THE CHEMICAL
HISTORY OF A CANDLE
A COURSE OF LECTURES DELIVERED BEFORE A JUVENILE
AUDIENCE AT THE ROYAL INSTITUTION
BY
MICHAEL FARADAY, D.C.L., F.R.S.
EDITED BY
WILLIAM CROOKES, F.C.S.
A NEW IMPRESSION, WITH ILLUSTRATIONS
LONDON
CHATTO & WINDUS
1908
PREFACE
From the primitive pine-torch to the paraffin candle, how wide an
interval! between them how vast a contrast! The means adopted by man to
illuminate his home at night, stamp at once his position in the scale of
civilisation. The fluid bitumen of the far East, blazing in rude vessels
of baked earth; the Etruscan lamp, exquisite in form, yet ill adapted to
its office; the whale, seal, or bear fat, filling the hut of the Esquimaux
or Lap with odour rather than light; the huge wax candle on the glittering
altar, the range of gas lamps in our streets,—all have their stories to
tell. All, if they could speak (and, after their own manner, they can),
might warm our hearts in telling, how they have ministered to man’s
comfort, love of home, toil, and devotion.
Surely, among the millions of fire-worshippers and fire-users who have
passed away in earlier ages, _some_ have pondered over the mystery of
fire; perhaps some clear minds have guessed shrewdly near the truth. Think
of the time man has lived in hopeless ignorance: think that only during a
period which might be spanned by the life of one man, has the truth been
known.
Atom by atom, link by link, has the reasoning chain been forged. Some
links, too quickly and too slightly made, have given way, and been
replaced by better work; but now the great phenomena are known—the
outline is correctly and firmly drawn—cunning artists are filling in the
rest, and the child who masters these Lectures knows more of fire than
Aristotle did.
The candle itself is now made to light up the dark places of nature; the
blowpipe and the prism are adding to our knowledge of the earth’s crust;
but the torch must come first.
Among the readers of this book some few may devote themselves to
increasing the stores of knowledge: the Lamp of Science _must_ burn.
“_Alere flammam._”
W. CROOKES.
CONTENTS.
LECTURE I.
A CANDLE: THE FLAME—ITS SOURCES—STRUCTURE—MOBILITY—BRIGHTNESS
LECTURE II.
BRIGHTNESS OF THE FLAME—AIR NECESSARY FOR COMBUSTION—PRODUCTION OF WATER
LECTURE III.
PRODUCTS: WATER FROM THE COMBUSTION—NATURE OF WATER—A COMPOUND—HYDROGEN
LECTURE IV.
HYDROGEN IN THE CANDLE—BURNS INTO WATER—THE OTHER PART OF WATER—OXYGEN
LECTURE V.
OXYGEN PRESENT IN THE AIR—NATURE OF THE ATMOSPHERE—ITS PROPERTIES—OTHER
PRODUCTS FROM THE CANDLE—CARBONIC ACID—ITS PROPERTIES
LECTURE VI.
CARBON OR CHARCOAL—COAL-GAS—RESPIRATION AND ITS ANALOGY TO THE BURNING
OP A CANDLE—CONCLUSION
LECTURE ON PLATINUM.
NOTES.
THE CHEMICAL HISTORY OF A CANDLE
LECTURE I.
A CANDLE: THE FLAME—ITS SOURCES—STRUCTURE—MOBILITY—BRIGHTNESS.
I purpose, in return for the honour you do us by coming to see what are
our proceedings here, to bring before you, in the course of these
lectures, the Chemical History of a Candle. I have taken this subject on a
former occasion; and were it left to my own will, I should prefer to
repeat it almost every year—so abundant is the interest that attaches
itself to the subject, so wonderful are the varieties of outlet which it
offers into the various departments of philosophy. There is not a law
under which any part of this universe is governed which does not come into
play, and is touched upon in these phenomena. There is no better, there is
no more open door by which you can enter into the study of natural
philosophy, than by considering the physical phenomena of a candle. I
trust, therefore, I shall not disappoint you in choosing this for my
subject rather than any newer topic, which could not be better, were it
even so good.
And before proceeding, let me say this also—that though our subject be so
great, and our intention that of treating it honestly, seriously, and
philosophically, yet I mean to pass away from all those who are seniors
amongst us. I claim the privilege of speaking to juveniles as a juvenile
myself. I have done so on former occasions—and, if you please, I shall do
so again. And though I stand here with the knowledge of having the words I
utter given to the world, yet that shall not deter me from speaking in the
same familiar way to those whom I esteem nearest to me on this occasion.
And now, my boys and girls, I must first tell you of what candles are
made. Some are great curiosities. I have here some bits of timber,
branches of trees particularly famous for their burning. And here you see
a piece of that very curious substance taken out of some of the bogs in
Ireland, called _candle-wood_,—a hard, strong, excellent wood, evidently
fitted for good work as a resister of force, and yet withal burning so
well that where it is found they make splinters of it, and torches, since
it burns like a candle, and gives a very good light indeed. And in this
wood we have one of the most beautiful illustrations of the general nature
of a candle that I can possibly give. The fuel provided, the means of
bringing that fuel to the place of chemical action, the regular and
gradual supply of air to that place of action—heat and light—all
produced by a little piece of wood of this kind, forming, in fact, a
natural candle.
