, and a round of
entertainments were given, consisting of tea and supper. Excepting at
these two periods, visiting was almost entirely confined to tea-parties,
which assembled at three o’clock and broke up at nine, and the amusement
of the evening was commonly some round game at cards, as Pope Joan or
Commerce.... Amongst the middle and higher classes there was little
taste for literature, still less for science, and their pursuits were
rarely of a dignified or intellectual kind. Hunting, shooting, wrestling,
cock-fighting, generally ending in drunkenness, were what they most
delighted in. Smuggling was carried on to a great extent, and drunkenness
and a low scale of morals were naturally associated with it.... Few
places have exhibited greater changes within the last half century than
Penzance. Not a single family belonging to the great gentry now in
existence west of Hayle, or in the Mount’s Bay, was known one hundred
years ago.”
“Carriages, it may be added, are of French invention. Under Francis I.
(A.D. 1515-1547), who was contemporary with our Henry VIII., there were
but two in Paris, one of which belonged to the Queen, and the other to
Diana, the natural daughter of the French Henry II. There were but three
in Paris in 1550; Henry IV. of France (A.D. 1589-1610) had one, but of
very rude construction, and without straps or springs. The first courtier
who set up this equipage in France was John de Laval de Bois-Dauphin, who
could not travel otherwise on account of his enormous bulk. Previously
to the use of carriages the kings of France travelled on horseback, the
princesses were carried in litters, and ladies rode behind their squires.
The first carriage seen in England was in the reign of Mary, about 1553;
but the art of making them was unknown in this country at that time.
Close carriages of good workmanship began to be used by persons of the
highest quality at the close of the sixteenth century; Fitz-Allen earl
of Arundel is said to have been the first who used them, and this was in
1580; their construction was various. They were first made in England
about the year 1590, when they were called ‘whirlicotes.’ In the year
1601, an Act was passed to prevent the effeminacy of men riding in
carriages (43d Elizabeth). The Duke of Buckingham, in 1619, was the first
who had a carriage with six horses to it; and the Duke of Northumberland,
on obtaining his liberation from the Tower (where he had been imprisoned
since the Gunpowder Plot) and hearing that Buckingham was drawn about
with six horses to his carriage, ordered, out of rivalry, eight horses
to be put to his, and in that manner passed from the Tower through the
City.”—_Haydn’s Dictionary._
“In the twelfth century carpets were articles of luxury. It is mentioned
by old English historians, as an instance of Becket’s splendid style of
living, that his sumptuous apartments were, every day in winter, strewn
with clean straw or hay. This was about the year 1160. The manufacture
of woollen carpets was introduced into France from Persia at the end of
the sixteenth or beginning of the seventeenth century. Some artisans, who
had quitted France in disgust, came over to England and established the
carpet manufacture among us about 1750. Our Kidderminster, Axminster, and
Wilton manufactures are the growth of the last hundred years.”—_Ibid._
[6] “Davy, it may be remarked, possessed, when a boy, a countenance
which, in its natural state, was very far from comely; while his round
shoulders, inharmonious voice, and insignificant manner, were calculated
to produce anything rather than a favourable impression. In riper years
he was what might be called ‘good-looking;’ although, as a wit of the
day observed, his aspect certainly was of the ‘bucolic’ character.”—_Dr.
Paris’ Life of Sir Humphry Davy_, p. 33. The Doctor afterwards describes
young Davy as an “extraordinary-looking boy.” “His manners were
retreating and modest,” says Mr. Poole (one of Davy’s oldest friends),
in a letter to Dr. Paris, speaking of Sir Humphry in early life; “he was
generally thought naturally graceful, and the upper part of his face
was beautiful. When he first lectured at the Royal Institution, the
ladies said, ‘Those eyes were made for something besides poring over
crucibles.’”—_Dr. John Davy’s Life of his Brother_, vol. i. p. 136. “I
was very young,” Lady Brownrigg says, in a letter to Dr. Davy, “when
I first had the pleasure of seeing your highly-gifted brother. We had
been invited by Dr. Richardson to go to his cottage at Portrush, to meet
the famous Mr. Davy. We arrived a short time before dinner; in passing
through a room we saw a youth, as he appeared,” (Davy was twenty-eight
years of age at this time) “who had come in from fishing, and who, with
a little note-book, was seated in a window-seat, having left a bag, rod,
&c. on the ground. He was very intent on this little book, and we passed
through unnoticed. When I went into the drawing-room I felt some little
awe at this great philosopher, annexing to such a character, at least,
the idea of an elderly grave gentleman—not, perhaps, with so large a wig
as Dr. Parr, or so sententious a manner as Dr. Johnson—but certainly
I never calculated on being introduced to the identical youth, with a
little brown head like a boy, that we had seen with his book at the
window-seat, and who when I came into the drawing-room was, in the most
animated manner, recounting an adventure which had entertained him on the
Causeway, and, from his mode of telling it, was causing loud laughing
in the whole room.”—Given in the _Life of Sir Humphry, by Dr. John
Davy_, who speaks of the above account as being “very descriptive of the
appearance and manner” of his brother “at this time.”
[7] These are called the “Long Ships’ Rocks,” and on one of them is
a “light.” British ships passing this pay one halfpenny a ton, and
foreigners one shilling each vessel; the annual revenue thus obtained
amounting to three thousand pounds.
[8] The Scilly Isles.
[9] Known by the name of “_Enys Dodnan_.”
[10] This is called in Cornish “_Tol-Pedn-Penwith_,” which signifies the
holed headland on the left hand.
[11] Davy is said to have delighted as a boy in visiting the Land’s End.
In one of his early poems occurs the following passage, in which the spot
is spoken of under its Latin name, “_Bolerium_:”
“Thy awful height, Bolerium, is not loved
By busy man; and no one wanders there
Save he who follows Nature—he who seeks
Amidst thy crags and storm-beat rocks to find
The marks of changes, teaching the great laws
That raised the globe from chaos; or he whose soul
Is warm with fire poetic.”
