They will be justified in saying: "You say that coal is transformed
vegetable matter; but can you show us how the transformation takes
place? Is it possible according to known natural laws?"
The chemist must answer that. And he tells us that wood can become
lignite, or wood-coal, by parting with its oxygen, in the shape of
carbonic acid gas, or choke-damp; and then common or bituminous coal,
by parting with its hydrogen, chiefly in the form of carburetted
hydrogen--the gas with which we light our streets. That is about as
much as the unscientific reader need know. But it is a fresh
corroboration of the theory that coal has been once vegetable fibre,
for it shows how vegetable fibre can, by the laws of nature, become
coal. And it certainly helps us to believe that a thing has been
done, if we are shown that it can be done.
This fact explains, also, why in mines of wood-coal carbonic acid,
i.e. choke-damp, alone is given off. For in the wood-coal a great
deal of the hydrogen still remains. In mines of true coal, not only
is choke-damp given off, but that more terrible pest of the miners,
fire-damp, or explosive carburetted hydrogen and olefiant gases. Now
the occurrence of that fire-damp in mines proves that changes are
still going on in the coal: that it is getting rid of its hydrogen,
and so progressing toward the state of anthracite or culm--stone-coal
as it is sometimes called.
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