The old view of the original matter of the nebulæ, 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 nebulæ 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 nebulæ 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?