ich forms a most important constituent of all
living matter, whether animal or vegetable. Woody fibre contains a large
quantity of this element, and the carbon of coal is thus accounted for; it
was accumulated during the growth of the plants of the Carboniferous
period.

Now carbon is one of those elementary substances which are said to be
_combustible_, which means that if we heat it in atmospheric air it gives
out heat and light, and gradually disappears, or, as we say, burns away.
The heat which is given out during combustion represents the chemical
energy stored up in the combustible, for combustion is in fact the
chemical union of one substance with another with the development of heat
and light. When carbon burns in air, therefore, a chemical combination
takes place, the air supplying the other substance with which the carbon
combines. That other substance is also an element--it is the invisible gas
which chemists call oxygen, and which forms one-fifth of the bulk of
atmospheric air, the remainder consisting of the gas nitrogen and small
quantities of other gases with which we shall have more to do
subsequently. When oxygen and carbon unite under the conditions described,
the product is an invisible gas known as carbon dioxide, and it is because
this gas is invisible that the carbon seems to disappear altogether on
combustion. In reality, however, the carbon is not lost, for matter is as
indestructible as energy, but it is converted into the dioxide which
escapes as gas under ordinary circumstances. If, however, we burn a given
weight of carbon with free access of air, and collect the product of
combustion and weigh it, we shall find that the product weighs more than
the carbon, by an amount which represents the weight of oxygen with which
the element has combined. By careful experiment it would be found that
one part by weight of carbon would give three and two-third parts by
weight of carbon dioxide. If, moreover, we could measure the quantity of
heat given out by the complete combustion of one pound of carbon, it would
be found that this quantity would raise 14,544 lbs. of water through 1 deg.
F., a quantity of heat corresponding to over eleven million foot-pounds of
work, or about seven and three-quarters horse-power per hour.

Here then is the chief source of the energy of coal--the carbon of the
plants which lived on this earth long ages ago has lain buried in the
earth, and when we ignite a coal fire this c