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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 matured stars which we see in the heavens。
When we duly take all these things into the account； the case of our solar system will appear as only one of a thousand cases of evolution and dissolution with which the heavens furnish us。 Other stars； like our sun； have undoubtedly started as vaporous masses； and have thrown off planets in contracting。 The inference may seem a bold one； but it after all involves no other assumption than that of the continuity of natural phenomena。 It is not likely； therefore； that the solar system will forever be left to itself。 Stars which strongly gravitate toward each other； while moving through a perennially resisting medium； must in time be drawn together。 The collision of our extinct sun with one of the Pleiades； after this manner； would very likely suffice to generate even a grander nebula than the one with which we started。 Possibly the entire galactic system may； in an inconceivably remote future； remodel itself in this way； and possibly the nebula from which our own g