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end。 Relatively to our powers of comprehension the practical difference is perhaps not great。 Scientifically speaking； Helmholtz and Thomson are as well entitled to reason upon the assumption of a perfectly frictionless fluid as geometers in general are entitled to assume perfect lines without breadth and perfect surfaces without thickness。 Perfect lines and surfaces do not exist within the region of our experience； yet the conclusions of geometry are none the less true ideally； though in any particular concrete instance they are only approximately realized。 Just so with the conception of a frictionless fluid。 So far as experience goes； such a thing has no more real existence than a line without breadth； and hence an atomic theory based upon such an assumption may be as true ideally as any of the theorems of Euclid； but it can give only an approximatively true account of the actual universe。 These considerations do not at all affect the scientific value of the theory； but they will modify the tenour of such transcendental inferences as may be drawn from it regarding； the probable origin and destiny of the universe。
The conclusions reached in the first part of this paper； while we were dealing only with gross visible matter； may have seemed bold enough； but they are far surpassed by the inference which our authors draw from the vortex theory as they interpret it。 Our authors exhibit various reasons； more or less sound； for attributing to the primordial fluid some slight amount of friction； and in support of this view they adduce Le Sage's explanation of gravitation as a differential result of pressure； and Struve's theory of the partial absorption of light…rays by the ether；questions with which our present purpose does not require us to meddle。 Apart from such questions it is every way probable that the primary assumption of Helmholtz and Thomson is only an approximation to the truth。 But if we accredit the primordial fluid with even an infinitesimal amount of friction； then we are required to conceive of the visible universe as developed from the invisible and as destined to return into the invisible。 The vortex…atom； produced by infinitesimal friction operating through wellnigh infinite time； is to be ultimately abolished by the agency which produced it。 In the words of our authors； 〃If the visible universe be developed from an invisible which is not a perfect fluid； then the argument deduced by Sir William Thomson in favour of the eternity of ordinary matter disappears； since this eternity depends upon the perfect fluidity of the invisible。 In fine； if we suppose the material universe to be composed of a series of vortex…rings developed from an invisible universe which is not a perfect fluid； it will be ephemeral； just as the smoke…ring which we develop from air； or that which we develop from water； is ephemeral； the only difference being in duration； these lasting only for a few seconds； and the others it may be for billions of years。〃 Thus； as our authors suppose that 〃the available energy of the visible universe will ultimately be appropriated by the invisible；〃 they go on to imagine； 〃at least as a possibility； that the separate existence of the visible universe will share the same fate； so that we shall have no huge； useless； inert mass existing in after ages to remind the passer…by of a form of energy and a species of matter that is long since out of date and functionally effete。 Why should not the universe bury its dead out of sight？〃
In one respect perhaps no more stupendous subject of contemplation than this has ever been offered to the mind of man。 In comparison with the length of time thus required to efface the tiny individual atom； the entire cosmical career of our solar system； or even that of the whole starry galaxy； shrinks into utter nothingness。 Whether we shall adopt the conclusion suggested must depend on the extent of our speculative audacity。 We have seen wherein its probability consists； but in reasoning upon such a scale we may fitly be cautious and modest in accepting inferences； and our authors； we may be sure； would be the first to recommend such modesty and caution。 Even at the dimensions to which our theorizing has here grown； we may for instance discern the possible alternative of a simultaneous or rhythmically successive generation and destruction of vortex…atoms which would go far to modify the conclusion just suggested。 But here we must pause for a moment； reserving for a second paper the weightier thoughts as to futurity which our authors have sought to enwrap in these sublime physical speculations。
PART SECOND。
UP to this point； however remote from ordinary every…day thoughts may be the region of speculation which we have been called upon to traverse； we have still kept within the limits of legitimate scientific hypothesis。 Though we have ventured for a goodly distance into the unknown； we have not yet been required to abandon our base of operations in the known。 Of the views presented in the preceding paper； some are wellnigh certainly established； some are probable； some have a sort of plausibility； othersto which we have refrained from giving assentmay possibly be true； but none are irretrievably beyond the jurisdiction of scientific tests。 No suggestion has so far been broached which a very little further increase of our scientific knowledge may not show to be either eminently probable or eminently improbable。 We have kept pretty clear of mere subjective guesses； such as men may wrangle about forever without coming to any conclusion。 The theory of the nebular origin of our planetary system has come to command the assent of all persons qualified to appreciate the evidence on which it is based； and the more immediate conclusions which we have drawn from that theory are only such as are commonly drawn by astronomers and physicists。 The doctrine of an intermolecular and interstellar
ether is wrapped up in the well…established undulatory theory of light。 Such is by no means the case with Sir William Thomson's vortex…atom theory； which to…day is in somewhat the same condition as the undulatory theory of Huyghens two centuries ago。 This； however； is none the less a hypothesis truly scientific in conception； and in the speculations to which it leads us we are still sure of dealing with views that admit at least of definite expression and treatment。 In other words； though our study of the visible universe has led us to the recognition of a kind of unseen world underlying the world of things that are seen； yet concerning the economy of this unseen world we have not been led to entertain any hypothesis that has not its possible justification in our experiences of visible phenomena。
We are now called upon； following in the wake of our esteemed authors； to venture on a different sort of exploration； in which we must cut loose altogether from our moorings in the world of which we have definite experience。 We are invited to entertain suggestions concerning the peculiar economy of the invisible portion of the universe which we have no means of subjecting to any sort of test of probability； either experimental or deductive。 These suggestions are； therefore； not to be regarded as properly scientific； but； with this word of caution； we may proceed to show what they are。
Compared with the life and death of cosmical systems which we have heretofore contemplated； the life and death of individuals of the human race may perhaps seem a small matter； yet because we are ourselves the men who live and die； the small event is of vastly greater interest to us than the grand series of events of which it is part and parcel。 It is natural that we should be more interested in the ultimate fate of humanity than in the fate of a world which is of no account to us save as our present dwelling…place。 Whether the human soul is to come to an end or not is to us a more important question than whether the visible universe； with its matter and energy； is to be absorbed in an invisible ether。 It is indeed only because we are interested in the former question that we are so curious about the latter。 If we could dissociate ourselves from the material universe； our habitat； we should probably speculate much less about its past and future。 We care very little what becomes of the black ball of the earth； after all life has vanished from its surface； or； if we care at all about it； it is only because our thoughts about the career of the earth are necessarily mixed up with our thoughts about life。 Hence in considering the probable ultimate destiny of the physical universe； our innermost purpose must be to know what is to become of all this rich and wonderful life of which the physical universe is the theatre。 Has it all been developed； apparently at almost infinite waste of effort； only to be abolished again before it has attained to completeness； or does it contain or shelter some indestructible element which having drawn sustenance for a while from the senseless turmoil of physical phenomena shall still survive their final decay？ This question is closely connected with the time…honoured question of the meaning； purpose； or tendency of the world。 In the career of the world is life an end； or a means toward an end； or onl