But we must speak of candles as they are in commerce. Here are a couple of
candles commonly called dips. They are made of lengths of cotton cut off,
hung up by a loop, dipped into melted tallow, taken out again and cooled,
then re-dipped until there is an accumulation of tallow round the cotton.
In order that you may have an idea of the various characters of these
candles, you see these which I hold in my hand—they are very small, and
very curious. They are, or were, the candles used by the miners in coal
mines. In olden times the miner had to find his own candles; and it was
supposed that a small candle would not so soon set fire to the fire-damp
in the coal mines as a large one; and for that reason, as well as for
economy’s sake, he had candles made of this sort—20, 30, 40, or 60 to the
pound. They have been replaced since then by the steel-mill, and then by
the Davy-lamp, and other safety-lamps of various kinds. I have here a
candle that was taken out of the _Royal George_[1], it is said, by Colonel
Pasley. It has been sunk in the sea for many years, subject to the action
of salt water. It shews you how well candles may be preserved; for though
it is cracked about and broken a good deal, yet, when lighted, it goes on
burning regularly, and the tallow resumes its natural condition as soon as
it is fused.
Mr. Field, of Lambeth, has supplied me abundantly with beautiful
illustrations of the candle and its materials. I shall therefore now refer
to them. And, first, there is the suet—the fat of the ox—Russian tallow,
I believe, employed in the manufacture of these dips, which Gay Lussac, or
some one who entrusted him with his knowledge, converted into that
beautiful substance, stearin, which you see lying beside it. A candle, you
know, is not now a greasy thing like an ordinary tallow candle, but a
clean thing, and you may almost scrape off and pulverise the drops which
fall from it without soiling anything. This is the process he
adopted[2]:—The fat or tallow is first boiled with quick-lime, and made
into a soap, and then the soap is decomposed by sulphuric acid, which
takes away the lime, and leaves the fat re-arranged as stearic acid,
whilst a quantity of glycerin is produced at the same time.
Glycerin—absolutely a sugar, or a substance similar to sugar—comes out
of the tallow in this chemical change. The oil is then pressed out of it;
and you see here this series of pressed cakes, shewing how beautifully the
impurities are carried out by the oily part as the pressure goes on
increasing, and at last you have left that substance which is melted, and
cast into candles as here represented. The candle I have in my hand is a
stearin candle, made of stearin from tallow in the way I have told you.
Then here is a sperm candle, which comes from the purified oil of the
spermaceti whale. Here also are yellow bees-wax and refined bees-wax, from
which candles are made. Here, too, is that curious substance called
paraffin, and some paraffin candles made of paraffin obtained from the
bogs of Ireland. I have here also a substance brought from Japan, since we
have forced an entrance into that out-of-the-way place—a sort of wax
which a kind friend has sent me, and which forms a new material for the
manufacture of candles.
And how are these candles made? I have told you about dips, and I will
shew you how moulds are made. Let us imagine any of these candles to be
made of materials which can be cast. “Cast!” you say. “Why, a candle is a
thing that melts; and surely if you can melt it, you can cast it.” Not so.
It is wonderful, in the progress of manufacture, and in the consideration
of the means best fitted to produce the required result, how things turn
up which one would not expect beforehand. Candles cannot always be cast. A
wax candle can never be cast. It is made by a particular process, which I
can illustrate in a minute or two: but I must not spend much time on it.
Wax is a thing which, burning so well, and melting so easily in a candle,
cannot be cast. However, let us take a material that can be cast. Here is
a frame, with a number of moulds fastened in it. The first thing to be
done is to put a wick through them. Here is one—a plaited wick, which
does not require snuffing[3]—supported by a little wire. It goes to the
bottom, where it is pegged in—the little peg holding the cotton tight,
and stopping the aperture, so that nothing fluid shall run out. At the
upper part there is a little bar placed across, which stretches the cotton
and holds it in the mould. The tallow is then melted, and the moulds are
filled. After a certain time, when the moulds are cool, the excess of
tallow is poured off at one corner, and then cleaned off altogether, and
the ends of the wick cut away. The candles alone then remain in the mould,
and you have only to upset them, as I am doing, when out they tumble, for
the candles are made in the form of cones, being narrower at the top than
at the bottom; so that what with their form and their own shrinking, they
only need a little shaking, and out they fall. In the same way are made
these candles of stearin and of paraffin. It is a curious thing to see how
wax candles are made. A lot of cottons are hung upon frames, as you see
here, and covered with metal tags at the ends to keep the wax from
covering the cotton in those places. These are carried to a heater, where
the wax is melted. As you see, the frames can turn round; and as they
turn, a man takes a vessel of wax and pours it first down one, and then
the next and the next, and so on. When he has gone once round, if it is
sufficiently cool, he gives the first a second coat, and so on until they
are all of the required thickness. When they have been thus clothed, or
fed, or made up to that thickness, they are taken off, and placed
elsewhere. I have here, by the kindness of Mr. Field, several specimens of
these candles. Here is one only half-finished. They are then taken down,
and well rolled upon a fine stone slab, and the conical top is moulded by
properly shaped tubes, and the bottoms cut off and trimmed. This is done
so beautifully that they can make candles in this way weighing exactly
four, or six, to the pound, or any number they please.
