solar radiation been, previously to that
epoch, in excess, it must at the more recent periods, counted backwards,
have been but slightly so, and ages unnumbered must have elapsed, before
the state of equilibrium which now exists could have been reached. The
earth too, at distant periods, must have been colder than at present,
while that the contrary is true is shown by numerous observations.

Neither could chemical action have had any great agency in establishing
the present temperature of the earth. The substances which burn are but
a small portion of the crust of the earth, and their combustion, if all
fired at a time, would cause no perceptible effect on the sensible heat
of the surface of our globe. Were combustible bodies even infinitely
more abundant, the supporters are insufficient to keep up their
combustion for any length of time, without sensible diminution, and this
would be the case, even were the whole of the oxygen that now exists as
a component of the waters of the ocean added to their present amount. It
is indeed possible that the outer shell of the earth, which is no more
than a crust of oxidated matter, may have existed at first in the
metallic state, but that crust has long intervened, and prevented any
contact between the air or ocean, and the metallic bases of the earths,
that in this case must lie beneath.

In spite of these obvious objections to their theory, some geologists
have madly fancied to themselves a great internal fire, maintained by
actual combustion, a fancy but little more rational than that which
seeks, in the present order of things, precipitation from some vast
quantity of a liquid menstruum, every trace of whose existence has now
vanished.

There is, however, yet another source of heat, if indeed solar heat be
not a mere case of its general action, far more general and universal,
which has its origin in the bodies themselves, and has no reference to
any extrinsic cause. All bodies are sensibly heated when condensed, and
lose sensible heat when they expand, so that their temperatures vary
with the greater or less distance of their particles. The atmosphere of
the earth furnishes a marked illustration of this fact. Of nearly
uniform chemical composition throughout, its elastic nature, conflicting
with its gravity, renders it more dense in its lower than in its higher
regions. The former are in consequence warmer than the latter, and the
mean temperature of our climates is in fact