OTHER SPECULATIONS.
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.