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…rings forever existing in such a frictionless fluid filling the whole of space？ Such a hypothesis is not less brilliant than Huyghens's conjectural identification of light with undulatory motion； and it is moreover a legitimate hypothesis； since it can be brought to the test of verification。 Sir William Thomson has shown that it explains a great many of the physical properties of matter： it remains to be seen whether it can explain them all。
Of course the ether which conveys thermal and luminous undulations is not the frictionless fluid postulated by Sir William Thomson。 The most conspicuous property of the ether is its enormous elasticity； a property which we should not find in a frictionless fluid。 〃To account for such elasticity；〃 says Professor Clifford （whose exposition of the subject is still more lucid than that of our authors）； 〃it has to be supposed that even where there are no material molecules the universal fluid is full of vortex…motion； but that the vortices are smaller and more closely packed than those of ＇ordinary＇ matter； forming altogether a more finely grained structure。 So that the difference between matter and ether is reduced to a mere difference in the size and arrangement of the component vortex…rings。 Now； whatever may turn out to be the ultimate nature of the ether and of molecules； we know that to some extent at least they obey the same dynamic laws； and that they act upon one another in accordance with these laws。 Until； therefore； it is absolutely disproved； it must remain the simplest and most probable assumption that they are finally made of the same stuff； that the material molecule is some kind of knot or coagulation of ether。〃＇5＇
＇5＇ Fortnightly Review； June； 1875； p。 784。
Another interesting consequence of Sir William Thomson's pregnant hypothesis is that the absolute hardness which has been attributed to material atoms from the time of Lucretius downward may be dispensed with。 Somewhat in the same way that a loosely suspended chain becomes rigid with rapid rotation； the hardness and elasticity of the vortex…atom are explained as due to the swift rotary motion of a soft and yielding fluid。 So that the vortex…atom is really indivisible； not by reason of its hardness or solidity； but by reason of the indestructibleness of its motion。
Supposing； now； that we adopt provisionally the vortex theory；the great power of which is well shown by the consideration just mentioned；we must not forget that it is absolutely essential to the indestructibleness of the material atom that the universal fluid in which it has an existence as a vortex…ring should be entirely destitute of friction。 Once admit even the most infinitesimal amount of friction； while retaining the conception of vortex…motion in a universal fluid； and the whole case is so far altered that the material atom can no longer be regarded as absolutely indestructible； but only as indefinitely enduring。 It may have been generated； in bygone eternity； by a natural process of evolution； and in future eternity may come to an 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 dimen