We must not, however, take up more time about the mere manufacture, but go
a little further into the matter. I have not yet referred you to luxuries
in candles (for there is such a thing as luxury in candles). See how
beautifully these are coloured: you see here mauve, magenta, and all the
chemical colours recently introduced, applied to candles. You observe,
also, different forms employed. Here is a fluted pillar most beautifully
shaped; and I have also here some candles sent me by Mr. Pearsall, which
are ornamented with designs upon them, so that as they burn you have as it
were a glowing sun above, and a bouquet of flowers beneath. All, however,
that is fine and beautiful is not useful. These fluted candles, pretty as
they are, are bad candles; they are bad because of their external shape.
Nevertheless, I shew you these specimens sent to me from kind friends on
all sides, that you may see what is done, and what may be done in this or
that direction; although, as I have said, when we come to these
refinements, we are obliged to sacrifice a little in utility.
Now, as to the light of the candle. We will light one or two, and set them
at work in the performance of their proper functions. You observe a candle
is a very different thing from a lamp. With a lamp you take a little oil,
fill your vessel, put in a little moss or some cotton prepared by
artificial means, and then light the top of the wick. When the flame runs
down the cotton to the oil, it gets extinguished, but it goes on burning
in the part above. Now, I have no doubt you will ask, how is it that the
oil, which will not burn of itself, gets up to the top of the cotton,
where it will burn? We shall presently examine that; but there is a much
more wonderful thing about the burning of a candle than this. You have
here a solid substance with no vessel to contain it; and how is it that
this solid substance can get up to the place where the flame is? How is it
that this solid gets there, it not being a fluid? or, when it is made a
fluid, then how is it that it keeps together? This is a wonderful thing
about a candle.
We have here a good deal of wind, which will help us in some of our
illustrations, but tease us in others; for the sake, therefore, of a
little regularity, and to simplify the matter, I shall make a quiet
flame—for who can study a subject when there are difficulties in the way
not belonging to it? Here is a clever invention of some costermonger or
street stander in the market-place for the shading of their candles on
Saturday nights, when they are selling their greens, or potatoes, or fish.
I have very often admired it. They put a lamp-glass round the candle,
supported on a kind of gallery, which clasps it, and it can be slipped up
and down as required. By the use of this lamp-glass, employed in the same
way, you have a steady flame, which you can look at, and carefully
examine, as I hope you will do, at home.
You see, then, in the first instance, that a beautiful cup is formed. As
the air comes to the candle it moves upwards by the force of the current
which the heat of the candle produces, and it so cools all the sides of
the wax, tallow, or fuel, as to keep the edge much cooler than the part
within; the part within melts by the flame that runs down the wick as far
as it can go before it is extinguished, but the part on the outside does
not melt. If I made a current in one direction, my cup would be lop-sided,
and the fluid would consequently run over,—for the same force of gravity
which holds worlds together holds this fluid in a horizontal position, and
if the cup be not horizontal, of course the fluid will run away in
guttering. You see, therefore, that the cup is formed by this beautifully
regular ascending current of air playing upon all sides, which keeps the
exterior of the candle cool. No fuel would serve for a candle which has
not the property of giving this cup, except such fuel as the Irish
bogwood, where the material itself is like a sponge, and holds its own
fuel. You see now why you would have had such a bad result if you were to
burn these beautiful candles that I have shewn you, which are irregular,
intermittent in their shape, and cannot therefore have that nicely-formed
edge to the cup which is the great beauty in a candle. I hope you will now
see that the perfection of a process—that is, its utility—is the better
point of beauty about it. It is not the best looking thing, but the best
acting thing, which is the most advantageous to us. This good-looking
candle is a bad burning one. There will be a guttering round about it
because of the irregularity of the stream of air and the badness of the
cup which is formed thereby. You may see some pretty examples (and I trust
you will notice these instances) of the action of the ascending current
when you have A little gutter run down the side of a candle, making it
thicker there than it is elsewhere. As the candle goes on burning, that
keeps its place and forms a little pillar sticking up by the side,
because, as it rises higher above the rest of the wax or fuel, the air
gets better round it, and it is more cooled and better able to resist the
action of the heat at a little distance. Now, the greatest mistakes and
faults with regard to candles, as in many other things, often bring with
them instruction which we should not receive if they had not occurred. We
come here to be philosophers; and I hope you will always remember that
whenever a result happens, especially if it be new, you should say, “What
is the cause? Why does it occur?” and you will in the course of time find
out the reason.
