s 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 sense--the relation of the life of man to a taper; and if you
will follow, I think I can make this clear. In order to make the relation
very plain, I have devised a little apparatus which we can soon build up
before you. Here is a board and a groove cut in it, and I can close the
groove at the top part by a little cover. I can then continue the groove
as a channel by a glass tube at each end, there being a free passage
through the whole. Suppose I take a taper or candle (we can now be liberal
in our use of the word "candle," since we understand what it means), and
place it in one of the tubes; it will go on, you see, burning very well.
You observe that the air which feeds the flame passes down the tube at one
end, then goes along the horizontal tube, and ascends the tube at the
other end in which the taper is placed.

[Illustration: Fig. 32]

If I stop the aperture through which the air enters, I stop combustion, as
you perceive. I stop the supply of air, and consequently the candle goes
out. But, now, what will you think of this fact? In a former experiment I
shewed you the air going from one burning candle to a second candle. If I
took the air proceeding from another candle, and sent it down by a
complicated arrangement into thi