glowing gas at a very high temperature. But this is by
no means necessary, as we have seen that the gradual contraction of the
vast mass supplies energy which may be converted into heat, and the
spectroscopic evidence seems also to point to the existence of a
moderate temperature in the gaseous nebulae, which must be considered to
be representatives of the hypothetical primitive chaos out of which our
sun and planets have been evolved. Another point which has been
reconsidered is the formation of the various planets. It was formerly
thought that the rotation of the original mass had by degrees caused a
number of rings of different dimensions to be separated from the central
part, the material of which rings in time collected into single planets.
The ring of Saturn was held to be a proof of this process, since we here
have a ring, the condensation of which into one or more satellites has
somehow been arrested. But while it is not impossible that matter in the
shape of rings may have been left behind during the contraction of the
nebulous mass (indeed, the minor planets between Mars and Jupiter have
perhaps originated in this way), it seems likely that the larger planets
were formed from the agglomeration of matter at a point on the equator
of the rotating nebula.

The actual steps of the process by which the primeval nebula became
transformed into the solar system seem to lie beyond reach of discovery.

CHAPTER XXVII.

THE TIDES.[44]

Mathematical Astronomy--Lagrange's Theories: how far they are
really True--The Solar System not Made of Rigid Bodies--Kepler's
Laws True to Observation, but not Absolutely True when the Bodies
are not Rigid--The Errors of Observation--The Tides--How the Tides
were Observed--Discovery of the Connection between the Tides and
the Moon--Solar and Lunar Tides--Work done by the Tides--Whence do
the Tides obtain the Power to do the Work?--Tides are Increasing
the Length of the Day--Limit to the Shortness of the Day--Early
History of the Earth-Moon System--Unstable Equilibrium--Ratio of
the Month to the Day--The Future Course of the System--Equality of
the Month and the Day--The Future Critical Epoch--The Constant Face
of the Moon accounted for--The other Side of the Moon--The
Satellites of Mars--Their Remarkable Motions--Have the Tides
Possessed Influence in Moulding the Solar System generally?--Moment
of Momen