Then, there is another point about these candles which will answer a
question,—that is, as to the way in which this fluid gets out of the cup,
up the wick, and into the place of combustion. You know that the flames on
these burning wicks in candles made of bees-wax, stearin, or spermaceti, do
not run down to the wax or other matter, and melt it all away, but keep to
their own right place. They are fenced off from the fluid below, and do
not encroach on the cup at the sides. I cannot imagine a more beautiful
example than the condition of adjustment under which a candle makes one
part subserve to the other to the very end of its action. A combustible
thing like that, burning away gradually, never being intruded upon by the
flame, is a very beautiful sight; especially when you come to learn what a
vigorous thing flame is—what power it has of destroying the wax itself
when it gets hold of it, and of disturbing its proper form if it come only
too near.
But how does the flame get hold of the fuel? There is a beautiful point
about that—_capillary attraction_[4]. “Capillary attraction!” you
say,—“the attraction of hairs.” Well, never mind the name: it was given
in old times, before we had a good understanding of what the real power
was. It is by what is called capillary attraction that the fuel is
conveyed to the part where combustion goes on, and is deposited there, not
in a careless way, but very beautifully in the very midst of the centre of
action which takes place around it. Now, I am going to give you one or two
instances of capillary attraction. It is that kind of action or attraction
which makes two things that do not dissolve in each other still hold
together. When you wash your hands, you wet them thoroughly; you take a
little soap to make the adhesion better, and you find your hand remains
wet. This is by that kind of attraction of which I am about to speak. And,
what is more, if your hands are not soiled (as they almost always are by
the usages of life), if you put your finger into a little warm water, the
water will creep a little way up the finger, though you may not stop to
examine it. I have here a substance which is rather porous—a column of
salt—and I will pour into the plate at the bottom, not water, as it
appears, but a saturated solution of salt which cannot absorb more; so
that the action which you see will not be due to its dissolving anything.
We may consider the plate to be the candle, and the salt the wick, and
this solution the melted tallow. (I have coloured the fluid, that you may
see the action better.) You observe that, now I pour in the fluid, it
rises and gradually creeps up the salt higher and higher; and provided the
column does not tumble over, it will go to the top.
[Illustration: Fig. 1.]
If this blue solution were combustible, and we were to place a wick at the
top of the salt, it would burn as it entered into the wick. It is a most
curious thing to see this kind of action taking place, and to observe how
singular some of the circumstances are about it. When you wash your hands,
you take a towel to wipe off the water; and it is by that kind of wetting,
or that kind of attraction which makes the towel become wet with water,
that the wick is made wet with the tallow. I have known some careless boys
and girls (indeed, I have known it happen to careful people as well) who,
having washed their hands and wiped them with a towel, have thrown the
towel over the side of the basin, and before long it has drawn all the
water out of the basin and conveyed it to the floor, because it happened
to be thrown over the side in such a way as to serve the purpose of a
syphon.[5] That you may the better see the way in which the substances act
one upon another, I have here a vessel made of wire gauze filled with
water, and you may compare it in its action to the cotton in one respect,
or to a piece of calico in the other. In fact, wicks are sometimes made of
a kind of wire gauze. You will observe that this vessel is a porous thing;
for if I pour a little water on to the top, it will run out at the bottom.
You would be puzzled for a good while if I asked you what the state of
this vessel is, what is inside it, and why it is there? The vessel is full
of water, and yet you see the water goes in and runs out as if it were
empty. In order to prove this to you, I have only to empty it. The reason
is this,—the wire, being once wetted, remains wet; the meshes are so
small that the fluid is attracted so strongly from the one side to the
other, as to remain in the vessel although it is porous. In like manner
the particles of melted tallow ascend the cotton and get to the top; other
particles then follow by their mutual attraction for each other, and as
they reach the flame they are gradually burned.
Here is another application of the same principle. You see this bit of
cane. I have seen boys about the streets, who are very anxious to appear
like men, take a piece of cane, and light it and smoke it, as an imitation
of a cigar. They are enabled to do so by the permeability of the cane in
one direction, and by its capillarity. If I place this piece of cane on a
plate containing some camphine (which is very much like paraffin in its
general character), exactly in the same manner as the blue fluid rose
through the salt will this fluid rise through the piece of cane. There
being no pores at the side, the fluid cannot go in that direction, but
must pass through its length. Already the fluid is at the top of the cane:
now I can light it and make it serve as a candle. The fluid has risen by
the capillary attraction of the piece of cane, just as it does through the
cotton in the candle.
Now, the only reason why the candle does not burn all down the side of the
wick is, that the melted tallow extinguishes the flame. You know that a
candle, if turned upside-down, so as to allow the fuel to run upon the
wick, will be put out. The reason is, that the flame has not had time to
make the fuel hot enough to burn, as it does above, where it is carried in
small quantities into the wick, and has all the effect of the heat
exercised upon it.
