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.
HELMHOLTZ’S DISCOVERY.
The old view of the original matter of the nebulae,
that it consisted of a “fiery mist,”
“a
tumultuous cloud,
Instinct with
fire and niter,”
fell at once with the rise of the science of thermodynamics.
In 1854, Helmholtz showed that the supposition of
an original fiery condition of the nebulous stuff
was unnecessary, since in the mutual gravitation of
widely separated matter we had a store of potential
energy sufficient to generate the high temperature
of the sun and stars. We could scarcely go wrong
in attributing the light of the nebulae to the conversion
of the gravitational energy of shrinkage into molecular
motion. The inquisitiveness of the human mind
did not allow us to remain content with the interpretation
of the present state of the cosmical masses, but suggested
the question—
What
see’st thou else
In the dark backward
and abysm of time?
What was the original state of things? How had
it come about that by the side of ageing worlds we
had nebulae in a relatively younger stage? Had
any of them received their birth from dark suns, which
had collided into new life, and so belonged to a second
or later generation of the heavenly bodies?
LOOKING BACKWARD.