“It is surely not difficult,” says Dr. Paris, “to understand how it
happened that a mind endowed with the genius and sensibility of Davy
should have been directed to the study of chemistry and mineralogy, when
we consider the nature and scenery of the country in which accident
had placed him.... Nor could he have wandered along the rocky coast,
nor have reposed for a moment to contemplate its wild scenery, without
being invited to geological inquiry.... ‘How often, when a boy,’ said
Davy to me (adds the Doctor), on my showing him a drawing of the wild
rock scenery of Botallack Mine, ‘have I wandered about those rocks in
search of new minerals, and, when fatigued, sat down upon the turf, and
exercised my fancy in anticipation of scientific renown.’” (Botallack
Mine is situate at St. Just, a town near to Cape Cornwall, and but a
short distance from the Land’s End.) “The granite and serpentine rocks
of his native county were, I believe,” says Dr. John Davy, “the first
he studied when he commenced the pursuit of geology, and both of them
were to him particularly attractive. The finest examples of these rocks
were within a day’s ride of Penzance; and when he visited home, a young
man, he never failed paying the Lizard and the Land’s End a visit, and
generally in company with some of his old school-fellows. I remember,
when a boy,” Dr. D. continues, “being allowed to join one of these
parties to the Land’s End, and it was a merry one—as youthful parties
commonly are. After exploring the cliff scenery, we dined at a tavern
at St. Just, and I well recollect the boisterous mirth indulged in when
the repast was concluded—the gymnastic feats attempted, the shouts
of applause, the unconstrained laughter, and all that abandonment of
spirit to mirth so common to young persons under excitement, and which,
excepting in youth, can scarcely be felt or enjoyed.”—_Life of Sir
Humphry Davy._
In the commencement of a work designed by Sir Humphry Davy upon “The
Geology of Cornwall,” the philosopher himself gives the following
description of the rocks at the Land’s End: “In the great arrangement
of the masses of granite of Cornwall, the rock appears composed of an
immense number of blocks of different sizes. This structure is nowhere
more perfectly exhibited than in the western cliffs. The incessant agency
of the Atlantic, its storms and its waves, have washed away or destroyed
all the loose materials of the shore, and left abrupt eminences of
rock from 50 to 360 feet in height. The arrangement of the granite is
in masses which approach to the cubical form, having, however, rounded
edges, heaped upon each other.... The masses are grand, their colours
uniform, and their uniformity increases the effect upon the eye; while
the arrangements of this kind have a peculiar wildness and sublimity.
Nowhere is it seen upon a greater scale, or in a more magnificent
assemblage of forms, than from a point between the Land’s End and Castle
Treene. Both these grand promontories appear extending into the Atlantic;
the cliffs between them are abrupt and lofty; the waves are broken by
a number of small island rocks, which are scattered along the shore.
The few portions of soil that appear above the cliff are covered with
short green grass, tufted with heath and furze, which, in the autumn,
present mixed hues of purple and gold. The rock throughout is of a
uniform yellowish red, the tint perfectly contrasted to the blue-greens
of the sea.” (Castle Treene, or “Castle Treryn,” as it is more correctly
written, is a headland beyond “St. Levan churchtown,” as it is sometimes
called—though the chapel which formerly stood there has been many years
since washed away by the sea, the steps alone now remaining.) St.
Levan lies a little to the eastward of the headland called “_Tol Pedn
Penwith_.” A short distance from St. Levan is “Port Carnow Cove,” which
is bounded on the eastern side by rocks that jut far into the waves,
and rise to a great height, being heaped one on another, in magnificent
order. Here stands the noble headland called “Castle Treryn,” above whose
summit two huge slanting and imposing masses of granite protrude. There
is a fissure between these masses leading to a smaller group of granite
rocks, on the top of which the huge Logan stone (weighing some 65 tons)
stands so delicately poised, that by clambering to a fearful height at
one of the angles, it may be made to sway to and fro with the least force.
[12] Dr. John Davy says, “The greater part of the year following” (the
period of his quitting Dr. Cardew’s school at Truro, which Humphry did at
the age of fifteen, when his school education was considered as complete)
“he was, I believe, in an unsettled state, studying in a desultory
manner, by fits and starts, and yielding to the allurements of occasional
dissipation, and the amusements which constitute the delight of active
youth—as fishing, shooting, swimming, and solitary rambles. _This was
perhaps the most dangerous period of his life, and in conversation with
me he has so spoken of it._ Amusement threatened for a time to obtain
the mastery, and keep him down to the common level; but his good genius
triumphed, and, after a few months’ vacillation, he applied himself
in earnest to the cultivation of his mind and to the acquisition of
knowledge; and the flame once kindled burnt on till it expired in death.”
Speaking of the circumstances which induced Humphry to relinquish all his
boyish habits about the period of his father’s death, his brother says:
“_This event, probably, had a powerful effect in giving steadfastness
to his resolution, and I am quite certain that the circumstances of his
family became with him an additional and powerful motive to exertion._”
“Like most young persons, Humphry when a boy,” says Dr. John Davy, “was
fond of declaiming, and indulged in it in his solitary walks and rambles.
On one occasion it is recorded of him, that, on his way to visit a poor
patient in the country” (during his apprenticeship), “in the fervour of
declamation, he threw out of his hand a phial of medicine which he had to
administer, and that when he arrived at the bed-side of the poor woman he
was surprised at the loss of it. The potion was found the next day in a
hay-field adjoining the path” (p. 55).
[13] The greater number of these resolves were fulfilled in after-life.
Dr. John Davy says: “The interest he took in me more resembled that of
a father than a brother, and it is with peculiar pleasure I reflect
on his various kindnesses—my numerous obligations, many of which were
delicately concealed at the time—his valuable hints and generous regard
to my studies, leaving me free to follow the bias of my own mind—and his
excellent advice in respect to my conduct, in which was always infused a
native nobleness of sentiment well adapted to stir up virtue in a young
mind.” In one of his letters to his brother, Sir Humphry says, “You must
study your own plans with respect to study. Pray do not care about the
expense, if it adds anything to the comfort or respectability of your
situation. I will, if you like, send £40 a year, in addition to what my
mother sends you. My dear John, let no difficulties alarm you; you may be
what you please. Let no example induce you to violate decorum—no ridicule
prevent you from guarding against sensuality or vice. Live in such a way
that you can always say the whole world may know what I am doing.”
Dr. Paris says: “No sooner had Davy found himself in a situation which
secured for him the necessaries of life, _than he renounced all claims
upon his paternal property in favour of his mother and sisters_.” In a
letter to one of his sisters, Davy says: “I enclose a one-pound note,
which you will lay out in books or in anything else you like. I enclose
another one-pound note, which I wish to have disposed of in the following
manner: To Mary Launder, 5_s._; to Betty White, 5_s._; and with the rest
you will buy some ribbons or little articles of dress for the Doctor’s
Jenny, my aunt Sampson’s Phillis, my aunt Millett’s maid, and my mother’s
servant, as New-years’s gifts.” In another letter he writes thus: “I
enclose a ten-pound note, which I beg you will lay out in the way you
think best for my sister’s children and any _old pensioners_ that knew
me in my youth.” “No Swiss peasant,” says Dr. Paris, “ever sighed more
deeply for his native mountains than did Davy for the scenes of his
early years. He entreated his nurse (when ill at the Royal Institution)
to convey to his friends his ardent wish to obtain some apples from a
particular tree which he had planted when a boy, and he remained in a
state of restlessness and impatience until their arrival.” Moreover, it
should be stated, that, in addition to his will, he left at his death a
paper of directions, which have been religiously observed by his widow.