There is another condition which you must learn as regards the candle,
without which you would not be able fully to understand the philosophy of
it, and that is the vaporous condition of the fuel. In order that you may
understand that, let me shew you a very pretty, but very common-place
experiment. If you blow a candle out cleverly, you will see the vapour
rise from it. You have, I know, often smelt the vapour of a blown-out
candle—and a very bad smell it is; but if you blow it out cleverly, you
will be able to see pretty well the vapour into which this solid matter is
transformed. I will blow out one of these candles in such a way as not to
disturb the air around it, by the continuing action of my breath; and now,
if I hold a lighted taper two or three inches from the wick, you will
observe a train of fire going through the air till it reaches the candle.
I am obliged to be quick and ready, because, if I allow the vapour time to
cool, it becomes condensed into a liquid or solid, or the stream of
combustible matter gets disturbed.
Now, as to the shape or form of the flame. It concerns us much to know
about the condition which the matter of the candle finally assumes at the
top of the wick—where you have such beauty and brightness as nothing but
combustion or flame can produce.
[Illustration: Fig. 2.]
You have the glittering beauty of gold and silver, and the still higher
lustre of jewels, like the ruby and diamond; but none of these rival the
brilliancy and beauty of flame. What diamond can shine like flame? It owes
its lustre at night-time to the very flame shining upon it. The flame
shines in darkness, but the light which the diamond has is as nothing
until the flame shine upon it, when it is brilliant again. The candle
alone shines by itself, and for itself, or for those who have arranged the
materials. Now, let us look a little at the form of the flame as you see
it under the glass shade. It is steady and equal; and its general form is
that which is represented in the diagram, varying with atmospheric
disturbances, and also varying according to the size of the candle. It is
a bright oblong—brighter at the top than towards the bottom—with the
wick in the middle, and besides the wick in the middle, certain darker
parts towards the bottom, where the ignition is not so perfect as in the
part above.
[Illustration: Fig. 3.]
I have a drawing here, sketched many years ago by Hooke, when he made his
investigations. It is the drawing of the flame of a lamp, but it will
apply to the flame of a candle. The cup of the candle is the vessel or
lamp, the melted spermaceti is the oil, and the wick is common to both.
Upon that he sets this little flame, and then he represents what is
true—a certain quantity of matter rising about it which you do not see,
and which, if you have not been here before, or are not familiar with the
subject, you will not know of. He has here represented the parts of the
surrounding atmosphere that are very essential to the flame, and that are
always present with it. There is a current formed, which draws the flame
out—for the flame which you see is really drawn out by the current, and
drawn upward to a great height—just as Hooke has here shewn you by that
prolongation of the current in the diagram. You may see this by taking a
lighted candle, and putting it in the sun so as to get its shadow thrown
on a piece of paper. How remarkable it is that that thing which is light
enough to produce shadows of other objects, can be made to throw its own
shadow on a piece of white paper or card, so that you can actually see
streaming round the flame something which is not part of the flame, but is
ascending and drawing the flame upwards. Now, I am going to imitate the
sunlight, by applying the voltaic battery to the electric lamp. You now
see our sun, and its great luminosity; and by placing a candle between it
and the screen, we get the shadow of the flame.
[Illustration: Fig. 4.]
You observe the shadow of the candle and of the wick; then there is a
darkish part, as represented in the diagram, and then a part which is more
distinct. Curiously enough, however, what we see in the shadow as the
darkest part of the flame is, in reality, the brightest part; and here you
see streaming upwards the ascending current of hot air, as shewn by
Hooke, which draws out the flame, supplies it with air, and cools the
sides of the cup of melted fuel.
I can give you here a little further illustration, for the purpose of
shewing you how flame goes up or down; according to the current. I have
here a flame—it is not a candle-flame—but you can, no doubt, by this
time, generalise enough to be able to compare one thing with another. What
I am about to do is to change the ascending current that takes the flame
upwards into a descending current. This I can easily do by the little
apparatus you see before me. The flame, as I have said, is not a candle
flame, but it is produced by alcohol, so that it shall not smoke too much.
I will also colour the flame with another substance[6], so that you may
trace its course; for with the spirit alone you could hardly see well
enough to have the opportunity of tracing its direction. By lighting this
spirit-of-wine, we have then a flame produced; and you observe that when
held in the air, it naturally goes upwards.
[Illustration: Fig. 5]
You understand now easily enough why flames go up under ordinary
circumstances—it is because of the draught of air by which the combustion
is formed. But now, by blowing the flame down, you see I am enabled to
make it go downwards into this little chimney—the direction of the
current being changed. Before we have concluded this course of lectures,
we shall shew you a lamp in which the flame goes up and the smoke goes
down, or the flame goes down and the smoke goes up. You see, then, that we
have the power in this way of varying the flame in different directions.
