at some future age they will doubtless present a single side to the sun, as our moon, from similar causes, now does to the earth.
Furthermore, it is thought that the medium which conveys light through space, extremely attenuated though it may be, is capable of opposing some resistance to planetary movements, so that the sun may at last unite with itself all the orbs now circling around it. The collision between the sun and its worlds would render the whole mass fiery hot; but radiation, in the course of time, would slowly bring its temperature lower and lower, until it would cease to shine altogether. The theory then supposes that the fate which shall have overtaken the solar system will then attack the sidereal heavens; that the causes which shall first make the planets unite with their primary will make stars unite with one another, until the ultimate result of all these changes may be that a solitary gigantic ball shall contain all the matter now interfused through space, its enormous store of energy, in the form of equably-diffused heat, being incapable of further change, and utterly unfit for the production or maintenance of life.
All this is assuredly very bold, and deduced most fairly from its premises; but premises in truth and completeness are of much more account and far more difficult of discovery than methods of logical inference. Let us briefly examine the grounds on which it is supposed that Nature is doomed to a death without resurrection, and see if they warrant the tremendous conclusions drawn from them.
The theory illustrates very pointedly the difficulties in which the finite mind of man becomes involved when it attempts to deal with what is not thinkably finite—or, if the term be preferred, infinite. In the first place, the theory under consideration assumes the finiteness in amount of matter and motion; but we do not know, nor can we imagine, that space has bounds, neither can we limit the extent of the orbs and movements which, as far as we can see, occupy it.
Secondly, the theory makes another conjecture in the realm of the absolute, when it presumes that heat is an absolutely homogeneous motion—that particles endowed with it move with so perfect a uniformity that there is an exclusion of any difference of motion which might serve as a starting-point for mechanical changes. But has science advanced far enough to make such a proposition tenable? Our knowledge of the ultimate structure of matter is very restricted; and as to what the modes of motion are which we call heat, electricity, and so on, we are entirely in the dark. Their quantities we know, but their qualities, their peculiar orbits, have scarcely been guessed at as yet. From a variety of reasons, however, the modern opinion, like the ancient one, is that matter is made up of atoms, which in the circumstances are units even if ideally divisible. Approximate measurements of them have been made by Prof. Sir William Thomson himself. (See his paper in Nature, vol. i, p. 551.)
Now, if atoms by virtue of their heat moved uninterruptedly in a