In these he desires that the interest arising from £100 stock may be
annually paid to the master of the Penzance Grammar-school, on _condition
that the boys may have a holiday on his birthday_. “There is something,”
adds Dr. Paris, “singularly interesting in this favourable recollection
of his native town and of the associations of his early youth. It adds
one more example to show, that, whatever may have been our destinies, and
however fortune may have changed our condition, where the heart remains
uncorrupted we shall, as the world closes upon us, fix our imaginations
upon the simplicities of our youth, and be cheered and warmed by the
remembrance of early pleasures.”
[14] “In all the various situations of life in which my mother was
placed,” says Dr. John Davy, “she so conducted herself as to gain
the regard and good-will of every one. She possessed a most kind and
affectionate heart, a pious mind, sound understanding, and perfect
integrity. She was devoted to the performance of all her duties, and
was remarkably free from all guile and foolish pride. When she became a
widow, she was in her thirty-fourth year, with five children, all of whom
were still to be educated, excepting Humphry, her eldest son. Her income
at this time was about £150 a-year, and it was encumbered with a debt of
about £1300, contracted by my father chiefly in consequence of losing
speculations in mining. Her good resolutions did not fail her on this
trying occasion; she met all her difficulties with courage and prudence.”
Dr. Paris, speaking of Davy’s mother, says: “She was remarkable for the
placidity of her temper and for the amiable and benevolent tendency of
her disposition.”
[15] “My brother,” says Dr. John Davy, “at the time of my father’s death
was sixteen years old. Seeing her (Mrs. Davy) in great affliction, he, in
a very affectionate manner, begged her ‘not to grieve,’ saying that ‘_he
would do all he could for his brother and sisters_.’”
[16] “It is remembered,” says Dr. John Davy, speaking of Sir Humphry’s
infancy, “that he walked off (to use a nursery phrase) when he was just
nine months old; and I have been told, that before he was two years of
age he could speak fluently. About this time my eldest sister was born,
and he was told by a servant, that on her appearance ‘his nose would be
put out of joint.’ On seeing the baby, it is related of him, that he put
his hand to his nose, and said, ‘Mamma, my nose not out of joint.’ Before
he had learned to write, he amused himself with copying the figures in
‘Æsop’s Fables,’ and under his drawings, in great letters, he contrived
to give them their names. His memory was very retentive, in proof of
which it is handed down in the family, that when very young he could
recite a great part of ‘Pilgrim’s Progress,’ even before he could well
read it. I believe that, like Pope, he lisped in numbers. I remember
hearing my mother say, that when scarcely five years old he made rhymes,
and recited them in the Christmas gambols. His disposition as a child
was remarkably sweet and affectionate.... The first school he was sent
to was that of a Mr. Bushell, at which reading and writing only were
taught. This master, then an old man, remarking the rapid progress of his
young pupil (he was then six years old), in a very disinterested manner
recommended my father to remove him to the Grammar-school.”
“It is a fact,” says Dr. Paris, “worthy, perhaps, of being recorded, that
Humphry Davy would, at the age of five years, turn over the pages of a
book as rapidly as if he were merely engaged in counting the leaves or in
hunting after pictures, and yet, on being questioned, he could generally
give a very satisfactory account of the contents. The same faculty was
retained by him through life.”
[17] “The Rev. Mr. Coryton (the master of the Grammar-school) was a man
of irregular habits, and ill-fitted for the office of teaching youth. He
was occasionally severe, and punished heavily slight offences. Pulling
the boys’ ears was practised by him in the most capricious manner, and
my brother was too frequently a sufferer from this infliction. It is
recorded of Humphry Davy, that he appeared before Mr. Coryton with a
large plaster on each ear, and that when asked by his master ‘what was
the matter with his ears,’ he replied with a very grave face, ‘that he
had put the plasters on to prevent a mortification.’”—_Dr. John Davy_, p.
14. In a letter to his mother, Sir Humphry Davy says, speaking of this
school, “I consider it fortunate that I was left much to myself as a
child, and put upon no particular plan of study, and that I enjoyed much
idleness at Mr. Coryton’s school. I perhaps owe to these circumstances
the little talents that I have, and their peculiar application. What I
am, I have made myself. I say this without vanity, and in pure simplicity
of heart.” Davy seems, indeed, to have been more distinguished out of
school, and by his comrades, than by any great advance in learning. “From
his facility in composing Latin and English verse,” says his brother,
“his assistance in these exercises was often requested, even by boys
much older than himself; and in writing valentines and love-letters, he
shone so pre-eminently, and gave his aid so willingly, that he is said
to have been generally resorted to on all emergencies of boyish loves.
Another cause of popularity among his comrades was his power of diverting
them by telling them stories; and so attractive were the tales, commonly
of wonder and terror, which he related, that they were in the habit
in an evening of collecting at a particular place to wait for him, as
under the balcony of the Star Inn, which afforded shelter, and where,
if there happened to be a cart, he would get into it, and hold forth
to his young audience.” (_Idem_, p. 15.) “The earliest indication that
I am aware of,” says Davy’s brother, “which he showed of his fondness
for experimenting, was in making fire-works. My eldest sister well
remembers that she was his assistant in this undertaking, and that their
workshop was an unfurnished room, in which, in bad weather, the Rev.
Dr. Tonkin (the elder brother of Mr. John Tonkin, the friend of our
family), then advanced in age and a valetudinarian, took exercise on his
chamber-horse—a large arm-chair attached to spring-boards, which boards
served for a table for compounding the ingredients of the squibs and
crackers.” “Davy,” says Dr. Paris, speaking of his youthful amusements,
“was in the habit of preparing a detonating composition, to which he gave
the name of ‘thunder powder,’ and which he would explode on a stone, to
the great delight of his young playfellows” (p. 5).
