the carbon, but we can
soon shew it to you. I will light it, and as long as there is any gas in
this cylinder it will go on burning. You see no carbon, but you see a
flame; and because that is bright, it will lead you to guess that there is
carbon in the flame. But I will shew it to you by another process. I have
some of the same gas in another vessel, mixed with a body that will burn
the hydrogen of the gas, but will not burn the carbon. I will light them
with a burning taper, and you perceive the hydrogen is consumed, but not
the carbon, which is left behind as a dense black smoke. I hope that by
these three or four experiments you will learn to see when carbon is
present, and understand what are the products of combustion, when gas or
other bodies are thoroughly burned in the air.

Before we leave the subject of carbon, let us make a few experiments and
remarks upon its wonderful condition as respects ordinary combustion. I
have shewn you that the carbon in burning burns only as a solid body, and
yet you perceive that, after it is burned, it ceases to be a solid. There
are very few fuels that act like this. It is, in fact, only that great
source of fuel, the carbonaceous series, the coals, charcoals, and woods,
that can do it. I do not know that there is any other elementary substance
besides carbon that burns with these conditions; and if it had not been
so, what would happen to us? Suppose all fuel had been like iron, which,
when it burns, burns into a solid substance. We could not then have such a
combustion as you have in this fire-place. Here also is another kind of
fuel which burns very well--as well as, if not better, than carbon--so
well, indeed, as to take fire of itself when it is in the air, as you see
[breaking a tube full of lead pyrophorus]. This substance is lead, and you
see how wonderfully combustible it is. It is very much divided, and is
like a heap of coals in the fireplace; the air can get to its surface and
inside, and so it burns. But why does it not burn in that way now, when it
is lying in a mass? [emptying the contents of the tube in a heap on to a
plate of iron]. Simply because the air cannot get to it. Though it can
produce a great heat, the great heat which we want in our furnaces and
under our boilers, still that which is produced cannot get away from the
portion which remains unburned underneath, and that portion, therefore, is
prevented from coming in contact with the atmosphere, and cannot be
consumed. How different is that from carbon. Carbon burns just in the same
way as this lead does, and so gives an intense fire in the furnace, or
wherever you choose to burn it; but then the body produced by its
combustion passes away, and the remaining carbon is left clear. I shewed
you how carbon went on dissolving in the oxygen, leaving no ash; whereas
here [pointing to the heap of pyrophorus] we have actually more ash than
fuel, for it is heavier by the amount of the oxygen which has united with
it. Thus you see the difference between carbon and lead or iron: if we
choose iron, which gives so wonderful a result in our application of this
fuel, either as light or heat. If, when the carbon burnt, the product went
off as a solid body, you would have had the room filled with an opaque
substance, as in the case of the phosphorus; but when carbon burns,
everything passes up into the atmosphere. It is in a fixed, almost
unchangeable condition before the combustion; but afterwards it is in the
form of gas, which it is very difficult (though we have succeeded) to
produce in a solid or a liquid state.

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; and I
must try to make that plain to you. For it is not merely true in a
poetical