a planet when it has reached so late a stage of
concentration that its equatorial portions contain matters capable of
easy precipitation into liquid and, finally, solid forms. Even then it
can be produced only under special conditions. Gaining a
rapidly-increasing preponderance as the gravitative force does during
the closing stages of concentration, the centrifugal force cannot, in
ordinary cases, cause the leaving behind of rings when the mass has
become dense. Only where the centrifugal force has all along been very
great, and remains powerful to the last, as in Saturn, can we expect
dense rings to be formed.

We find, then, that besides those most conspicuous peculiarities of the
Solar System which first suggested the theory of its evolution, there
are many minor ones pointing in the same direction. Were there no other
evidence, these mechanical arrangements would, considered in their
totality, go far to establish the Nebular Hypothesis.

* * * * *

From the mechanical arrangements of the Solar System, turn we now to its
physical characters; and, first, let us consider the inferences
deducible from relative specific gravities.

The fact that, speaking generally, the denser planets are the nearer to
the Sun, has been by some considered as adding another to the many
indications of nebular origin. Legitimately assuming that the outermost
parts of a rotating nebulous spheroid, in its earlier stages of
concentration, must be comparatively rare; and that the increasing
density which the whole mass acquires as it contracts, must hold of the
outermost parts as well as the rest; it is argued that the rings
successively detached will be more and more dense, and will form planets
of higher and higher specific gravities. But passing over other
objections, this explanation is quite inadequate to account for the
facts. Using the Earth as a standard of comparison, the relative
densities run thus:--

Neptune. Uranus. Saturn. Jupiter. Mars. Earth. Venus. Mercury. Sun.
0.17 0.25 0.11 0.23 0.45 1.00 0.92 1.26 0.25

Two insurmountable objections are presented by this series. The first
is, that the progression is but a broken one. Neptune is denser than
Saturn, which, by the hypothesis, it ought not to be. Uranus is denser
than Jupiter, which it ought not to be. Uranus is denser than Saturn,
and the Earth is denser than Venus--facts which not only give no
countenance to,