ion of
rotation. But such a mass, filling all space as far as or beyond Saturn,
although containing the materials of the whole solar system in itself,
must have been of very rare consistency. Occupying so much bulk it could
not have been solid, nor yet liquid, but it might have been gaseous.

Are there any such gigantic rotating masses of gas in the heaven now?
Certainly there are; there are the nebulae. Some of the nebulae are now
known to be gaseous, and some of them at least are in a state of
rotation. Laplace could not have known this for certain, but he
suspected it. The first distinctly spiral nebula was discovered by the
telescope of Lord Rosse; and quite recently a splendid photograph of the
great Andromeda nebula, by our townsman, Mr. Isaac Roberts, reveals what
was quite unsuspected--and makes it clear that this prodigious mass also
is in a state of extensive and majestic whirl.

Very well, then, put this problem:--A vast mass of rotating gas is left
to itself to cool for ages and to condense as it cools: how will it
behave? A difficult mathematical problem, worthy of being attacked
to-day; not yet at all adequately treated. There are those who believe
that by the complete treatment of such a problem all the history of the
solar system could be evolved.

[Illustration: FIG. 80.--Lord Rosse's drawing of the spiral nebula in
Canes Venatici, with the stub marks of the draughtsman unduly emphasised
into features by the engraver.]

Laplace pictured to himself this mass shrinking and thereby whirling
more and more rapidly. A spinning body shrinking in size and retaining
its original amount of rotation, as it will unless a brake is applied,
must spin more and more rapidly as it shrinks. It has what
mathematicians call a constant moment of momentum; and what it loses in
leverage, as it shrinks, it gains in speed. The mass is held together by
gravitation, every particle attracting every other particle; but since
all the particles are describing curved paths, they will tend to fly off
tangentially, and only a small excess of the gravitation force over the
centrifugal is left to pull the particles in, and slowly to concentrate
the nebula. The mutual gravitation of the parts is opposed by the
centrifugal force of the whirl. At length a point is reached where the
two forces balance. A portion outside a certain line will be in
equilibrium; it will be left behind, and the rest must contract without
it. A ring is formed,