[18] The above are Davy’s own words, taken from a fragment of a letter
which his brother says exists in one of Sir Humphry’s note-books, kept
during his youth, and which was addressed to one of his early home
friends, the letter itself being descriptive of his escape from the vices
which are the most seductive to youth in towns. “An active mind,” he
writes, “a deep ideal feeling of good, _a look towards future greatness_,
has preserved me from these.”
[19] When Davy was offered the appointment of Superintendant of the
Pneumatic Institution at Bristol, “he accepted it,” says his brother,
“with the consent of all his friends, excepting Mr. John Tonkin, who had
hoped he would have settled at Penzance; and who,” Dr. Davy tells us in
another place, “was so angry with Humphry for accepting the appointment,
that he made some alteration in his will in consequence.” Dr. Paris’s
version of the affair is as follows: “His old and valued friend, Mr.
Tonkin, not only expressed his disapprobation of the scheme, but was
so vexed and irritated at having _his favourite plan of fixing Davy in
his native town as a surgeon_ thus thwarted, that he actually altered
his will, and revoked the legacy of his house, which he had previously
bequeathed him. Mr. Tonkin died on the 24th December, 1801; so that,
although he lived long enough to witness Davy’s appointment to the Royal
Institution, _he could never have anticipated the elevation to which his
genius and talents ultimately raised him.”—Life of Sir Humphry Davy_, p.
39.
[20] The little zoophyte here described is a kind of small “jelly-fish”
known by the name of the “Beroe.” It may be often seen on our shores.
[21] This memorable passage was written in a diary kept by Humphry Davy
during his youth. His brother, after quoting it, adds, “and this early
sentiment never forsook him; even in his last days he had a feeling
of the same kind, looking forward, were his life spared, to greater
exertions.”
[22] The details of this accident are taken from the account of an
explosion which occurred at Felling Colliery, near Sunderland, on the
12th of May, 1812, and of which a narrative was prefixed by the Rev. John
Hodgson to the published form of the funeral sermon he preached on the
occasion. It was this fearful explosion which led to the formation of
the Society for the Prevention of Accidents in Coal Mines; and it was at
the request of the members of this body that Humphry Davy was induced to
perfect his safety-lamp.
[23] The above generous sentiments are taken, almost verbatim, from a
letter of Mr. Buddle—“a person,” says the biographer of our hero, “whose
extensive practical knowledge justly entitled him to be considered as the
highest authority on all subjects connected with the art of mining,” and
who was of great service to Davy in carrying out his invention of the
safety-lamp. That gentleman, writing to Dr. Paris, says, “Sir Humphry
Davy accompanied me into some of our fiery mines, to prove the efficacy
of his lamp. Nothing could be more gratifying than the result of the
experiments, as they inspired everybody with perfect confidence in the
security which his invention had afforded. Sir Humphry was delighted,
and I was overpowered with feelings of gratitude to the great genius
which had produced it. I felt, however,” continues Mr. Buddle, “that he
did not contemplate any pecuniary reward, and in a private conversation
I remonstrated with him on the subject. I said, ‘You might as well
have secured this invention by a patent, and received your five or ten
thousand a-year for it.’ The reply of this great and noble-minded man
was, ‘No, my good friend, I never thought of such a thing,’” etc. as
above given. “I expostulated,” adds Mr. Buddle, “saying, that his ideas
were much too philosophic and refined for the occasion. He replied,
‘I have enough for all my views and purposes; more wealth might be
troublesome,’” and so on, the remainder of the speech being nearly word
for word with that which young Humphry has been here made to deliver.
[24] Watt’s first patent was in the year 1769, and that for his double
engine was in 1781. Dr. Davy, in speaking of the Wherry Mine as being a
place of favourite resort with his brother during his youth, says, “The
steam-engine there (an invention,” he adds, “which had only a short time
before been perfected by Mr. Watt) was one of the earliest that had been
introduced into Cornwall.”
[25] W. Symington (according to Haydn’s Dictionary) made a passage on
the Forth and Clyde Canal in 1789. In 1807, Fulton started a steam-boat
in America, on the river Hudson. In 1812, steam-vessels first began
plying on the Clyde; but it was not till the year 1815 that the first
steam-vessel made its appearance on the Thames. This was a boat from
Glasgow; for it was only in that year the first steamer was built in
England. Ten years afterward (1825) Captain Johnston received £10,000 for
making the first steam voyage to India in the _Enterprise_. In the year
1852 there were 1227 steam-vessels belonging to the United Kingdom.
[26] Gas was first evolved from coal by Clayton, in 1739. Its application
to the purposes of illumination was first tried by Mr. Murdoch, in
Cornwall, in 1792. Ten years after this (in 1802) Bolton and Watt’s
foundry, at Birmingham, exhibited the first display of gas-lights during
the rejoicings for peace. The first manufactory permanently lighted by
gas was a cotton mill at Manchester—this was in 1805. Gas was first used
for lighting Pall Mall, in London, in 1809, and in 1814 it had become
general throughout the metropolis. The gas-pipes in and round London are
now said to be more than 1100 miles in length.
[27] Mail-coaches were first set up at Bristol, Aug. 2, 1784, and at the
end of 1785 they became general in England. This plan for the conveyance
of letters was the invention of Mr. Palmer, at Bath. The mails had
previously been conveyed by carts with a single horse, or by boys on
horseback. From the establishment of mail-coaches the prosperity of the
Post-office commenced. The year before their introduction the postal
revenue was only £146,000, and it ultimately increased to £2,500,000.
[28] See Sir John Herschel’s “Introduction to the Study of Natural
Philosophy.”
[29] These are Davy’s own words, being the concluding passage of his work
“On the Safety-Lamp.”
[30] Professor Challis, of the Cambridge University, calculated the
height of the bow of light proceeding from the aurora (seen at Cambridge,
March 9, 1847) to be 177 miles above the surface of the earth. The limits
of our own atmosphere are placed, by Sir John Herschel, at the 100th part
of the diameter of the globe, or, in round numbers, 80 miles above the
surface.
[31] To give a just idea of this fearful event it should be added, that,
at the most moderate calculation, 1300 human beings lost their lives
through the eruption; it likewise caused the death of 20,000 horses, 7000
horned cattle, and 130,000 sheep. The fisheries on the southern coast of
the island, moreover, were destroyed by it; and Iceland has not to this
day, it is said, recovered from the disastrous event of the year of the
eruption of Skaptaa Jokul.
