parent solar mass seems to be after all but a temporary separation. So nicely balanced are they now in their orbits that they may well seem capable of rolling on in their present courses forever. But this is not the case. Two sets of circumstances are all the while striving, the one to drive the planets farther away from the sun, the other to draw them all into it. On the one hand, every body in our system which contains fluid matter has tides raised upon its surface by the attraction of neighbouring bodies. All the planets raise tides upon the surface of the sun and the periodicity of sun-spots (or solar cyclones) depends upon this fact. These tidal waves act as a drag or brake upon the rotation of the sun, somewhat diminishing its rapidity. But, in conformity with a principle of mechanics well known to astronomers, though not familiar to the general reader, all the motion of rotation thus lost by the sun is added to the planets in the shape of annual motion of revolution, and thus their orbits all tend to enlarge,--they all tend to recede somewhat from the sun. But this state of things, though long-enduring enough, is after all only temporary, and will at any rate come to an end when the sun and planets have become solid. Meanwhile another set of circumstances is all the time tending to bring the planets nearer to the sun, and in the long run must gain the mastery. The space through which the planets move is filled with a kind of matter which serves as a medium for the transmission of heat and light, and this kind of matter, though different in some respects from ordinary ponderable matter, is yet like it in exerting friction. This friction is almost infinitely little, yet it has a wellnigh infinite length of time to work in, and during all this wellnigh infinite length of time it is slowly eating up the momentum of the planets and diminishing their ability to maintain their distances from the sun. Hence in course of time the planets will all fall into the sun, one after another, so that the solar system will end, as it began, by consisting of a single mass of matter. But this is by no means the end of the story. When two bodies rush together, each parts with some of its energy of motion, and this lost energy of motion reappears as heat. In the concussion of two cosmical bodies, like the sun and the earth, an enormous quantity of motion is thus converted into heat. Now heat, when not allowed to radiate, or when generated faster than it can be radiated, is transformed into motion of expansion. Hence the shock of sun and planet would at once result in the vaporization of both bodies; and there can be no doubt that by the time the sun has absorbed the outermost of his attendant planets, he will have resumed something like his original nebulous condition. He will have been dilated into a huge mass of vapour, and will have become fit for a new process of contraction and for a new production of life-bearing planets.