tion; heat is par excellence the communist of our universe, and it will no doubt ultimately bring the system to an end. This universe may in truth be compared to a vast heat engine, and this is the reason why we have brought such engines so prominently before our readers. The sun is the furnace or source of high-temperature heat of our system, just as the stars are for other systems, and the energy which is essential to our existence is derived from the heat which the sun radiates, and represents only a very small portion of that heat. But while the sun thus supplies us with energy he is himself getting colder, and must ultimately, by means of radiation into space, part with the life-sustaining power which he at present possesses. Besides the cooling of the sun we must also suppose that owing to something analogous to ethereal friction' the earth and the other planets of our system will be drawn spirally nearer and nearer to the sun, and will at length be engulfed in his mass. In each such case there will be, as the result of the collision, the conversion of visible energy into heat, and a partial and temporary restoration of the power of the sun. At length, however, this process will have come to an end, and he will be extinguished until, after long but not immeasurable ages, by means of the same ethereal friction his black mass is brought into contact with that of his nearest neighbour. 115. Not much further need we dilate on this. It is absolutely certain that life, so far as it is physical, depends essentially upon transformations of energy; it is also absolutely certain that age after age the possibility of such transformations is becoming less and less; and, so far as we 1 Stewart and Tait on the Heating of a Disk by Rotation in vacuo (Proceedings of the Royal Society). yet know, the final state of the present universe must be an aggregation (into one mass) of all the matter it contains, i.e. the potential energy gone, and a practically useless state of kinetic energy, i.c. uniform temperature throughout that mass. But the present potential energy of the solar system is so enormous, approaching in fact possibly to what in our helplessness we call infinite, that it may supply for absolutely incalculable future ages what is required for the physical existence of life. Again, the fall together, from the distance of Sirius, let us say, of the sun and an equal star would at once supply the sun with at least thirty times as much energy for future radiation to possible planets as could possibly have been acquired by his own materials in falling together from practically infinite diffusion as a cloud of stones or dust, or a nebula; so that it is certain that, if the present physical laws remain long enough in operation, there will be (at immense intervals of time) mighty catastrophes due to the crashing together of defunct suns-the smashing of the greater part of each into nebulous dust surrounding the remainder, which will form an intensely heated nucleus then, possibly, the formation of a new and larger set of planets with a proportionately larger and hotter sun, a solar system on a far grander scale than the present. And so on, growing in grandeur but diminishing in number till the exhaustion of energy is complete, and after that eternal rest, so far at least as visible motion is concerned. 116. The study of the necessary future has prepared us for an inquiry into the long remote past. Just as the present discrete stellar systems must finally come together, so the materials which now form them must have originally been widely separate. Our modern knowledge enables us to look back with almost certitude to the time when there was nothing but gravitating matter and its potential energy throughout the expanse of space-ready, as slight local differences of distribution predisposed it, to break up into portions, each converging to one or more nuclei of its own, and thus forming in time separate solar or stellar systems. We have thus reached the beginning as well as the end of the present visible universe, and have come to the conclusion that it began in time and will in time come to an end. Immortality is therefore impossible in such a universe. CHAPTER IV. MATTER AND ETHER. "Felix qui potuit rerum cognoscere causas, subjecit pedibus, strepitumque Acherontis avari.” "Who shall tempt with wandering feet The happy isle ?"-MILTON, Paradise Lost. VERGIL 117. THE next portion of the preliminary inquiry necessary to our concluding argument is that which relates to the intimate nature of matter; and more especially of that very wonderful form of matter which is the vehicle of all the energy we receive from the sun, as it is that of all the information we obtain about the position, motion, nature, mass, condition, and properties of the almost infinitely more distant bodies, which are scattered through cosmical space. To use the comparison of a writer on energy, we have hitherto spoken only of the laws of working of that machine called the physical universe; let us now endeavour to study the structure of that material of which it is composed. 118. Various hypotheses have been proposed as to the ultimate nature of matter. To give even a general account of all the less absurd of these would require a large volume, so we content ourselves with a few of the more reasonable or historically more important. (1.) The foremost place must of course be taken by the old Greek notion of the Atom. The outlines of the atomic theory were laid down very precisely by Democritus and Leukippus (circa 400 B.C.), who taught that the whole universe is made up of empty space and eternal atoms, differing only in form (as A and N), order (as AN and NA), and posture (as Z and N). The atoms are endued with a primitive motion in virtue of their weight, and clashing together, produce vortices from which the world is formed. The gradual progress of this whirl of atoms brings similar elements together, as in the sifting of grain, and so the atoms are grouped into homogeneous masses. The great weakness of this theory lay in the very false ideas then held as to the nature of motion by weight, which was supposed to be necessarily in parallel lines, and with a velocity greater for heavy than for light bodies. The difficulty which arose from this notion led Epicurus to give to the atoms a perfectly arbitrary and capricious side movement, as well as the rectilineal motion due to their weight, and thus, in his school, the theory became really a metaphysical one, reducing the order of the universe to pure chance. It is such a medley of physical speculations, with metaphysical notions, that we find in the greatest exponent of the system, the "poet philosopher" Lucretius. With the help of Munro's splendid edition of the text of Lucretius, and his very valuable translation and notes, it is now a comparatively easy matter to give a concise summary of the principal points of this most remarkable early physical speculation. In attempting to do so we will endeavour, so |