1893.] the Electric and Luminiferous Medium. 457 



This question may also be instructively illustrated from another 

 side, by the consideration of an actual medium which possesses 

 precisely the rotational elasticity of MacCullagh's aether. I allude to 

 a solid medium with small magnets interspersed through it in any 

 arbitrary manner, but so that in any single element of volume there 

 is some regularity in their orientation. If this medium when un- 

 strained is in equilibrium in a magnetic field, then when an element 

 of it is displaced rotationally it will be acted on by a bodily couple 

 arising from this external field ; and therefore the surface tractions 

 on the element would, in the presence of this couple, be unbalanced. 

 Here the disturbing cause is a magnetic forcive arising either from 

 the medium as a whole or from some external system ; it has to be 

 considered as of a statical character, that is, the velocity of propaga- 

 tion of the magnetic action is supposed to be indefinitely great 

 compared with the velocity of propagation of any disturbances that 

 are under discussion ; the magnetic influence of the whole system is 

 supposed to be instantaneously brought to bear on the element, and 

 not merely the influence of the surrounding parts. On this saving 

 hypothesis, the magnetic energy is here also correctly localised, for 

 dynamical purposes, in the element of volume of the medium, and 

 the Lagrangian method has perfect application to the mathematical 

 analysis of its phenomena. 



Now, in the case of the aether we have at hand a vera causa 

 precisely of this kind. The cause of the phenomena of gravitation 

 has hitherto remained perfectly inscrutable. Though the present 

 order of ideas forbids us to consider it otherwise than as propagated 

 in time, yet all we know of its velocity of propagation is the demon- 

 stration by Laplace that it must, at the very least, exceed the velocity 

 of propagation of light in the same kind of proportion as the latter 

 velocity exceeds that of ordinary motions of matter. It is not un- 

 philosophical to assume that an explanation of gravitation might 

 carry along with it the explanation of the fact that the tangential 

 tractions on an element of the strained aether are unbalanced. The 

 dynamical phenomena of mass in matter would appear to be ana- 

 lytical^ explicable by the addition of a rotational part to the kinetic 

 energy of the element of the medium ; such a term is of course prac- 

 tically null except in the vortex rings. 



In all that has been hitherto said we have kept clear of the com- 

 plication of viscous forces ; but in order to extend our account to 

 the phenomena of opacity in the theory of radiation and of electric 

 currents in ordinary conductors, it is necessary to introduce such 

 forces and make what we can of them on general principles. It is 

 shown that the introduction of the dissipation function into dynamics 

 by Lord K-ayleigh enables us to amend the statement of the funda- 

 mental dynamical principle, the law of Least Action, so as to include 



