are the secondary contrasts between the velocities
and periods of the several planets, and between their simple revolutions
and the double ones of their satellites, which have to move round their
primaries while moving round the sun. There is the yet further strong
contrast between the sun and the planets in respect of temperature; and
there is reason to suppose that the planets and satellites differ from
each other in their proper heat, as well as in the heat they receive
from the sun.

When we bear in mind that, in addition to these various contrasts, the
planets and satellites also differ in respect to their distances from
each other and their primary; in respect to the inclinations of their
orbits, the inclinations of their axes, their times of rotation on their
axes, their specific gravities, and their physical constitutions; we see
what a high degree of heterogeneity the solar system exhibits, when
compared with the almost complete homogeneity of the nebulous mass out
of which it is supposed to have originated.

Passing from this hypothetical illustration, which must be taken for
what it is worth, without prejudice to the general argument, let us
descend to a more certain order of evidence. It is now generally agreed
among geologists that the Earth was at first a mass of molten matter;
and that it is still fluid and incandescent at the distance of a few
miles beneath its surface. Originally, then, it was homogeneous in
consistence, and, in virtue of the circulation that takes place in
heated fluids, must have been comparatively homogeneous in temperature;
and it must have been surrounded by an atmosphere consisting partly of
the elements of air and water, and partly of those various other
elements which assume a gaseous form at high temperatures. That slow
cooling by radiation which is still going on at an inappreciable rate,
and which, though originally far more rapid than now, necessarily
required an immense time to produce any decided change, must ultimately
have resulted in the solidification of the portion most able to part
with its heat--namely, the surface. In the thin crust thus formed we
have the first marked differentiation. A still further cooling, a
consequent thickening of this crust, and an accompanying deposition of
all solidifiable elements contained in the atmosphere, must finally have
been followed by the condensation of the water previously existing as
vapour. A second marked differentiation m