If we examine the solid crust of the earth we shall not long be at a
loss in regard to the origin of this internal heat. We are all familiar
with the burning of coal. Now coal is mainly a substance called
_carbon_, and when it burns it unites with _oxygen_, one of the gases in
the air. Many rarer substances, such as silicon, and the metals
magnesium, calcium, and sodium, are even more inflammable than carbon,
and in burning give rise to solid products. Now the rocks in the earth
are found to be made up almost wholly of these very inflammable
substances combined with oxygen. The solid portions of the earth, then,
are nothing but the ashes and cinders of a great conflagration. Even the
waters are made up of hydrogen, one of the most inflammable substances,
united with this same oxygen, and, strange as it may seem, they too, are
the products of combustion. When, therefore, the materials of which the
earth is formed were burning, our planet must have been a fiery star,
and the great heat must have reduced all the products of the
conflagration to a liquid state.