18. The fluid vortex atom of Lord Kelvin faithfully represents in various ways the permanence and mobility of these subatoms of matter: but it entirely fails to include an electric charge as part of their constitution. According to any aether-theory static electric attraction must be conveyed by elastic action across the aether, and an electric field must be a field of strain: hence each subatom with its permanent electric charge must be surrounded by a field of permanent or intrinsic aethereal strain, which implies elastic quality in the aether instead of complete fluidity: the protion must therefore be in whole or in part a nucleus of intrinsic strain in the aether, a place at which the continuity of the medium has been broken and cemented together again (to use a crude but effective image) without accurately fitting the parts, so that there is a residual strain all round the place.
The assumption of elasticity of some kind in the aether is of course absolutely essential to its optical functions: and the elucidation from the optical phenomena, as a purely abstract problem in analytical dynamics, of the mathematical type of this elasticity, was acomplished in 1839 by MacCullagh[1] in an investigation which may fairly claim to rank amongst the classical achievements of mathematical physics. The type of elasticity which he arrived at was one wholly rotational, so that the aether would be perfectly fluid for all motions of irrotational type, but would resist elastically, by a reacting torque, any differential rotations of the elements of volume, somewhat after the manner that a spinning fly-wheel resists any angular deflexion of its axis. Here then we have the specification of an ideal medium that would behave as a fluid to solid bodies moving through it, because its irrotational motion would be precisely the same as that of a fluid in the corresponding circumstances: it would not resist the motion of such solids any more than the aether resists the motion of the heavenly bodies or of material masses generally: moreover vortex rings could permanently exist in it and persist according to the well-known laws of abstract hydrodynamics. But these tempting
- ↑ 'An Essay towards a Dynamical Theory of Crystalline Reflexion and Refraction,' Trans. R. I. A., vol. xxi: Collected Works, p. 145.