[32] The note-books of Davy during the time of his apprenticeship have
been preserved, and show (says his brother) “the ardour with which he
entered upon his studies, and the extensive reach of his mind in the
various branches of knowledge which he proposed to pursue. One of them,
bearing the date of 1795, is,” (Dr. John Davy adds,) “on many accounts, a
literary curiosity. It is a small quarto, with parchment covers. Outside
one of the covers is the figure of an ancient lyre drawn with his pen,
and on the other an olive leaf encircling a lamp; as if,” remarks his
biographer, “_in anticipation of his great discovery of confining flame
in the safety-lamp_. At the commencement of it is the following plan of
study:
1. Theology;
Or Religion, Ethics, } { Taught by Nature.
or moral virtues, } { ” by Revelation.
2. Geography.
3. My Profession.
1. Botany.
2. Pharmacy.
3. Nosology.
4. Anatomy.
5. Surgery.
6. Chemistry.
4. Logic.
5. Language.
1. English.
2. French.
3. Latin.
4. Greek.
5. Italian.
6. Spanish.
7. Hebrew.
6. Physics.
1. The doctrines and properties of natural bodies.
2. Of the operations of nature.
3. Of the doctrines of fluids.
4. Of the properties of organised matter.
5. Of the organisation of matter.
6. Simple astronomy.
7. Mechanics.
8. Rhetoric and Oratory.
9. History and Chronology.
10. Mathematics.
To give some distinct idea of the bent of his studies at this time,”
continues his brother, “I shall briefly notice the principal topics
which appear in this MS. volume. It opens with ‘_Hints towards the
Investigation of Truth in Religious and Political Opinions_, composed
as they occurred, to be placed in a more regular manner hereafter.’ His
first essay is ‘_On the Immortality and Immateriality of the Soul_;’ the
second bears the title of ‘_Body, organised matter_;’ and his third is
‘_On Governments_.’ Then there follow a variety of essays on metaphysical
and moral subjects. These topics occupy more than one half of the book;
the other part, which appears to have been written after, commences
at the opposite end, inverted. This is devoted partly to religious
essays; but besides these,” adds Dr. Davy, “there are some verses and
the beginning of a romance, called ‘_An Idyl_,’ which is in the form of
dialogue, the characters being ‘TREVELIS a warrior and friend of Prince
Arthur, and MORROBIN a Druid;’ the scene, ‘A cliff at the Land’s End in
Cornwall.’
“From the same source of information—his note-books—it appears, that in
the beginning of the year 1796 he entered on the study of mathematics.
One book is almost entirely confined to this subject, and he seems to
have finished the elementary course in more than twelve months, when he
was commencing the eleventh book of Euclid, having gone through most of
the other branches. He engaged in these studies without a master, and
perfectly voluntary on his part, from the conviction of their usefulness
preliminary to the study of chemical and physical sciences. His passion
for poetry at the same time appears to have kept pace with the expansion
of his faculties, and not to have been damped even by the application
to mathematics, for his early note-books contain many desultory verses.
His early chemical reading was confined to two works of a very different
description, ‘_Lavoisier’s Elements of Chemistry,_’ and ‘_Nicholson’s
Dictionary of Chemistry_.’ This new study seems very soon to have excited
in his mind a most lively interest. He was not satisfied with merely
reading and acquiring the ideas of others. He criticised the theoretical
views of the great French philosopher” (Lavoisier, the author of the
new Theory of Combustion, which was propounded only some few years
before); “he doubted, rejected, and advanced speculations of his own.
Speculation appears to have led him to experiment, and experiment to
further speculation, with such rapid progress that in a few months he had
formed a new hypothesis (concerning the principle of heat), and flattered
himself that he had triumphed over an important part of the doctrine of
the French school.
“Humphry Davy himself writes in one of his note-books, dated 1799,
‘About twenty months ago I began the study of Chemistry. The system of
Lavoisier, almost the only elementary book in my possession, was the
first that I studied.’” That the subject of heat was the first chemical
principle that engaged the boy’s attention we have further evidence
in the fact, that, a few years afterwards, his “_Researches on Heat
and Light_” were published in the form of essays, in a miscellaneous
volume edited by Dr. Beddoes. In the preface to this work the editor
says, referring to the views propounded by Davy, ‘It is not necessary,
in praise or excuse of his system, to add that at the time the theory
was formed the author was under twenty years of age, pupil to a
surgeon-apothecary in the most remote town of Cornwall, with little
access to philosophical books, and none at all to philosophical men.’”
[33] A lucifer match may be conveniently employed for the same purpose,
and it will be found that the match, by means of the cone, may be
inflamed at a greater distance from the fire than it could possibly be
without it.
[34] Young gentlemen of an experimental turn are cautioned against
attempting the same feat; for, should there be the least part of the
metal, at the bottom of the kettle, left unprotected by the soot, they
will assuredly experience considerable pain.
[35] Evaporation goes on more or less rapidly at all temperatures.
[36] Dr. Paris, in his “Life of Davy” (p. 37), relates the following
anecdote concerning the construction of this apparatus: “A French vessel
having been wrecked off the Land’s End, the surgeon escaped, and found
his way to Penzance. Accident brought him acquainted with Humphry Davy,
who showed him many civilities, and in return received, as a present from
the surgeon, a case of instruments which had been saved from the ship.
The contents were eagerly turned out and examined by the young chemist;
not, however, with any professional view as to their utility, but in
order to ascertain how far they might be convertible to experimental
purposes. The old-fashioned and clumsy glyster apparatus was viewed with
exultation, and seized in triumph. What reverses may not be suddenly
effected by a simple accident! So says the moralist. Reader, behold an
illustration: in the brief space of an hour did this long-neglected and
unobtrusive machine, emerging from its obscurity and insignificance,
figure away in all the pomp of a complicated piece of pneumatic
apparatus. Nor did its fortunes end here; it was destined for greater
things; and we shall hereafter learn that it actually performed the
duties of an air-pump in an original experiment on the nature and sources
of heat.” It is but right to add, that Dr. Davy doubts the truth of the
above story.
[37] The highest temperature of a good blast-furnace is, according to
Daniel, about equal to 3300°. This constitutes a “high white heat.”
[38] The above is Dr. Davy’s account of his brother’s first chemical
studies: “Such was the commencement of Humphry Davy’s career of
original research,” he adds, “which in a few years, by a succession
of discoveries, accomplished more in relation to change of theory and
extension of science than, in the most ardent and ambitious moments of
youth, he could either have hoped to effect or imagined possible.”
[39] A chemical philosopher in America ignited a piece of paper by one of
these lights on a still night. The breath, however, had to be held during
the operation, for the least agitation of the air wafted them from the
spot.
