This nebula would be
gradually drawn in again by gravitation, just as the dust falls
to the floor of a room. The collisions of its particles as they
fell toward the centre would raise its temperature and give it a
rotating movement. A time would come when the centrifugal force
at the outer ring of the rotating disk would equal the
centripetal (or inward) pull of gravity, and this ring would be
detached, still spinning round the central body. The material of
the ring would slowly gather, by gravitation, round some denser
area in it; the ring would become a sphere; we should have the
first, and outermost, planet circling round the sun. Other rings
would successively be detached, and form the rest of the planets;
and the sun is the shrunken and condensed body of the nebula.
So simple and beautiful a theory of the solar system could not
fail to captivate astronomers, but it is generally rejected
to-day, in the precise form which Laplace gave it. What the
difficulties are which it has encountered, and the modifications
it must suffer, we shall see later; as well as the new theories
which have largely displaced it. It will be better first to
survey the universe from the evolutionary point of view.
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