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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.

We are now, however, confronted by an interesting but difficult question. Throughout all this grand past and future career of the solar system which we have just briefly traced, we have been witnessing a most prodigal dissipation of energy in the shape of radiant heat. At the outset we had an enormous quantity of what is called “energy of position,” that is, the outer parts of our primitive nebula had a very long distance through which to travel towards one another in the slow process of concentration; and this distance was the measure of the quantity of work possible to our system. As the particles of our nebula drew nearer and nearer together, the energy of position continually lost reappeared continually as heat, of which the greater part was radiated off, but of which a certain amount was retained. All the gigantic amount of work achieved in the geologic development of our earth and its companion planets, and in the development of life wherever life may exist in our system, has been the product of this retained heat. At the present day the same wasteful process is going on. Each moment the sun’s particles are losing energy of position as they draw closer and closer together, and the heat into which this lost energy is metamorphosed is poured out most prodigally in every direction. Let us consider for a moment how little of it gets used in our system. The earth’s orbit is a nearly circular figure more than five hundred million miles in circumference, while only eight thousand miles of this path are at any one time occupied by the earth’s mass. Through these eight thousand miles the sun’s radiated energy is doing work, but through the remainder of the five hundred million it is idle and wasted. But the case is far more striking when we reflect that it is not in the plane of the earth’s orbit only that the sun’s radiance is being poured out. It is not an affair of a circle, but of a sphere. In order to utilize all the solar rays, we should need to have an immense number of earths arranged so as to touch each other, forming a hollow sphere around the sun, with the present radius of the earth’s orbit. We may well believe Professor Tyndall, therefore, when he tells us that all the solar radiance we receive is less than a two-billionth part of what is sent flying through the desert regions of space. Some of the immense residue of course hits other planets stationed in the way of it, and is utilized upon their surfaces; but the planets, all put together, stop so little of the total quantity that our startling illustration is not materially altered by taking them into the account. Now this two-billionth part of the solar radiance poured out from moment to moment suffices to blow every wind, to raise every cloud, to drive every engine, to build up the tissue of every plant, to sustain the activity of every animal, including man, upon the surface of our vast and stately globe. Considering the wondrous richness and variety of the terrestrial life wrought out by the few sunbeams which we catch in our career through space, we may well pause overwhelmed and stupefied at the thought of the incalculable possibilities of existence which are thrown away with the potent actinism that darts unceasingly into the unfathomed abysms of immensity. Where it goes to or what becomes of it, no one of us can surmise.