[40] A melancholy proof of this was furnished by the death of Mr. Hennell
(the chemist at Apothecaries’ Hall) on the 4th June, 1842. He was in
the act of mixing two separate portions of the powder in a moist state
with an ivory knife, when the whole quantity, amounting to above 6 lbs.
exploded, and shattered his head, breast, and right arm to atoms. A man,
however, who was standing within four yards of him, was not injured,
but the windows of the surrounding buildings were broken; while a large
wooden block, upon which one of the basins was placed, was shivered, as
was also the pavement on which it stood.
[41] In the ordinary lucifer-match (an invention since Davy’s time) we
have a striking illustration of the different temperatures required for
various substances to enter into combustion. The phosphorus with which
these lucifers are tipped becomes inflamed at a very low heat (60°),
so that mere friction is sufficient to ignite it. This substance, in
burning, produces heat sufficient to kindle the sulphur next to it; for
this, as we have seen, enters into combustion at about 500°; and the
sulphur again, in burning, raises the temperature sufficient to ignite
the wood, which requires a heat of at least 1000° before it can be made
to burn. Had the match been tipped with phosphorus alone, the phosphorus
acid produced by the combustion would have incrusted the wood and
prevented it inflaming. Before the use of phosphorus matches had become
general, others were introduced, the tips of which were coated with a
mixture of chlorate of potash and sulphur, and these had to be drawn
forcibly through a piece of folded sand-paper, for this mixture requires
a much higher heat in order to inflame it. Previous to the introduction
of these, again, the common brimstone match was in general use. This,
as is well known, was kindled by means of a spark in tinder—the tinder
consisting of charred rag, and the spark, therefore, being merely a
particle of charcoal at a red heat, while the ignition of the tinder
itself was originally produced by the percussion of a piece of flint and
steel, which evolved so much heat that small splinters of the metal were
fused by it and fell upon the tinder in a red-hot state.
[42] Chlorine gas was called “_oxy-muriatic acid_” when Davy was a boy,
from being supposed to be a compound of _oxygen_ and _muriatic acid
gas_. Davy, however, afterwards proved that this same oxy-muriatic acid
contained no oxygen whatever, and was rather an elementary substance,
being incapable of being resolved into any two other bodies.
[43] A piece of blotting-paper dipped in _oil of turpentine_, and
introduced into a jar of _chlorine gas_, immediately becomes inflamed.
[44] A few _flowers_ have also the property of emitting light at ordinary
temperatures. Among these may be cited the _tube-rose_, _nasturtium_,
and _marigold_, which occasionally give out flashes of light on a warm
summer’s evening. This, probably, arises from the combination of the
oxygen of the atmosphere with the vapour of the volatile oils upon which
the perfumes of flowers are known to depend. Again, other substances
emit light during the act of _crystallization_. This phenomenon is most
distinctly observed during the gradual deposit of arsenious acid, when
dissolved in hot hydrochloric acid (spirits of salts). In a dark place
each crystal, as it is formed, may be seen to emit a spark of light; and
on shaking the flask, so many crystals are sometimes suddenly produced,
that vivid flashes become perceptible. The cause of this phenomenon is
probably dependent on the fixation of oxygen by the arsenic at the moment
of precipitation.
[45] The blue colour of the oxy-hydrogen flame is due, probably, to the
particles of water formed by the combustion of the two gases.
[46] Dr. Paris’s “Life of Sir Humphry Davy,” p. 352.
[47] “Edinburgh Review,” January, 1816.
[48] “It will hereafter be scarcely credited,” says Dr. Paris, “that
an invention so eminently philosophic, and which could never have been
derived but from the sterling treasury of science, should have been
claimed in behalf of an enginewright of Killingworth colliery, of the
name of Stephenson; _a person not even professing a knowledge of the
elements of chemistry_.” The “enginewright” here sneered at for his
ignorance, ultimately rose to be the great George Stephenson, the
inventor of the locomotive. How would the fashionable physician speak
of the quondam enginewright of Killingworth colliery now! Stephenson’s
lamp was formed on the principle of admitting the fire-damp by narrow
tubes, and “in such small detached portions, that it would be consumed by
combustion.” The two lamps were, doubtlessly, distinct inventions, though
Davy, in all justice, appears to be entitled to precedence—not only in
point of date, but as regards the long chain of inductive reasoning
concerning the nature of flame by which his result was arrived at.
[49] “Davy’s passion for angling betrayed itself upon all occasions,”
says Dr. Paris; “whenever I had the honour of dining at his table,
the conversation, however it might have commenced, invariably ended
on fishing; and when a brother of the angle happened to be present,
you had the pleasure of hearing all his encounters with the finny
tribe—how he had lured them by his treachery, and vanquished them by
his perseverance. He would occasionally strike into a most eloquent and
impassioned strain upon some subject which warmed his fancy; such, for
example, as the beauties of mountain scenery: but before you could fully
enjoy the prospect which his imagination had pictured, down he carried
you into some sparkling stream, or rapid current, to flounder for the
next half-hour with a hooked salmon.... Nothing irritated him so much as
to find that his companions had caught more fish than himself, and if,
during conversation, a brother fisherman surpassed him in the relation of
his success, he betrayed similar impatience.”
[50] See Dr. Paris’s description of Davy’s ordinary fishing costume, p.
189.
[51] “This favourite dog is well remembered in Penzance,” says Davy’s
brother. “My sister writes,” he adds, “that ‘on his first return from
Bristol, after an absence of about twelve months, Chloe did not remember
him, till he called her by name, and then she was in a transport of joy.’
Her descendants are now numerous in the Mount’s Bay, and prized for good
qualities.” Vol. i. p. 54.
[52] See Dr. Paris’s “Life of Davy,” p. 12.
[53] See Dr. Paris’s account of Davy’s first interview with Gregory Watt,
p. 35.
[54] For the sake of simplicity, the nitric acid is here made to consist
(according to the new theory) of NO₆+H (instead of NO₅+HO), and so to
combine directly with the metal as NO₆+Ag (instead of with the oxide as
NO₅+OAg).
[55] The range of the eye, or diameter of the field of vision, is
110°; consequently, this is the largest angle under which an object
can be seen. The largest angle, however, is here made 120°, for the
simplification of the numbers. The range of vision is from 110° to 1′.
[56] See illustration at p. 378.
[57] The arrangement of lenses, above described, constitutes the
principle of what is termed the “_astronomical_ telescope;” for this
makes the objects appear upside down. But, though the inversion of a
star or planet is a matter of no moment in astronomical observation,
such an effect is most disagreeable when applied to terrestrial objects.
