100 Mr. J. Larmor on Wiener s Localization of the 



forcive k1*x is required. If now instead of comparing the 

 total forcives in the two cases we compare the forcives per 

 unit volume, an increase of linear dimensions in the ratio of 

 k to one diminishes this forcive in the ratio of k 2 to one. 

 Thus, if only the atoms are taken small enough, an aether of 

 very slight rigidity can exert a forcive on them which, esti- 

 mated per unit volume, is of any order of magnitude we 

 please. The features of the case are in fact analogous to 

 those of the suspension of small bodies, such as motes, in a 

 viscous fluid medium like the atmosphere : if only the par- 

 ticles are small enough they will float for an indefinitely 

 great time against the force of gravity, even be they as 

 dense as platinum, — the only limit being in that case the one 

 imposed by the molecular discreteness of the air itself. 



It would appear that the application of this principle does 

 much to vivify the notion of an elastic solid aether. A medium 

 of this kind, which is excessively rare and, as a consequence, 

 of very feeble elasticity, would exert practically negligible 

 tractions on the surfaces of a mass of matter in bulk, while 

 it may exert relatively very powerful ones on the individual 

 atoms of which the mass is composed if only they are suf- 

 ficiently small, it being of course supposed that the structure 

 of the medium itself is absolutely continuous. And it would 

 even appear that a medium of very small density and 

 rigidity may be competent to excite powerful vibrations in 

 the molecules notwithstanding the strength of the forcives 

 which hold them together. 



We may thus imagine a working illustration of a ponder- 

 able transparent medium of elastic solid type as made up of 

 very small spherical nodules of great density and rigidity 

 dispersed through the aether and imbedded in it. We may 

 even imagine these nodules to be collected into more or less 

 independent groups, each of which w T ill have free periods of 

 relative vibrations of its own nearly independent of other 

 groups, in the manner now well known in connexion with 

 Prof. E wing's model of a magnetic medium. A wave running 

 across such a medium may excite these groups, and thus 

 illustrate the theory of selective absorption by means of a 

 system in which only the elasticity of the ambient medium is 

 operative, but no other internal forcive. 



The explanation of a very weak medium exciting such 

 powerful tractions implies of course strains of enormous 

 intensity, so that its limits of perfect elasticity must be taken 

 enormously wide compared with anything we know in ordi- 

 nary matter. The magnitude of the strains also requires 

 that the displacement of an atom relative to the aether must 



