Photographic Action of Stationary Light- Waves. 103 



appreciable force on finite bodies moving through it. But these 

 very properties would incapacitate it for acquiring the very 

 large volume-densities of energy that would have to be 

 associated with it in order to explain electrodynamic phe- 

 nomena. 



Any representation which would make the aether consist 

 of molecules of ordinary matter is open to the objection that 

 the thermal kinetic energy of gases and other material systems 

 must then, in accordance with Maxwell's law of distribution 

 of energy, largely reside in it. But, on the other hand, if 

 we hold to the view of matter which was first rendered pre- 

 cise by Lord Kelvin's theory of vortex atoms, namely, that 

 the aether is the single existing medium and that atoms of 

 matter are intrinsic singularities of motion or strain which 

 belong to it, then there is no inducement to assume for the 

 aether a molecular structure at all, or to make its inertia any- 

 thing comparable with the inertia of the atoms on whose play 

 the thermal energy of the movements of the matter consists. 

 On such a theory the inertia, and the resulting kinetic energy, 

 of the matter may be hard to explain, but it is certainly 

 something different from the inertia of the underlying 

 medium in which the atom is merely a form of strain or 

 motion. On such a theory refraction, and also double refrac- 

 tion, will be caused by the atmosphere of intrinsic strain 

 which represents the electric charge on the atom ; and only 

 dispersion will be assigned to the influence of sympathetic 

 vibrations in the atoms or molecules, thus doing away with 

 any difficulty of the kind mentioned above. 



In the theory of gases the ordinary kinetic energy of the 

 molecules represents sensible heat, and as such may be derived 

 for example from the dissipation by friction or otherwise of 

 the mechanical energy of ordinary masses : it is of the nature 

 of kinetic energy of the masses of the atoms. But the store 

 of energy which keeps up radiation is of electromotive kind, 

 is concerned with displacing electricity, not with moving 

 matter except indirectly ; at least no consistent scheme has 

 yet been forthcoming which includes both. It is quite con- 

 ceivable that the disturbances which occur in the ordinary 

 encounters of molecules are of far too gentle a character to 

 excite the very powerful elasticity which on a certain form of 

 the electric theory binds together the continuous medium 

 taking part in optical propagation, any more than a system 

 of solid balls rushing about in an enclosure bounded by a 

 heavy continuous rigid solid can excite sensibly the elastic 

 qualities of that body. The opinion has been widely sup- 

 ported, both on theoretical and experimental grounds, that a 