The ordinary telescope for _land_ purposes, therefore—or the “_day_
telescope,” as it is usually styled—has two other lenses behind the
eye-lens. These lenses have both the same focus as the eye-lens itself,
and are placed at a fixed distance from each other, such distance being
equal to the sum of their focal lengths: that is to say, if the eye-lens
have a focus of 2 inches, then each of the two other glasses should have
the same length of focus, and be placed at 4 (2 + 2) inches apart from
one another. The magnifying power of the day telescope may be calculated
in the same manner as that of the astronomical one above explained; for
the two additional lenses in the day instruments, having the same focal
length as the eye-lens itself, produce no further enlargement of the
objects, but serve only to cross the rays a second time, and so to render
the image _erect_ instead of _inverted_.
[58] Within the last 30 years the diamond has been used for the purpose
of microscopic lenses; for, owing to the refractive power of this
precious stone being greater than almost any other known substance,
and nearly double that of glass, lenses can be produced from it of a
great degree of magnifying power, and that with a comparatively small
curvature, so that increased distinctness is obtained; while the lens
itself, being nearly “_achromatic_,” the image produced by it is untinged
by prismatic colours. Mr. Pritchard constructed the first diamond
microscope in 1826. The diamond lens of this was double convex, and
had a focus of ¹⁄₃₀th of an inch, so that its magnifying power was 150
times. Dr. Goring, an eminent authority on the subject, says—“I conceive
diamond lenses to constitute the ultimatum of perfection in the single
microscope.” The sapphire has also been used for the construction of
microscopic lenses with considerable advantage, its magnifying power
being much greater than that of glass. Mr. Pritchard says, that the
sapphire, next to the diamond, possesses all qualities requisite for the
formation of a perfect magnifier, and presents less difficulties in the
construction.
INDEX.
Air, currents of, how caused, 186.
Air-pump, experiments with, 236.
Animal mechanism, wonders of, 88.
Argand-burners, why superior in brilliancy, 300.
Artificial light dependent on heat, 297.
Bath wells, temperature of the, 125.
Blow-pipe, cause of the increased heat produced by the, 300.
Brocken, spectre of the, in the Hartz mountains, 380;
philosophy of, 383.
Camera, images produced in, 342;
why reversed, 372;
how the lens intensifies images in the, 377;
cause of variation in the size of objects in, 378;
pictures must be projected on an opaque body, 380.
Cellini, Benvenuto, and the necromancer, 417.
Chlorine, experiments with, 279.
Coal, power concentrated in, 110.
Coal-mine, destructive explosion in, 94.
Colours, curious result from mixing in certain proportions, 425.
Combustion, laws of, 113;
phenomena of, 245;
nature of, 250;
experiments in, 247;
philosophy of, 284.
Combustion, spontaneous, 262, 272.
_Corpse candles_, philosophy of, 261.
Creation, the wondrous story of, 116.
Daguerreotype process of photography, 436.
Dew, deposition of, 144;
less plentiful in cloudy weather, 146;
its laws, 147.
Drummond light, the, 296.
Earthquakes, 126.
Electric light, 296.
Ether, its powers of vaporization, 224.
Explosive substances, 261, 264, 266.
Eye, the, wonderful construction of, 87.
Fire-damp not inflammable by red heat, 273;
rapidly explodes at white heat, 274;
explosible only when mixed with atmospheric air, 303.
Flame, subterranean, 125.
Flame, nature of, 285, 293;
of a candle hollow, 298;
experiments with, 301.
Fluids, expansive power of, 202.
Fulminates of the precious metals, 267;
gold, 268;
mercury, 268;
silver, 269;
platinum, 270.
Gas, its first application to illumination, 109.
Glass _absorbs artificial_, but transmits _solar_ heat, 159.
Hastings, French coast sometimes visible from, by refraction, 358.
Heat, natural sources of, 116;
celestial, 118;
subterraneous, 120-124;
mechanical production of, 129;
chemical, 130;
combustion, 132;
respiration, 131;
communication of, 134;
radiation, 137;
reflexion, 148;
difference between _solar_ and _terrestrial_, 158;
transmission of, 158-161;
absorption of, 163;
degrees of, in the spectrum, 162;
relative absorbing and radiating powers of surfaces, 165;
radiation by different colours, 166;
_solar_ more powerful reflected than direct, 168;
conduction, 169;
wonderful effects of, 193;
expansive power of, 196;
latent, 212;
white, 244;
artificial, curious changes in its character at high rates of
temperature, 244;
then assumes all the properties of solar, 244.
Hot-springs and wells, 125.
_Ignes fatui_, how produced, 260.
_Jack-o’-lanterns_, causes of, 260.
Land’s End described, 41.
Lens, magnifying power of the, how determined, 391.
Light, electric, 296;
artificial, dependent on heat, 297;
rays of, travel in straight lines, 344, 419;
refraction of, 350, 357;
experiments, 351;
rays of, assume the colour of objects from which they are
reflected, 367;
reflexion of, 407;
compound nature of a ray of, 422;
composed of seven colours, 423.
Lightning, varieties of, 128.
Liquids imperfect conductors of heat, 177;
merely solids whose particles are kept apart by heat, 212.
Lucifer-matches, why so readily inflammable, 272.
Luminosity, temperature at which bodies assume, 243.
Magician’s mirror, the, explained, 412.
Mail-coaches, the first, 109.
Metals, their relative power of conducting heat, 172;
expansion and contraction of, 197;
practical application of this power, 198;
cooling of, 239.
Microscope, principle of the, 395, 401;
the single, 397;
the compound, 400.
_Mirage_, an optical illusion caused by refraction, 357.
Mirrors, concave, experiments with, 148;
wonders produced by, 413;
different effects produced by metallic and glass, 154.
Mont Blanc, ascent of, 110;
ebullition on summit of, 230.
Objects, why they diminish in size in proportion to distance, 386;
magnifying of, by lenses, 388.
Oceanic currents, direction of the, 190.
Oils, lamp, philosophy of the combustion of, 275, 295.
Palissy, Bernard, his discoveries in pottery, 333.
Pendulum, compensation, principle of the, 200.
Phlogiston, an imaginary principle, 245.
Photography, first experiments in, 335, 433;
practice of, 435.
Prism, the spectrum produced by, merely an oblong figure of the sun,
421;
its colours the decomposition of the sunbeam into its elementary
tints, 422.
