The nebular hypothesis of Laplace required a rotating mass of fluid
which at successive epochs became unstable from excess of motion, and
left behind rings, or more probably, perhaps, lumps, of matter from
the equatorial regions. To some thinkers was suggested a different
view of things, according to which it was not necessary to suppose
that one part of the system gravitationally supported another. The
whole might consist of a congeries of discrete bodies, even if these
bodies were the ultimate molecules of matter. The planets might have
been formed by the gradual accretion of such discrete bodies. On the
view that the material of the condensing solar system consisted of
separate particles or masses, we had no longer the fluid pressure
which was an essential part of Laplace's theory. Faye, in his theory
of evolution from meteorites, had to throw over his fundamental idea
of the nebular hypothesis, and formulated instead a different
succession of events of which the outer planets were formed last, a
theory which had difficulties of its own. Professor George Darwin had
recently shown, from an investigation of the mechanical conditions of
a swarm of meteorites, that on certain assumptions a meteoric swarm
might behave as a coarse gas, and in this way bring back the fluid
pressure exercised by one part of the system on the other, which was
required by Laplace's theory. One chief assumption consisted in
supposing that such inelastic bodies as meteoric stones might attain
the effective elasticity of a high order which was necessary to the
theory through the sudden volatilization of a part of their mass at an
encounter, by which what was virtually a violent explosive was
introduced between the two colliding stones. Professor Darwin was
careful to point out that it must necessarily be obscure as to how a
small mass of solid matter could take up a very large amount of energy
in a small fraction of a second.