The Mosaic cosmogony is not unworthy of the great people among whom it took its rise; it recognises the fact that the earth had a history antecedent to the advent of man, and its account of the order of events in this history is not only remarkable as a feat of a priori reasoning, but accords in some respects with the results achieved after much labour by modern science.
It was not until the middle of the eighteenth century that the reign of evolution began, and attempts were made to trace the history of a planetary system from its source in a primeval nebula on purely mechanical grounds. Swedenborg (1735) was the pioneer in this direction, then came Thomas Wright (1750) of Durham, whose work furnished inspiration to Emanuel Kant (1755), and led him to construct a consistent scheme of the Universe. The last of this group of cosmic philosophers is Laplace (1796), whose admirable description of the evolution of the solar system was arrived at independently, and without knowledge of the previous work of Kant.
Laplace assumed as his starting-point the existence of a nebula formed of incandescent gas, and extending beyond the limits of the outermost planet of our system. It was in rotation about a central axis, and possessed in consequence a disc-like or lenticular form. Radiating its heat away in all directions through surrounding space, it grew continually colder, and in cooling diminished in bulk. As a consequence of this contraction its rate of rotation increased, till at length the centrifugal force of the outermost part became so great that this could no longer continue to follow the contracting mass within, and thus remained behind as a great rotating ring. The continued contraction of the internal mass, and the resulting increase in the velocity of rotation, again brought about the same condition of things, and a fresh ring was left behind.
Cooling of the Nebula
This process was repeated time after time, till as many rings were formed as there are planets in the solar system; the central mass which survived within the innermost ring condensed to form the sun. The rings were highly unstable—that is to say, a slight disturbing force was sufficient to destroy their continuity; they broke across and rolled up into great nebulous globes, which revolved round the sun in the same direction as the original nebula, and rotated on their axes in the same direction as that in which they revolved. Most of them repeated the behaviour of the original nebulæ, leaving behind rings as they contracted, and these rings either rolled up to form moons or satellites, or, in the solitary instance of Saturn’s rings, retained their annular form. The rings are now known to consist of a multitude of solid bodies, as proved by Clerk-Maxwell.
The Temperature of the Earth
By this hypothesis, so beautiful in its simplicity, an explanation was afforded embracing all the more important facts of our system; the revolution of all the planets in nearly circular orbits and in the same direction as that in which the sun rotates, and the revolution of their satellites, also in circular orbits and in the same direction as their primaries; the comparatively high temperature and consequent low density of the larger planets and the sun, as well as a variety of other phenomena, all seem to follow naturally from it. The fundamental assumption seems to be in harmony with a number of known facts. Thus in the case of our own planet the volcanoes distributed around the margins of the oceans, and the hot springs scattered irregularly over the whole terrestrial surface, suggest that great stores of heat exist beneath our feet, a presumption which finds confirmation in the fact that whenever we descend towards the interior of the earth, as in deep mines or wells, the temperature continues steadily to rise after we have passed a depth below which seasonal and diurnal changes of temperature cease to be felt, the rise being in some cases as much as 3 deg. for 100 ft., in others only 1 deg. for the same distance, but on the average 1 deg. for 60 ft. or 70 ft. If this increase of temperature continues down to great depths, and there seems to be no reason why it should not, then a point will be reached, say, at thirty or forty miles down, where the interior will attain a white heat.
The Earth as a Star
Thus the earth might be regarded as a white hot body surrounded with a film of rock growing continually cooler towards the surface. But such a hot body suspended in space must be cooling, just as all bodies which are hotter than their surroundings. It is cooler to-day than it was yesterday, or—what is the same thing—it was hotter yesterday than it is to-day, and so of all previous yesterdays. And thus as we travel backwards in time we perceive that the earth will be growing hotter, the level of white heat will be mounting upwards towards the surface, and will at last reach it, so that the earth, instead of being, as it now is, a dark body shining only with the reflected light of the sun, will be self-luminous, a tiny star of a magnitude so diminutive as to have awakened resentment on the part of some terrestrial inhabitants, who have regarded it as disproportionate to their dignity. But we cannot arrest imagination at this stage; our thought still extends its retrospective glance into the abyss of past time, and we perceive the earth still growing hotter, till its temperature transcends those limits at which it can exist in the solid state. It becomes molten—nay, more, it becomes gaseous, and thus resumes the nebular state from which it sprang. Precisely the same argument applies to the sun; our mighty luminary is also a cooling body, and if we could restore to it the heat which it has lost in the course of past æons it would resume a completely gaseous state. Modified in one way or another, this chain of reasoning seemed irrefragable in those happy days which preceded the discovery of radium.