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Supplement to '' Nature J' August 4, 1923 



lines of force ; attention may be directed, however, 

 to papers descril)ing my early attempts at such experi- 

 ments, as partly described in the Philosophical Magazine 

 for April 1907 and May 1919. In making experi- 

 ments on the ether we must recognise that what we 

 set out to look for we may not find : but we can 

 also remember that careful and conscientious experi- 

 ment, conducted with good apparatus, must lead us 

 somewhere, and may result in a discovery exceeding 

 in importance and interest any property we had 

 set out to examine. 



Kinetic Elasticity. 



The contrast between the kinetic and the static 

 mode of regarding things runs all through physics. 

 Most physicists are imbued with the more fundamental 

 character of a kinetic explanation, and never feel 

 really satisfied with an explanation in terms of static 

 or potential energy. Of the two kinds of energy, 

 kinetic appears to them the more fundamental kind. 



So, as we all know. Lord Kelvin tried to explain 

 the elasticity of a spring balance, or any spiral spring, 

 by means of gyrostats, or spinning tops. He was 

 able to devise, at any rate theoretically, two concealed 

 mechanisms, one of which was static and the other 

 kinetic ; that is to say, one of which contained a 

 spiral spring with a protruding hook at the end, while 

 the other contained a precessing system of gyrostats, 

 also with a protruding hook. The observer was 

 challenged to hang things on the hook, or to feel its 

 recoil, and to say which was which. Or, in more 

 general terms. Lord Kelvin endeavoured to devise 

 a kinetic theory of elasticity. His famous theory of 

 the vortex atom, in which he tried to explain some 

 of the properties of atoms in terms of vortex rings 

 and their collisions and interactions, was of this nature. 

 It was extended by J. J. Thomson at an early date 

 into almost chemical regions, in his early Adams' 

 prize essay. 



In his later life, Lord Kelvin was inclined to abandon 

 this view of elasticity as regards solids ; and his vortex 

 atom declined to satisfy either him or others, on 

 further development. But none of these ideas should 

 be completely abandoned. In so far as they success- 

 fully illustrated any of the properties of matter, 

 they are worthy of consideration. Although we now 

 know that the atom is not a vortex ring, or any- 

 thing like it, I would challenge any one to say the 

 same of an electron. The electron has become the 

 fundamental material unit : and what its constitu- 

 tion may be, we none of us know. It must be in 

 close relation with the ether, and must ultimately, 

 as I think, be explicable in terms of the ether. But 

 the fundamental properties of the ether are too little 

 known at present to enable this to be done. We 

 cannot say whether the electron is to be explained 

 statically as a knot or other geometrical configuration 

 or strain centre, on one hand, or as some kind of 

 circulating or vortex movement on the other. The 

 constitution of the electron remains for discovery, 

 in spite of all the work of Larmor on the subject in 

 his brilliant book " Ether and Matter," and other 

 papers imbedded in the Phil. Trans. If it should 

 turn out that an electron can be thought of as a knot 

 or any kind of static strain, then I for one feel that 



that cannot be regarded as an ultimate explanation, 

 though a most useful approximate one, and that the 

 strain will have to be resolved into or accounted for 

 by some kind of etherial vortex motion. 



Not only have the electric and magnetic forces 

 belonging to an electron, respectively at rest and in 

 motion, to be explained ; but also the slight residual 

 strain depending on the square of the charge, and 

 therefore irrespective of sign, which we call gravitation, 

 has in a complete theory to be explained also. For 

 few can doubt, I think, that gravitation must noN\ 

 be regarded as a function of the electron and th< 

 proton, that it is not something which springs inti 

 being when these units are associated so as to con 

 stitute an atom of matter ; it is unlikely that th< 

 tight packing of a large number of hypothetical 

 positive and negative units in the proton could 

 account for it. More likely gravitation will turn out 

 to be an etherial phenomenon explicable in terms of 

 the beknottedness which distinguishes the singular 

 point of an electron from the rest of the ether. TIk 

 rest of the ether is not affected by gravity, but 

 possesses qualities akin to what in mechanics we call 

 elasticity and inertia. Otherwise the unspecialised 

 ether of space could not transmit radiation, or sustain 

 a magnetic field — as we know it does. For it is 

 necessary always to remember that though electric 

 lines of force terminate on material units, most of 

 their course lies in undisturbed ether : while magnetic 

 lines of force do not terminate at all, but are always 

 closed curves, surrounding electrons in motion, but 

 themselves existing, I presume, wholly in the ether, 

 and showing every sign of being essentially a kinetic 

 phenomenon, demonstrative of inertia. 



Inertia itself I see no way of explaining in an\ 

 fundamental manner. It seems to be a property 

 that we must postulate as existing in the ether, — a 

 property akin to density ; though it is true we can 

 explain the inertia of matter — that is, of any material 

 unit — in terms of the concealed magnetic fields in- 

 evitably associated with its motion. 



To leave these more transcendental regions for the 

 present, we may recall that although the kinetic 

 theory of elasticity has hitherto failed to develop in 

 connexion with solids, it holds perfectly for the case 

 of gases. The elasticity or recoil of compressed air 

 used to be thought of as analogous to the recoil of an 

 elastic spring. But Waterston first, and then Joul' 

 and others — including especially Maxwell and Clausius 

 and Loschmidt, — explained it brilliantly, together 

 with many other of the metrical properties of gases, 

 as the result of molecular motions and bombardment ; 

 so that it has become a familiar and elaborate theory 

 — the Kinetic Theory of Gases. Hence, in that form 

 of matter about which we know most, the kinetic 

 theory of elasticity holds the field. 



Radiation and Matter. 



Now come a series of questions which it is diflfi- 

 cult to formulate precisely because of our inadequate 

 knowledge, and concerning which we must make the 

 best of the hints which from time to time are afforded 

 us by Nature, — questions which are mainly concerned 

 with the nature of radiation, and with the interactions 

 between ether waves and ordinary matter. 



