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. [Sidenote: Universe still in Evolution] The question may be considered from another point of view. On searching the heavens we find that many of the stages which are assumed in Laplace’s hypothesis are still represented by actual existences. There are, to begin with, those immense diffused nebulæ, almost incapable of definition, which are proved, on spectroscopic examination, to emit that kind of light which is characteristic of glowing gas; from these we pass to others which are resolvable by the telescope into a central and more condensed nucleus, with two mighty nebulous arms whirled round in a spiral, and bearing more condensed masses in their midst; even ring nebulæ are known to exist; and, finally, there are nebulous halos which surround some of the stars. Then we come to the stars themselves, which are suns of various degrees of magnitude, some immensely larger than our own luminary, and these are evidently in various stages of existence. Some are blue, and afford evidence of a higher temperature than that of our sun; others are yellow, and make a nearer approach to the solar temperature; while, again, others are red, and certainly colder.