There are now some other points that I must bring before you. Many of the
flames you see here vary very much in their shape by the currents of air
blowing around them in different directions; but we can, if we like, make
flames so that they will look like fixtures, and we can photograph
them—indeed, we have to photograph them—so that they become fixed to us,
if we wish to find out everything concerning them. That, however, is not
the only thing I wish to mention. If I take a flame sufficiently large, it
does not keep that homogeneous, that uniform condition of shape, but it
breaks out with a power of life which is quite wonderful. I am about to
use another kind of fuel, but one which is truly and fairly a
representative of the wax or tallow of a candle. I have here a large ball
of cotton, which will serve as a wick. And, now that I have immersed it in
spirit and applied a light to it, in what way does it differ from an
ordinary candle? Why, it differs very much in one respect, that we have a
vivacity and power about it, a beauty and a life entirely different from
the light presented by a candle. You see those fine tongues of flame
rising up. You have the same general disposition of the mass of the flame
from below upwards; but, in addition to that, you have this remarkable
breaking out into tongues which you do not perceive in the case of a
candle. Now, why is this? I must explain it to you, because when you
understand that perfectly, you will be able to follow me better in what I
have to say hereafter. I suppose some here will have made for themselves
the experiment I am going to shew you. Am I right in supposing that
anybody here has played at snapdragon? I do not know a more beautiful
illustration of the philosophy of flame, as to a certain part of its
history, than the game of snapdragon. First, here is the dish; and let me
say, that when you play snapdragon properly, you ought to have the dish
well-warmed; you ought also to have warm plums and warm brandy, which,
however, I have not got. When you have put the spirit into the dish, you
have the cup and the fuel; and are not the raisins acting like the wicks?
I now throw the plums into the dish, and light the spirit, and you see
those beautiful tongues of flame that I refer to. You have the air
creeping in over the edge of the dish forming these tongues. Why? Because,
through the force of the current and the irregularity of the action of the
flame, it cannot flow in one uniform stream. The air flows in so
irregularly that you have what would otherwise be a single image, broken
up into a variety of forms, and each of these little tongues has an
independent existence of its own. Indeed, I might say, you have here a
multitude of independent candles. You must not imagine, because you see
these tongues all at once, that the flame is of this particular shape. A
flame of that shape is never so at any one time. Never is a body of flame,
like that which you just saw rising from the ball, of the shape it appears
to you. It consists of a multitude of different shapes, succeeding each
other so fast that the eye is only able to take cognisance of them all at
once. In former times, I purposely analysed a flame of that general
character, and the diagram shews you the different parts of which it is
composed. They do not occur all at once: it is only because we see these
shapes in such rapid succession, that they seem to us to exist all at one
time.
[Illustration: Fig. 6.]
It is too bad that we have not got further than my game of snapdragon; but
we must not, under any circumstances, keep you beyond your time. It will
be a lesson to me in future to hold you more strictly to the philosophy of
the thing, than to take up your time so much with these illustrations.
LECTURE II.
A CANDLE: BRIGHTNESS OF THE FLAME—AIR NECESSARY FOR
COMBUSTION—PRODUCTION OF WATER.
We were occupied the last time we met in considering the general character
and arrangement as regards the fluid portion of a candle, and the way in
which that fluid got into the place of combustion. You see, when we have a
candle burning fairly in a regular, steady atmosphere, it will have a
shape something like the one shewn in the diagram, and will look pretty
uniform, although very curious in its character. And now, I have to ask
your attention to the means by which we are enabled to ascertain what
happens in any particular part of the flame—why it happens, what it does
in happening, and where, after all, the whole candle goes to: because, as
you know very well, a candle being brought before us and burned,
disappears, if burned properly, without the least trace of dirt in the
candlestick—and this is a very curious circumstance. In order, then, to
examine this candle carefully, I have arranged certain apparatus, the use
of which you will see as I go on. Here is a candle: I am about to put the
end of this glass tube into the middle of the flame—into that part which
old Hooke has represented in the diagram as being rather dark, and which
you can see at any time, if you will look at a candle carefully, without
blowing it about. We will examine this dark part first.
[Illustration: Fig. 7.]
Now, I take this bent glass tube, and introduce one end into that part of
the flame, and you see at once that something is coming from the flame,
out at the other end of the tube; and if I put a flask there, and leave it
for a little while, you will see that something from the middle part of
the flame is gradually drawn out, and goes through the tube and into that
flask, and there behaves very differently from what it does in the open
air. It not only escapes from the end of the tube, but falls down to the
bottom of the flask like a heavy substance, as indeed it is. We find that
this is the wax of the candle made into a vaporous fluid—not a gas. (You
must learn the difference between a gas and a vapour: a gas remains
permanent, a vapour is something that will condense.) If you blow out a
candle, you perceive a very nasty smell, resulting from the condensation
of this vapour. That is very different from what you have outside the
flame; and, in order to make that more clear to you, I am about to produce
and set fire to a larger portion of this vapour—for what we have in the
small way in a candle, to understand thoroughly, we must, as philosophers,
produce in a larger way, if needful, that we may examine the different
parts. And now Mr. Anderson will give me a source of heat, and I am about
to shew you what that vapour is. Here is some wax in a glass flask, and I
am going to make it hot, as the inside of that candle-flame is hot, and
the matter about the wick is hot. [The Lecturer placed some wax in a glass
flask, and heated it over a lamp.] Now, I dare say that is hot enough for
me. You see that the wax I put in it has become fluid, and there is a
little smoke coming from it. We shall very soon have the vapour rising up.
