It is not as likely that the condensation of the sun was so sudden
as that of the planets, and therefore in this case this distance is only
approximate.

[40] Mechanique Celeste. Theory of the Moon.

[41] Mechanique Celeste. Masses of the planets.

SECTION FIFTH.

COMETARY PHENOMENA.

The planetary arrangements of the solar system are all _a priori_
indications of the theory of vortices, not only by the uniform direction
of the motions, the circular orbits in which these motions are
performed, the near coincidence of the planes of these orbits, and the
uniform direction of the rotation of the planets themselves; but, also,
by the law of densities and distances, which we have already attempted
to explain. In the motions of comets we find no such agreement. These
bodies move in planes at all possible inclinations in orbits extremely
eccentrical and without any general direction--as many moving contrary
to the direction of the planets as in the opposite direction; and when
we consider their great volume, and their want of mass, it appears, at
first sight, that comets do present a serious objection to the theory.
We shall point out, however, a number of _facts_ which tend to
invalidate this objection, and which will ultimately give the
preponderance to the opposite argument.

Every fact indicative of the nature of comets proves that the nuclei are
masses of material gases, similar, perhaps (at least in the case of the
short-period comets), to the elementary gases of our own planet, and,
consequently, these masses must be but small. In the nascent state of
the system, the radial stream of the vortex would operate as a fan,
purging the planetary materials of the least ponderable atoms, and, as
it were, separating the wheat from the chaff. It is thus we conceive
that the average atomic density of each planet has been first determined
by the radial stream, and, subsequently, that the solidification of the
nebulous planets has, by their atomic density, assigned to each its
position in the system, from the consequent relation which it
established between the density of the ether within the planet, and the
density of the ether external to it, so that, according to this view, a
single isolated atom of the same density as the mean atomic density of
the earth could (_ceteris paribus_) revolve in an orbit at the distance
of the earth, and in the same periodic time. This, however, is only
advanced by way of illustration.