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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 group of planets has been formed may have owed its origin to the disintegration of systems which had accomplished their career in the depths of the bygone eternity。
When the problem is extended to these huge dimensions； the prospect of an ultimate cessation of cosmical work is indefinitely postponed； but at the same time it becomes impossible for us to deal very securely with the questions we have raised。 The magnitudes and periods we have introduced are so nearly infinite as to baffle speculation itself： One point； however； we seem dimly to discern。 Supposing the stellar universe not to be absolutely infinite in extent； we may hold that the day of doom； so often postponed； must come at last。 The concentration of matter and dissipation of energy； so often checked； must in the end prevail； so that； as the final outcome of things； the entire universe will be reduced to a single enormous ball； dead and frozen； solid and black； its potential energy of motion having been all transformed into heat and radiated away。 Such a conclusion has been suggested by Sir William Thomson； and it is quite forcibly stated by the authors of 〃The Unseen Universe。〃 They remind us that 〃if there be any one form of energy less readily or less completely transformable than the others； and if transformations constantly go on； more and more of the whole energy of the universe will inevitably sink into this lower grade as time advances。〃 Now radiant heat； as we have seen； is such a lower grade of energy。 〃At each transformation of heat…energy into work； a large portion is degraded； while only a small portion is transformed into work。 So that while it is very easy to change all of our mechanical or useful energy into heat； it is only possible to transform a portion of this heat…energy back again into work。 After each change； too； the heat becomes more and more dissipated or degraded； and less and less available for any future transformation。 In other words；〃 our authors continue； 〃the tendency of heat is towards equalization； heat is par excellence the communist of our universe； and it will no doubt ultimately bring the system to an end。 。。。。 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 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。 e。 uniform temperature throughout that mass。〃 Thus our authors conclude that the visible universe began in time and will in time come to an end； and they add that under the physical conditions of such a universe 〃immortality is impossible。〃
Concerning the latter inference we shall by and by have something to say。 Meanwhile this whole speculation as to the final cessation of cosmical work seems to meas it does to my friend； Professor Clifford＇3＇by no means trustworthy。 The conditions of the problem so far transcend our grasp that any such speculation must remain an unverifiable guess。 I do not go with Professor Clifford in doubting whether the laws of mechanics are absolutely the same throughout eternity； I cannot quite reconcile such a doubt with faith in the principle of continuity。 But it does seem to me needful； before we conclude that radiated energy is absolutely and forever wasted； that we should find out what becomes of it。 What we call radiant heat is simply transverse wave…motion； propagated with enormous velocity through an ocean of subtle ethereal matter which bathes the atoms of all visible or palpable bodies and fills the whole of space； extending beyond the remotest star which the telescope can reach。 Whether there are any bounds at all to this ethereal ocean； or whether it is as infinite as space itself； we cannot surmise。 If it be limited； the possible dispersion of radiant energy is limited by its extent。 Heat and light cannot travel through emptiness。 If the ether is bounded by surrounding emptiness； then a ray of heat； on arriving at this limiting emptiness； would be reflected back as surely as a ball is sent back when thrown against a solid wall。 If this be the case； it will not affect our conclusions concerning such a tiny region of space as is occupied by the solar system； but it will seriously modify Sir William Thomson's suggestion as to the fate of the universe as a whole。 The radiance thrown away by the sun is indeed lost so far as the future of our system is concerned； but not a single unit of it is lost from the universe。 Sooner or later； reflected back in all directions； it must do work in one quarter or another； so that ultimate stagnation be comes impossible。 It is true that no such return of radiant energy has been detected in our corner of the world； but we have not yet so far disentangled all the force…relations of the universe that we are entitled to regard such a return as impossible。 This is one way of escape from the consummation of things depicted by our authors。 Another way of escape is equally available； if we suppose that while the ether is without bounds the stellar universe also extends to infinity。 For in this case the reproduction of nebulous masses fit for generating new systems of worlds must go on through space that is endless； and consequently the process can never come to an end and can never have had a beginning。 We have； therefore； three alternatives： either the visible universe is finite； while the ether is infinite； or both are finite； or both are infinite。 Only on the first supposition； I think； do we get a universe which began in time and must end in time。 Between such stupendous alternatives we have no grounds for choosing。 But it would seem that the third； whether strictly true or not； best represents the state of the case relatively to our feeble capacity of comprehension。 Whether absolutely infinite or not； the dimensions of the universe must be taken as practically infinite； so far as human thought is concerned。 They immeasurably transcend the capabilities of any gauge we can bring to bear on them。 Accordingly all that we are really entitled to hold； as the outcome of sound speculation； is the conception of innumerable systems of worlds concentrating out of nebulous masses； and then rushing together and dissolving into similar masses； as bubbles unite and break upnow here； now therein their play on the surface of a pool； and to this tremendous series of events we can assign neither a beginning nor an end。
＇3＇ Fortnightly Review； April； 1875。
We must now make some more explicit mention of the ether which carries through space the rays of heat and light。 In closest connection with the visible stellar universe； the vicissitudes of which we have briefly traced； the all…pervading ether constitutes a sort of unseen world remarkable enough from any point of view； but to which the theory of our authors ascribes capacities hitherto unsuspected by science。 The very existence of an ocean of ether enveloping the molecules of material bodies has been doubted or denied by many eminent physicists； though of course none have called in question the necessity for some interstellar medium for the transmission of thermal and luminous vibrations。 This scepticism has been； I think； partially justified by the many difficulties encompassing the conception； into which； however； we need not here enter。 That light and heat cannot be conveyed by any of the ordinary sensible forms of matter is unquestionable。 None of the forms of sensible matter can be imagined sufficiently elastic to propagate wave…motion at the rate of one hundred and eighty…eight thousand miles per second。 Yet a ray of light is a series of waves； and implies some substance in which the waves occur。 The substance required is one which seems to possess strangely contradictory properties。 It is commonly regarded as an 〃ether〃 or infinitely rare substance； but； as Professor Jevons observes； we might as well regard it as an infinitely solid 〃adamant。〃 〃Sir John Herschel ha