The Chemical History of a Candle

"The Chemical History of a Candle" was the most famous course in

the long and remarkable series of Christmas lectures, "adapted to a

juvenile auditory," at the Royal Institution, and remains a

rarely-approached model of what such lectures should be. They were

illustrated by experiments and specimens, but did not depend upon

these for coherence and interest. They were delivered in 1860-61,

and have just been translated, though all but half-a-century old,

into German.

_I.--Candles and their Flames_

There is not a law under which any part of this universe is governed

that does not come into play in the phenomena of the chemical history of

a candle. There is no better door by which you can enter into the study

of natural philosophy than by considering the physical phenomena of a

candle.

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. However, 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.

The candle I have in my hand is a stearine candle, made of stearine from

tallow. Then here is a sperm candle, which comes from the purified oil

of the spermaceti whale. Here, also, are yellow beeswax and refined

beeswax 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, a sort of wax which a kind friend has sent me, and which

forms a new material for the manufacture of candles.

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 function. 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 stopped, 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? Or, when it is made a fluid, then how is it that it keeps

together? This is a wonderful thing about a candle.

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 stopped, but the part on the

outside does not melt. If I made a current in one direction, my cup

would be lopsided, and the fluid would consequently run over--for the

same force of gravity which holds worlds together, holds this fluid in a

horizontal position. 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 have such a bad result if you burn those beautiful

fluted candles, 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 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. 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 to the wick, and into the place of combustion. You know that the

flames on these burning wicks in candles made of beeswax, stearine, 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. 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.

_II.--The Brightness of the Candle_

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, showing

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 for combustion.

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 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.

It evidently existed in the candle, or else we should not have had it

here. 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. 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, it quells and quenches it at once, and allows a volume of

smoke to rise up.

Whenever a substance burns without assuming the vaporous state--whether

it becomes liquid or remains solid--it becomes exceedingly luminous.

What I 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 when heated, and that it is to this presence of solid

particles in the candle-flame that it owes its brilliancy.

I have here a piece of carbon, or charcoal, which will burn and give us

light exactly in the same manner as if it were burnt as part of a

candle. The heat that is in the flame of a candle decomposes the vapour

of the wax, and sets free the carbon particles--they rise up heated and

glowing as this now glows, and then enter into the air. But the

particles when burnt never pass off from a candle in the form of carbon.

They go off into the air as a perfectly invisible substance, about which

we shall know hereafter.

Is it not beautiful to think that such a process is going on, and that

such a dirty thing as charcoal can become so incandescent? You see, it

comes to this--that all bright flames contain these solid particles; all

things that burn and produce solid particles, either during the time

they are burning, as in the candle, or immediately after being burnt, as

in the case of the gunpowder and iron-filings--all these things give us

this glorious and beautiful light.

_III.--The Products of Combustion_

We observe that there are certain products as the result of the

combustion of a candle, and that of these products one portion may be

considered as charcoal, or soot; that charcoal, when afterwards burnt,

produces some other product--carbonic acid, as we shall see; and it

concerns us very much now to ascertain what yet a third product is.

Suppose I take a candle and place it under a jar. You see that the sides

of the jar become cloudy, and the light begins to burn feebly. It is the

products, you see, which make the light so dim, and this is the same

thing which makes the sides of the jar so opaque. If you go home and

take a spoon that has been in the cold air, and hold it over a

candle--not so as to soot it--you will find that it becomes dim, just as

that jar is dim. If you can get a silver dish, or something of that

kind, you will make the experiment still better. It is _water_ which

causes the dimness, and we can make it, without difficulty, assume the

form of a liquid.

And so we can go on with almost all combustible substances, and we find

that if they burn with a flame, as a candle, they produce water. You may

make these experiments yourselves. The head of a poker is a very good

thing to try with, and if it remains cold long enough over the candle,

you may get water condensed in drops on it; or a spoon, or a ladle, or

anything else may be used, provided it be clean, and can carry off the

heat, and so condense the water.

And now--to go into the history of this wonderful production of water

from combustibles, and by combustion--I must first of all tell you that

this water may exist in different conditions; and although you may now

be acquainted with all its forms, they still require us to give a little

attention to them for the present, so that we may perceive how the

water, whilst it goes through its protean changes, is entirely and

absolutely the same thing, whether it is produced from a candle, by

combustion, or from the rivers or ocean.

