, we should find the earth and all the planets
red-hot; and back further still, to an exceedingly remote epoch, when
the planets would be heated just as much as our sun is now. In a still
earlier stage the whole solar system is thought to have been one vast
mass of glowing gas, from which the present forms of the sun, with the
planets and their satellites, have been gradually evolved. We cannot be
sure that the course of events has been what is here indicated; but
there are sufficient grounds for thinking that this doctrine
substantially represents what has actually occurred.

Many of the features in the solar system harmonise with the supposition
that the origin of the system has been that suggested by the nebular
theory. We have already had occasion in an earlier chapter to allude to
the fact that all the planets perform their revolutions around the sun
in the same direction. It is also to be observed that the rotation of
the planets on their axes, as well as the movements of the satellites
around their primaries, all follow the same law, with two slight
exceptions in the case of the Uranian and Neptunian systems. A
coincidence so remarkable naturally suggests the necessity for some
physical explanation. Such an explanation is offered by the nebular
theory. Suppose that countless ages ago a mighty nebula was slowly
rotating and slowly contracting. In the process of contraction, portions
of the condensed matter of the nebula would be left behind. These
portions would still revolve around the central mass, and each portion
would rotate on its axis in the same direction. As the process of
contraction proceeded, it would follow from dynamical principles that
the velocity of rotation would increase; and thus at length these
portions would consolidate into planets, while the central mass would
gradually contract to form the sun. By a similar process on a smaller
scale the systems of satellites were evolved from the contracting
primary. These satellites would also revolve in the same direction, and
thus the characteristic features of the solar system could be accounted
for.

The nebular origin of the solar system receives considerable countenance
from the study of the sidereal heavens. We have already dwelt upon the
resemblance between the sun and the stars. If, then, our sun has passed
through such changes as the nebular theory requires, may we not
anticipate that similar phenomena should be met with in other stars? If
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