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.
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