228 Professor Josejili Larmor [^l^ay 1, 



years before. That investigator had, in fact, shown that all the 

 optical functions of the aether were consistent with Lagrange's 

 abstract relations, in which the essential content of Newtonian 

 dynamics had been concisely formulated free from the material acci- 

 dents attaching to special systems and mechanisms. MacCuUagh's 

 abstract mathematical solution had, however, been widely repudiated, 

 because it went beyond such conceptions as could properly belong to 

 ordinary static material phenomena. But rotational momentum, as 

 a possible endowment of matter, had been overlooked in this criticism : 

 and Lord Kelvin's ideal model was thus the justification of Mac- 

 CuUagh's theory of light — which, once it became favourably appre- 

 ciated, now extended itself most readily and naturally to include the 

 whole field of electric phenomena, which appeared as non-vibratory 

 modes of disturbance of the same aether — thus in fact exhibiting the 

 Maxwellian theory developed in the reverse direction from optics to 

 electrics. 



All his life Lord Kelviii was keenly interested in such potentiali- 

 ties of intrinsic latent spinning masses, as a possible factor in the 

 ultimate explanation of the sensible qualities of bodies. In ordinary 

 physics, the elasticity of solids is merely taken as something existing, 

 without attempt at explanation of its origin, but with full utilisation 

 of the restrictions as to ty]ie which the principle of energy imposes. 

 Hence the significance of the Koyal Institution Lecture of 1881, 

 with its copious and beautiful illustrations, entitled ' Elasticity viewed 

 as possibly a Mode of Motion ' — viewed in fact as possibly stiffness 

 induced in matter, itself non-elastic, by latent intrinsic spin — and 

 more fully developed in the British Association Address of 1884, 

 ' Steps towards a Kinetic Theory of Matter.'* 



It has been recalled that the elastic properties of the nether can 

 be imitated by a model composed of connected rotating masses, too 

 complex, of course, to be actually made, but which can quite definitely 

 be imagined. The fascinating question whether all physical action 

 can thus be ascribed to latent phenomena of motion was always to 

 the fore in Thomson's thought. The refined experimental proofs by 

 Joule that mechanical energy never vanishes, but when apparently 

 lost is still traceable as the diffuse energy of heat, afforded to him 

 the strongest presumption that all manifestations of energy are sub- 

 ject to the principles of dynamics as resolved into their essential 

 elements by Newton. This conviction would be the stronger, from 

 the fact that for two years or more he was at a loss to reconcile 

 Joule's principle with the doctrine t of Carnot, which had very early 

 obtained a firm hold on his mind. The reconciliation came through 

 the distinction drawn between total energy and available energy ; 

 the 'deeper meaning of the principle of Carnot was simply (1874, 



,^ * Popular Lectures and Addresses, i. 



