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s。 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。
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。
Now when； in the remote future； our sun is reduced to vapour by the impact of the several planets upon his surface； the resulting nebulous mass must be a very insignificant affair compared with the nebulous mass with which we started。 In order to make a second nebula equal in size and potential energy to the first one； all the energy of position at first existing should have been retained in some form or other。 But nearly all of it has been lost； and only an insignificant fraction remains with which to endow a new system。 In order to reproduce； in future ages； anything like that cosmical development which is now going on in the solar system； aid must be sought from without。 We must endeavour to frame some valid hypothesis as to the relation of our solar system to other systems。
Thus far our view has been confined to the career of a single star；our sun；with the tiny； easily…cooling balls which it has cast off in the course of its development。 Thus far； too； our inferences have been very secure； for we have been dealing with a circumscribed group of phenomena； the beginning and end of which have been brought pretty well within the compass of our imagination。 It is quite another thing to deal with the actual or probable career of the stars in general； inasmuch as we do not even know how many stars there are； which form parts of a common system； or what。 are their precise dynamic relations to one another。 Nevertheless we have knowledge of a few facts which may support some cautious inferences。 All the stars which we can see are undoubtedly bound together by relations of gravitation。 No doubt our sun attracts all the other stars within our ken； and is reciprocally attracted by them。 The stars； too； lie mostly in or around one great plane； as is the case with the members of the solar system。 Moreover； the stars are shown by the spectroscope to consist of chemical elements identical with those which are found in the solar system。 Such facts as these make it probable that the career of other stars； when adequately inquired into； would be found to be like that of our own sun。 Observation daily enhances this probability； for our study of the sidereal universe is continually showing us stars in all stages of development。 We find irregular nebulae； for example； we find spiral and spheroidal nebulae； we find stars which have got beyond the nebulous stage； but are still at a whiter heat than our sun； and we also find many stars which yield the same sort of spectrum as our sun。 The inference seems forced upon us that the same process of concentration which has gone on in the case of our solar nebula has been going on in the case of other nebulae。 The history of the sun is but a type of the history of stars in general。 And when we consider that all other visible stars and nebulae are cooling and contracting bodies； like our sun； to what other conclusion could we very well come？ When we look at Sirius； for instance； we do not see him surrounded by planets； for at such a distance no planet could be visible； even Sirius himself； though fourteen times larger than our sun； appearing only as a 〃twinkling little star。〃 But a comparative survey of the heavens assures us that Sirius can hardly have arrived at his present stage of concentration without detaching； planet…forming rings； for there is no reason for supposing that mechanical laws out there are at all different from what they are in our own system。 And the same kind of inference must apply to all the