July 2, 193s] 



NA TV RE 



199 



Hicks, J. J. Thomson, and others, as well as Lord 

 Kelvin himself, have, from various points of view, 

 endeavoured to devise a scheme of spinning motion in 

 a perfect fluid plenum which should be able to accom- 

 plish in tfeneral terms all that the jethcr is known 

 to perform : more particularly that it should be able 

 to imitate its faculty of transmitting the transverse 

 or solid quiverings that we call light, yet without 

 resisting the motion of bodies through it ; and at the 

 .same time that it should be able to maintain its own 

 turbulent or whirlpool motion in an unconfused and 

 regularly stable condition throughout infinite time. 

 And in this difficult undertaking they have from time 

 to time seemed partially successful ; at any rate, 

 they have reached suggestive results and opened up 

 stimulating vistas. 



The ether must be incompressible, too, being per- 

 fectly continuous without breaks or any kind of atomic 

 or granular structure, save such as may be conferred 

 upon it by reason of its infra-material internal motion. 

 An infinitesimally turbulent liquid of some kind seemed 

 the desideratum, and many have been the attempts to 

 devise such a liquid. An interlaced system of vortex 

 fibres or filaments has to some seemed the most likely 

 device ; a similar scheme was a system of plates or 

 laminar vortices ; while a third modification con- 

 ceived it as a collection of connected filaments all in 

 a state of rapid internal motion, though stationary as 

 regards locomotion in space; — what might be called 

 a vortex sponge. By some such means it was hoped 

 to be able to combine the elastic rigidity appropriate 

 to a solid, with the penetrable unresistance to motion 

 of solids through it, characteristic of a perfect fluid, 

 and with the complete incompressibility of an ideal 

 liquid. But the mathematical difficulties of all such 

 treatment have been rather overwhelming; and an un- 

 certainty about the stability or permanence of such a 

 medium has always obtruded itself in a discouraging 

 manner. 



In fact, there has always been a troublesome 

 amount of instability in all the schemes that have 

 hitherto been devised, so that none of the expounders 

 of the motion doctrine was able to announce a finally 

 satisfactory result. 



Still it was felt by most of those who have worked 

 at the subject that the outlook in this direction would 

 be so bright, if initial difficulties could be overcome, 

 that it was worth a long-continued effort to see if a 

 coherent scheme could be planned on these lines, so 

 as to secure what, if it turned out to be the truth, 

 would surely be a magnificent generalisation. 



Indeed, it has sometimes seemed unlikely that a 

 mode of explanation which offered such attractive 

 features, and led so far in the right direction, 

 could, after all, be a blind alley leading nowhere; or, 

 to vary the metaphor, a mere will-of-the-wisp which 

 it was waste of time to pursue. 



What has certainly been made out is that motion 

 of atomic structures, in an aether with elasticitv postu- 

 lated, supplies a complete working scheme on which 

 we can rest without inquiring further as to the origin 

 of this elasticity. Beyond this, the attempt to explain 

 the material universe on a purely kinetic basis has 

 not made much progress in quite recent vears ; and, 

 to those competent to attack it, it has probably 

 seemed better to let the problem lie dormant for a 

 time, until future discoveries in mathematics or in 

 physics threw more light upon the rockv path or pro- 

 vided us with better instruments for climbing it. 



During the epoch of waiting it now appears that 

 our venerated chief was deflected from further attempts 

 in this direction, and directed his attention elsewhere. 

 Other methods seemed to him more immediately hope- 



NO. 2018, VOL. 78] 



ful ; and whereas it had been hoped to explain force 

 in terms of latent motion. Lord Kelvin in later years 

 sought to expound motion in terms of force, giving 

 up the kinetic unification of the material universe in 

 favour of a conception more arbitrary and descriptive, 

 and permitting himself to regard force as perhaps an 

 equally fundamental, perhaps a more fundamental, 

 conception than motion. 



It may be that philosophers will concede the (to me) 

 somewhat improbable proposition that an explanation 

 in terms of force and action-at-a-distance wsill be as 

 satisfactory as an elucidation in terms of motion and 

 a continuous medium. To Lord Kelvin it would 

 appear that both solutions were equally satisfactory, 

 and that it was only a question of which was the 

 most tractable. In any case it is noteworthy that 

 he took up so clear and definite a position ; it is 

 the key to much of his recent work, and to the diffi- 

 culties which he felt in accepting some of the hypo- 

 theses which are a natural consequence of the 

 electrical theory of matter and of some of the facts 

 of radio-activity. It now seems not unnatural that he 

 should have sought to express and explain these great 

 results otherwise. His attitude is both coherent and 

 reasonable ; though I would urge that most 

 theoretical advance and discovery (in the hands of 

 Maxwell and others) has been along the continuous 

 and medium line, which, if not the line of ultimate 

 explanation, is at any rate that of achievement. 



At the same time, it must be admitted that, if a 

 longitudinal impulse is transmitted bv an incompres- 

 sible medium at an infinite pace, the process becomes 

 barely distinguishable from action al: a distance, 

 through a force varying according to a specified law. 

 Or — putting what is virtually the same thought in 

 another way — the influence of an electron, or matter- 

 unit, whose field of force extends infinitely in all 

 directions, need not be conceived as limited by some 

 arbitrary boundary beyond which things can be said 

 to be at a distance from it. 



It will be remembered that some of the old philo- 

 sophers saw great difficulties in the abstract con- 

 ception of motion. It appears as a curious evanescent 

 transition from one place to another, involving the 

 attribute of " time "; it is indeed " not a being but a 

 becoming," when position is taken as the primary 

 conception. 



But I urge that it is simplest to regard " position " 

 and " distance " as secondary conceptions, sub- 

 I ordinate to and arising out of our perception of 

 I motion. Unless motion is supposed to be a thing 

 directly apprehended, it is truly rather an elusive 

 idea. To me it seems a direct apprehension — direct 

 information conveyed by our muscular sense. Space 

 itself seems a consequence deduced from our percep- 

 tion of motion ; and the idea of time follows from our 

 i direct perception of rapidity of motion. But prob- 

 ably to Lord Kelvin these things appeared other- 

 I wise. 



The conclusion of the discussion on the constitu- 

 tion of the atom may be summed up thus : — 



The internal energy of Lord Kelvin's model atom is 

 static or potential. The internal energy of the hypo- 

 thetical atom at which others are working is kinetic. 



The disintegration of radium in the former case 

 is comparable to the explosion of an unstable chemical 

 compound, like gun-cotton. In the latter case it must be 

 represented by something more akin to the flying to 

 pieces of a single rapidly spinning unit, such as a fly- 

 wheel. 



And so for the present the matter stands. 



Oliver Lodge. 



