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