Pyramids of Egypt, size of the, 110.
Pyrophorus, how produced, 261.
Radiation, power of, in different substances, 142.
Ramsgate, Dover Castle rendered visible from, by refraction, 360.
Refraction of light, curious property of, 357;
illusions caused by extraordinary instances of, 358, 360.
Respiration, philosophy of, 86.
Safety-lamp, first glimmer of the, 93;
experiments with, 285;
completion of the first, 305;
its perfected form, 311;
value and importance of the invention, 303.
St. Michael’s Mount, Cornwall, 73.
Science, true, its nature, 87.
Scoresby’s, Captain, observation of a distant ship by refraction, 365.
Secretion, marvels of, 85.
Spectrum, proportions of prismatic tints in the, 426;
circular, 427.
Spontaneous combustion, 262, 272.
Steam, difference of heat in high and low pressure, 178.
Steam-boat, the first, 107.
Sun-pictures, the earliest, 335.
Talbot, Mr. Fox, his curious experiment of photographing a rapidly
revolving wheel, 443.
Talbotype process of photography, 436.
Telescope, principle of the, 393, 400.
Temperature, rate of increase below the surface of the earth, 122.
Vaporizable liquids readily explosible, 274.
Vision, range of, 386.
Volcanoes, 126.
_Will-o’-the-wisp_, philosophy of, 260.
Wire-gauze, its power of resisting flame, 310.
THE END.
BOOKS BY THE ABBOTTS.
THE FRANCONIA STORIES.
By JACOB ABBOTT. In Ten Volumes. Beautifully Illustrated. 16mo, Cloth, 90
cents per Vol.; the set complete, in case, $9 00.
1. =Malleville.=
2. =Mary Bell.=
3. =Ellen Linn.=
4. =Wallace.=
5. =Beechnut.=
6. =Stuyvesant.=
7. =Agnes.=
8. =Mary Erskine.=
9. =Rodolphus.=
10. =Caroline.=
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Marco Paul’s Voyages and Travels in the Pursuit of Knowledge. By JACOB
ABBOTT. Beautifully Illustrated. Complete in 6 Volumes, 16mo, Cloth, 90
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=In New York.=
=On the Erie Canal.=
=In the Forests of Maine.=
=In Vermont.=
=In Boston.=
=At the Springfield Armory.=
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set complete, in case, $4 50.
=Handie.=
=Rainbow’s Journey.=
=The Three Pines.=
=Selling Lucky.=
=Up the River.=
YOUNG CHRISTIAN SERIES.
By JACOB ABBOTT. In Four Volumes. Richly Illustrated with Engravings, and
Beautifully Bound. 12mo, Cloth, $1 75 per Vol. The set complete, Cloth,
$7 00; in Half Calf, $14 00.
=1. The Young Christian.=
=2. The Corner Stone.=
=3. The Way to Do Good.=
=4. Hoaryhead and M’Donner.=
HARPER’S STORY BOOKS.
A Series of Narratives, Biographies, and Tales, for the Instruction and
Entertainment of the Young. By JACOB ABBOTT. Embellished with more than
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“HARPER’S STORY BOOKS” can be obtained complete in Twelve Volumes, bound
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VOL. I.
=BRUNO=; or, Lessons of Fidelity, Patience, and Self-Denial Taught by a
Dog.
=WILLIE AND THE MORTGAGE=: showing How Much may be Accomplished by a Boy.
=THE STRAIT GATE=; or, The Rule of Exclusion from Heaven.
VOL. II.
=THE LITTLE LOUVRE=; or, The Boys’ and Girls’ Picture-Gallery.
=PRANK=; or, The Philosophy of Tricks and Mischief.
=EMMA=; or, The Three Misfortunes of a Belle.
VOL. III.
=VIRGINIA=; or, A Little Light on a Very Dark Saying.
=TIMBOO AND JOLIBA=; or, The Art of Being Useful.
=TIMBOO AND FANNY=; or, The Art of Self-Instruction.
VOL. IV.
=THE HARPER ESTABLISHMENT=; or, How the Story Books are Made.
=FRANKLIN=, the Apprentice-Boy.
=THE STUDIO=; or, Illustrations of the Theory and Practice of Drawing,
for Young Artists at Home.
VOL. V.
=THE STORY OF ANCIENT HISTORY=, from the Earliest Periods to the Fall of
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Revolution.
=THE STORY OF AMERICAN HISTORY=, from the Earliest Settlement of the
Country to the Establishment of the Federal Constitution.
VOL. VI.
=JOHN TRUE=; or, The Christian Experience of an Honest Boy.
=ELFRED=; or, The Blind Boy and his Pictures.
=THE MUSEUM=; or, Curiosities Explained.
VOL. VII.
=THE ENGINEER=; or, How to Travel in the Woods.
=RAMBLES AMONG THE ALPS.=
=THE THREE GOLD DOLLARS=; or, An Account of the Adventures of Robin Green.
VOL. VIII.
=THE GIBRALTAR GALLERY=: being an Account of various Things both Curious
and Useful.
=THE ALCOVE=: containing some Farther Account of Timboo, Mark, and Fanny.
=DIALOGUES= for the Amusement and Instruction of Young Persons.
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=THE GREAT ELM=; or, Robin Green and Josiah Lane at School.
=AUNT MARGARET=; or, How John True kept his Resolutions.
=VERNON=; or, Conversations about Old Times in England.
VOL. X.
=CARL AND JOCKO=; or, The Adventures of the Little Italian Boy and his
Monkey.
=LAPSTONE=; or, The Sailor turned Shoemaker.
=ORKNEY, THE PEACEMAKER=; or, The Various Ways of Settling Disputes.
VOL. XI.
=JUDGE JUSTIN=; or, The Little Court of Morningdale.
=MINIGO=; or, The Fairy of Cairnstone Abbey.
=JASPER=; or, The Spoiled Child Recovered.
VOL. XII.
=CONGO=; or, Jasper’s Experience in Command.
=VIOLA= and her Little Brother Arno.
=LITTLE PAUL=; or, How to be Patient in Sickness and Pain.
Some of the Story Books are written particularly for girls, and some for
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The successive volumes of the series, though they each contain
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present age back to the remotest times.
The narratives are intended to be succinct and comprehensive,
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them I am indebted for about all the historical knowledge I have.”_Project Gutenberg
The wonders of science : $b Or, young Humphry Davy (the Cornish apothecary's boy, who taught himself natural philosophy, and eventually became President of the Royal Society)
Mayhew, Henry