I will make it still hotter, and now we get more of it, so that I can
actually pour the vapour out of the flask into that basin, and set it on
fire there. This, then, is exactly the same kind of vapour as we have in
the middle of the candle; and that you may be sure this is the case, let
us try whether we have not got here, in this flask, a real combustible
vapour out of the middle of the candle. [Taking the flask into which the
tube from the candle proceeded, and introducing a lighted taper.] See how
it burns. Now, this is the vapour from the middle of the candle, produced
by its own heat; and that is one of the first things you have to consider
with respect to the progress of the wax in the course of its combustion,
and as regards the changes it undergoes. I will arrange another tube
carefully in the flame, and I should not wonder if we were able, by a
little care, to get that vapour to pass through the tube to the other
extremity, where we will light it, and obtain absolutely the flame of the
candle at a place distant from it. Now, look at that. Is not that a very
pretty experiment? Talk about laying on gas—why, we can actually lay on a
candle! And you see from this that there are clearly two different kinds
of action—one the _production_ of the vapour, and the other the
_combustion_ of it—both of which take place in particular parts of the
candle.
[Illustration: Fig. 8]
I shall get no vapour from that part which is already burnt. If I raise
the tube (fig. 7) to the upper part of the flame, so soon as the vapour
has been swept out, what comes away will be no longer combustible: It is
already burned. How burned? Why, burned thus:—In the middle of the flame,
where the wick is, there is this combustible vapour; on the outside of the
flame is the air which we shall find necessary for the burning of the
candle; between the two, intense chemical action takes place, whereby the
air and the fuel act upon each other, and at the very same time that we
obtain light the vapour inside is destroyed. If you examine where the heat
of a candle is, you will find it very curiously arranged. Suppose I take
this candle, and hold a piece of paper close upon the flame, where is the
heat of that flame? Do you not see that it is _not_ in the inside? It is
in a ring, exactly in the place where I told you the chemical action was;
and even in my irregular mode of making the experiment, if there is not
too much disturbance, there will always be a ring. This is a good
experiment for you to make at home. Take a strip of paper, have the air in
the room quiet, and put the piece of paper right across the middle of the
flame (I must not talk while I make the experiment), and you will find
that it is burnt in two places, and that it is not burnt, or very little
so, in the middle; and when you have tried the experiment once or twice,
so as to make it nicely, you will be very interested to see where the heat
is, and to find that it is where the air and the fuel come together.
This is most important for us as we proceed with our subject. Air is
absolutely necessary for combustion; and, what is more, I must have you
understand that _fresh_ air is necessary, or else we should be imperfect
in our reasoning and our experiments. Here is a jar of air. I place it
over a candle, and it burns very nicely in it at first, shewing that what
I have said about it is true; but there will soon be a change. See how the
flame is drawing upwards, presently fading, and at last going out. And
going out, why? Not because it wants air merely, for the jar is as full
now as it was before; but it wants pure, fresh air. The jar is full of
air, partly changed, partly not changed; but it does not contain
sufficient of the fresh air which is necessary for the combustion of a
candle. These are all points which we, as young chemists, have to gather
up; and if we look a little more closely into this kind of action, we
shall find certain steps of reasoning extremely interesting. For instance,
here is the oil-lamp I shewed you—an excellent lamp for our
experiments—the old Argand lamp. I now make it like a candle [obstructing
the passage of air into the centre of the flame]; there is the cotton;
there is the oil rising up it; and there is the conical flame. It burns
poorly, because there is a partial restraint of air. I have allowed no air
to get to it, save round the outside of the flame, and it does not burn
well. I cannot admit more air from the outside, because the wick is large;
but if, as Argand did so cleverly, I open a passage to the middle of the
flame, and so let air come in there, you will see how much more
beautifully it burns. If I shut the air off, look how it smokes; and why?
