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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links, too quickly and too slightly made, have given way, and been
replaced by better work; but now the great phenomena are known - the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 beeswax, 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 camphin (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


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