First of all, water, when at the coldest, is ice. Now, we speak of water

as water; whether it be in its solid, or liquid, or gaseous state, we

speak of it chemically as water.

We shall not in future be deceived, therefore, by any changes that are

produced in water. Water is the same everywhere, whether produced from

the ocean or from the flame of the candle. Where, then, is this water

which we get from a candle? It evidently comes, as to part of it, from

the candle; but is it within the candle beforehand? No! It is not in the

candle; and it is not in the air round about the candle, which is

necessary for its combustion. It is neither in one nor the other, but it

comes from their conjoint action, a part from the candle, a part from

the air. And this we have now to trace.

If we decompose water we can obtain from it a gas. This is hydrogen--a

body classed amongst those things in chemistry which we call elements,

because we can get nothing else out of them. A candle is not an

elementary body, because we can get carbon out of it; we can get this

hydrogen out of it, or at least out of the water which it supplies. And

this gas has been so named hydrogen because it is that element which, in

association with another, generates water.

Hydrogen gives rise to no substance that can become solid, either during

combustion or afterwards, as a product of its combustion. But when it

burns it produces water only; and if we take a cold glass and put it

over the flame, it becomes damp, and you have water produced immediately

in appreciable quantity, and nothing is produced by its combustion but

the same water which you have seen the flame of a candle produce. This

hydrogen is the only thing in Nature that furnishes water as the sole

product of combustion.

Water can be decomposed by electricity, and then we find that its other

constituent is the gas oxygen in which, as can easily be shown, a candle

or a lamp burns much more brilliantly than it does in air, but produces

the same products as when it burns in air. We thus find that oxygen is

a constituent of the air, and by burning something in the air we can

remove the oxygen therefrom, leaving behind for our study the nitrogen,

which constitutes about four-fifths of the air, the oxygen accounting

for nearly all the rest.

The other great product of the burning of a candle is carbonic acid--a

gas formed by the union of the carbon of the candle and the oxygen of

the air. Whenever carbon burns, whether in a candle or in a living

creature, it produces carbonic acid.

_IV.--Combustion and Respiration_

Now I must take you to a very interesting part of our subject--to the

relation between the combustion of a candle and that living kind of

combustion which goes on within us. In every one of us there is a living

process of combustion going on very similar to that of a candle. For it

is not merely true in a poetical sense--the relation of the life of man

to a taper. A candle will burn some four, five, six, or seven hours.

What, then, must be the daily amount of carbon going up into the air in

the way of carbonic acid? What a quantity of carbon must go from each of

us in respiration! A man in twenty-four hours converts as much as seven

ounces of carbon into carbonic acid; a milch cow will convert seventy

ounces, and a horse seventy-nine ounces, solely by the act of

respiration. That is, the horse in twenty-four hours burns seventy-nine

ounces of charcoal, or carbon, in his organs of respiration to supply

his natural warmth in that time.

All the warm-blooded animals get their warmth in this way, by the

conversion of carbon; not in a free state, but in a state of

combination. And what an extraordinary notion this gives us of the

alterations going out in our atmosphere! As much as 5,000,000 pounds of

carbonic acid is formed by respiration in London alone in twenty-four

hours. And where does all this go? Up into the air. If the carbon had

been like lead or iron, which, in burning, produces a solid substance,

what would happen? Combustion would not go on. As charcoal burns, it

becomes a vapour and passes off into the atmosphere, which is the great

vehicle, the great carrier, for conveying it away to other places. Then,

what becomes of it?

Wonderful is it to find that the change produced by respiration, which

seems so injurious to us, for we cannot breathe air twice over, is the

very life and support of plants and vegetables that grow upon the

surface of the earth. It is the same also under the surface in the great

bodies of water, for fishes and other animals respire upon the same

principle, though not exactly by contact with the open air. They respire

by the oxygen which is dissolved from the air by the water, and form

carbonic acid; and they all move about to produce the one great work of

making the animal and vegetable kingdoms subservient to each other.

All the plants growing upon the surface of the earth absorb carbon.

These leaves are taking up their carbon from the atmosphere, to which we

have given it in the form of carbonic acid, and they are prospering.

Give them a pure air like ours, and they could not live in it; give them

carbon with other matters, and they live and rejoice. So are we made

dependent not merely upon our fellow-creatures, but upon our

fellow-existers, all Nature being tied by the laws that make one part

conduce to the good of the other.