We have now some very interesting points to study. We have the case of the
combustion of a candle; we have the case of a candle being put out by the
want of air; and we have now the case of imperfect combustion; and this is
to us so interesting, that I want you to understand it as thoroughly as
you do the case of a candle burning in its best possible manner. I will
now make a great flame, because we need the largest possible
illustrations. Here is a larger wick [burning turpentine on a ball of
cotton]. All these things are the same as candles, after all. If we have
larger wicks, we must have a larger supply of air, or we shall have less
perfect combustion. Look now at this black substance going up into the
atmosphere; there is a regular stream of it. I have provided means to
carry off the imperfectly burned part, lest it should annoy you. Look at
the soots that fly off from the flame: see what an imperfect combustion it
is, because it cannot get enough air. What, then, is happening? Why,
certain things which are necessary to the combustion of a candle are
absent, and very bad results are accordingly produced; but we see what
happens to a candle when it is burnt in a pure and proper state of air. At
the time when I shewed you this charring by the ring of flame on the one
side of the paper, I might have also shewn you, by turning to the other
side, that the burning of a candle produces the same kind of
soot—charcoal or carbon.
But, before I shew that, let me explain to you—as it is quite necessary
for our purpose—that, though I take a candle and give you, as the general
result, its combustion in the form of a flame, we must see whether
combustion is always in this condition, or whether there are other
conditions of flame; and we shall soon discover that there are, and that
they are most important to us. I think, perhaps, the best illustration of
such a point to us, as juveniles, is to shew the result of strong
contrast. Here is a little gunpowder. You know that gunpowder burns with
flame—we may fairly call it flame. It contains carbon and other
materials, which altogether cause it to burn with a flame. And here is
some pulverised iron, or iron filings. Now, I purpose burning these two
things together. I have a little mortar in which I will mix them. (Before
I go into these experiments, let me hope that none of you, by trying to
repeat them, for fun’s sake, will do any harm. These things may all be
very properly used if you take care; but without that, much mischief will
be done.) Well, then, here is a little gunpowder, which I put at the
bottom of that little wooden vessel, and mix the iron filings up with it,
my object being to make the gunpowder set fire to the filings and burn
them in the air, and thereby shew the difference between substances
burning with flame and not with flame. Here is the mixture; and when I set
fire to it, you must watch the combustion, and you will see that it is of
two kinds. You will see the gunpowder burning with a flame, and the
filings thrown up. You will see them burning too, but without the
production of flame. They will each burn separately. [The Lecturer then
ignited the mixture.] There is the gunpowder, which burns with a flame;
and there are the filings—they burn with a different kind of combustion.
You see, then, these two great distinctions; and upon these differences
depend all the utility and all the beauty of flame which we use for the
purpose of giving out light. When we use oil, or gas, or candle, for the
purpose of illumination, their fitness all depends upon these different
kinds of combustion.
There are such curious conditions of flame, that it requires some
cleverness and nicety of discrimination to distinguish the kinds of
combustion one from another. For instance, here is a powder which is very
combustible, consisting, as you see, of separate little particles. It is
called _lycopodium_[7], and each of these particles can produce a vapour,
and produce its own flame; but, to see them burning, you would imagine it
was all one flame. I will now set fire to a quantity, and you will see the
effect. We saw a cloud of flame, apparently in one body; but that rushing
noise [referring to the sound produced by the burning] was a proof that
the combustion was not a continuous or regular one. This is the lightning
of the pantomimes, and a very good imitation. [The experiment was twice
repeated by blowing lycopodium from a glass tube through a spirit-flame.]
This is not an example of combustion like that of the filings I have been
speaking of, to which we must now return.
Suppose I take a candle, and examine that part of it which appears
brightest to our eyes. Why, there I get these black particles, which
already you have seen many times evolved from the flame, and which I am
now about to evolve in a different way. I will take this candle and clear
away the gutterage, which occurs by reason of the currents of air; and if
I now arrange a glass tube so as just to dip into this luminous part, as
in our first experiment, only higher, you see the result. In place of
having the same white vapour that you had before, you will now have a
black vapour. There it goes, as black as ink. It is certainly very
different from the white vapour; and when we put a light to it, we shall
find that it does not burn, but that it puts the light out. Well, these
particles, as I said before, are just the smoke of the candle; and this
brings to mind that old employment which Dean Swift recommended to
servants for their amusement, namely, writing on the ceiling of a room
with a candle. But what is that black substance? Why, it is the same
carbon which exists in the candle. How comes it out of the candle? It
evidently existed in the candle, or else we should not have had it here.
And now I want you to follow me in this explanation. You would hardly
think that all those substances which fly about London, in the form of
soots and blacks, are the very beauty and life of the flame, and which are
burned in it as those iron filings were burned here. Here is a piece of
wire gauze, which will not let the flame go through it; and I think you
will see, almost immediately, that when I bring it low enough to touch
that part of the flame which is otherwise so bright, that it quells and
quenches it at once, and allows a volume of smoke to rise up.
I want you now to follow me in this point,—that whenever a substance
burns, as the iron filings burnt in the flame of gunpowder, without
assuming the vaporous state (whether it becomes liquid or remains solid),
it becomes exceedingly luminous. I have here taken three or four examples
apart from the candle, on purpose to illustrate this point to you; because
what I have to say is applicable to all substances, whether they burn or
whether they do not burn,—that they are exceedingly bright if they retain
their solid state, and that it is to this presence of solid particles in
the candle-flame that it owes its brilliancy.