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Jevons observes； we might as well regard it as an infinitely solid 〃adamant。〃 〃Sir John Herschel has calculated the amount of force which may be supposed； according to the undulatory theory of light； to be exerted at each point in space； and finds it to be 1；148；000；000；000 times the elastic force of ordinary air at the earth's surface； so that the pressure of the ether upon a square inch of surface must be about 17；000；000；000；000； or seventeen billions of pounds。〃＇4＇ Yet at the same time the resistance offered by the ether to the planetary motions is too minute to be appreciable。 〃All our ordinary notions；〃 says Professor Jevons； 〃must be laid aside in contemplating such an hypothesis； yet ＇it is＇ no more than the observed phenomena of light and heat force us to accept。 We cannot deny even the strange suggestion of Dr。 Young； that there may be independent worlds； some possibly existing in different parts of space； but others perhaps pervading each other； unseen and unknown； in the same space。 For if we are bound to admit the conception of this adamantine firmament； it is equally easy to admit a plurality of such。〃
＇4＇ Jevons's Principles of Science； Vol。 II。 p。 145。 The figures； which in the English system of numeration read as seventeen billions； would in the American system read as seventeen trillions。
The ether； therefore； is unlike any of the forms of matter which we can weigh and measure。 In some respects it resembles a fluid； in some respects a solid。 It is both hard and elastic to an almost inconceivable degree。 It fills all material bodies like a sea in which the atoms of the material bodies are as islands； and it occupies the whole of what we call empty space。 It is so sensitive that a disturbance in any part of it causes a 〃tremour which is felt on the surface of countless worlds。〃 Our old experiences of matter give us no account of any substance like this； yet the undulatory theory of light obliges us to admit such a substance； and that theory is as well established as the theory of gravitation。 Obviously we have here an enlargement of our experience of matter。 The analysis of the phenomena of light and radiant heat has brought us into mental relations with matter in a different state from any in which we previously knew it。 For the supposition that the ether may be something essentially different from matter is contradicted by all the terms we have used in describing it。 Strange and contradictory as its properties may seem； are they any more strange than the properties of a gas would seem if we were for the first time to discover a gas after heretofore knowing nothing but solids and liquids？ I think not； and the conclusion implied by our authors seems to me eminently probable； that in the so…called ether we have simply a state of matter more primitive than what we know as the gaseous state。 Indeed； the conceptions of matter now current； and inherited from barbarous ages； are likely enough to be crude in the extreme。 It is not strange that the study of such subtle agencies as heat and light should oblige us to modify them； and it will not be strange if the study of electricity should entail still further revision of our ideas。
We are now brought to one of the profoundest speculations of modern times； the vortex…atom theory of Helmholtz and Thomson； in which the evolution of ordinary matter from ether is plainly indicated。 The reader first needs to know what vortex…motion is； and this has been so beautifully explained by Professor Clifford； that I quote his description entire： 〃Imagine a ring of india…rubber； made by joining together the ends of a cylindrical piece （like a lead…pencil before it is cut）； to be put upon a round stick which it will just fit with a little stretching。 Let the stick be now pulled through the ring while the latter is kept in its place by being pulled the other way on the outside。 The india…rubber has then what is called vortex…motion。 Before the ends were joined together； while it was straight； it might have been made to turn around without changing position； by rolling it between the hands。 Just the same motion of rotation it has on the stick； only that the ends are now joined together。 All the inside surface of the ring is going one way； namely； the way the stick is pulled； and all the outside is going the other way。 Such a vortex…ring is made by the smoker who purses his lips into a round hole and sends out a puff of smoke。 The outside of the ring is kept back by the friction of his lips while the inside is going forwards； thus a rotation is set up all round the smoke…ring as it travels out into the air。〃 In these cases； and in others as we commonly find it； vortex…motion owes its origin to friction and is after a while brought to an end by friction。 But in 1858 the equations of motion of an incompressible frictionless fluid were first successfully solved by Helmholtz； and among other things he proved that； though vortex…motion could not be originated in such a fluid； yet supposing it once to exist； it would exist to all eternity and could not be diminished by any mechanical action whatever。 A vortex…ring； for example； in such a fluid； would forever preserve its own rotation； and would thus forever retain its peculiar individuality； being； as it were； marked off from its neighbour vortex…rings。 Upon this mechanical truth Sir William Thomson based his wonderfully suggestive theory of the constitution of matter。 That which is permanent or indestructible in matter is the ultimate homogeneous atom； and this is probably all that is permanent； since chemists now almost unanimously hold that so…called elementary molecules are not really simple； but owe their sensible differences to the various groupings of an ultimate atom which is alike for all。 Relatively to our powers of comprehension the atom endures eternally； that is； it retains forever unalterable its definite mass and its definite rate of vibration。 Now this is just what a vortex…ring would do in an incompressible frictionless fluid。 Thus the startling question is suggested； Why may not the ultimate atoms of matter be vortex…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。 Scient