and away goes the inner nucleus contracting
further and further towards a centre. After a time another ring will be
left behind in the same way, and so on. What happens to these rings?
They rotate with the motion they possess when thrown or shrunk off; but
will they remain rings? If perfectly regular they may; if there be any
irregularity they are liable to break up. They will break into one or
two or more large masses, which are ultimately very likely to collide
and become one. The revolving body so formed is still a rotating gaseous
mass; and it will go on shrinking and cooling and throwing off rings,
like the larger nucleus by which it has been abandoned. As any nucleus
gets smaller, its rate of rotation increases, and so the rings last
thrown off will be spinning faster than those thrown off earliest. The
final nucleus or residual central body will be rotating fastest of all.

The nucleus of the whole original mass we now see shrunk up into what we
call the sun, which is spinning on its axis once every twenty-five days.
The rings successively thrown off by it are now the planets--some large,
some small--those last thrown off rotating round him comparatively
quickly, those outside much more slowly. The rings thrown off by the
planetary gaseous masses as they contracted have now become satellites;
except one ring which has remained without breaking up, and is to be
seen rotating round Saturn still.

One other similar ring, an abortive attempt at a planet, is also left
round the sun (the zone of asteroids).

Such, crudely and baldly, is the famous nebular hypothesis of Laplace.
It was first stated, as has been said above, by the philosopher Kant,
but it was elaborated into much fuller detail by the greatest of French
mathematicians and astronomers.

The contracting masses will condense and generate great quantities of
heat by their own shrinkage; they will at a certain stage condense to
liquid, and after a time will begin to cool and congeal with a
superficial crust, which will get thicker and thicker; but for ages they
will remain hot, even after they have become thoroughly solid. The small
ones will cool fastest; the big ones will retain their heat for an
immense time. Bullets cool quickly, cannon-balls take hours or days to
cool, planets take millions of years. Our moon may be nearly cold, but
the earth is still warm--indeed, very hot inside. Jupiter is believed by
some observers still to glow with a dull red hea