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. But I may observe, in passing, that the sceptical remarks one
hears at times about scientific theories contradicting and superseding
each other are frivolous. One great idea pervades all the theories of
the evolution of worlds, and that idea is firmly established. The
stars and their planets are enormous aggregations of cosmic dust, swept
together and compressed by the action of gravitation. The precise nature
of this cosmic dust--whether it was gas, meteorites and gas, or other
particles--is open to question.

As we saw in the first chapter, the universe has the word evolution
written, literally, in letters of fire across it. The stars are of all
ages, from sturdy youth to decrepit age, and even to the darkness of
death. We saw that this can be detected on the superficial test of
colour. The colours of the stars are, it is true, an unsafe ground to
build upon. The astronomer still puzzles over the gorgeous colours
he finds at times, especially in double stars: the topaz and azure
companions in beta Cygni, the emerald and red of alpha Herculis, the
yellow and rose of eta Cassiopeiae, and so on. It is at the present time
under discussion in astronomy how far these colours are objective at
all, or whether, if they are real, they may not be due to causes other
than temperature. Yet the significance of the three predominating
colours--blue-white, yellow, and red--has been sustained by the
spectroscope. It is the series of colours through which a white-hot bar
of iron passes as it cools. And the spectroscope gives