arbon combines with
atmospheric oxygen, and restores some of the energy that was stored up at
that remote period. But the whole of the energy dormant in coal is not due
to the carbon, for this fuel contains another combustible element,
hydrogen, which is also a gas when in the free state, and which is one of
the constituents of water, the other constituent being oxygen. In fact,
there is more latent energy in hydrogen, weight for weight, than there is
in carbon, for one pound of hydrogen on complete combustion would give
enough heat to raise 62,032 lbs. of water through 1 deg. F. Hydrogen in
burning combines with oxygen to form water, so that the products of the
complete combustion of coal are carbon dioxide and water. The amount of
heat contributed by the hydrogen of coal is, however, comparatively
insignificant, because there is only a small percentage of this element
present, and we thus come to the conclusion that nearly all the work that
is done by our steam-engines of the present time is drawn from the latent
energy of the carbon of the fossilized vegetation of the Carboniferous
period.

The conclusion to which we have now been led leaves us with the question
as to the _origin_ of the energy of coal still unanswered. We shall have
to go a step further before this part of our story is complete, and we
must form some kind of idea of the way in which a plant grows. Carbon
being the chief source of energy in coal, we may for the present confine
ourselves to this element, of which woody fibre contains about 50 per
cent. Consider the enormous gain in weight during the growth of a plant;
compare the acorn, weighing a few grains, with the oak, weighing many
tons, which arises from it after centuries of growth. If matter is
indestructible, and never comes into existence spontaneously, where does
all this carbon come from? It is a matter of common knowledge that the
carbon of plants is supplied by the atmosphere in the form of carbon
dioxide--the gas which has already been referred to as resulting from the
combustion of carbon. This gas exists in the atmosphere in small
quantity--about four volumes in 10,000 volumes of air; but insignificant
as this may appear, it is all important for the life of plants, since it
is from this source that they derive their carbon. The origin of the
carbon dioxide, which is present as a normal constituent of the
atmosphere, does not directly concern us at present, but it is important
to bear