THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
767 
the decay of the oscillations, will be a new state of equilibi-iinn, with cliai’ges on both 
the conductors, precisely as under electrostatic circumstances. 
64. The phenomenon of specific inductive capacity has been explained or illustrated 
at different times by Faraday, Mossotti, Lord Kelvin, and Maxwell, by the 
behaviour of a medium composed of small polar elements which partially orientate 
themselves under the action of the electric force ; and these ^'i^nsfemagnetic elements 
have been identified with the molecules, each composed of a positive and a negative 
ion. Another illustration* * * § which leads to the same mathematical consequences 
supposes the dielectric field to be filled with small conducting bodies, in each of which 
electric induction occurs, thus making it a polar element so long as it is under the 
influence of the electrie force. The qRasi-magnetic theory is adopted by von 
Helmholtz in his generalization, on the notions of action at a distance, of Maxwell’s 
theory of electrodynamics ; and it is shown by him that such a hypothesis destroys 
the cireuital character of the electric current, a conclusion which may also lie arrived 
at by elementary reasoning.t The molecules must therefore on such a theory be 
arranged with their positive and negative elements in some fomi of symmetry so that 
they shall have no appreciable resultant electric moments and the specific inductive 
capacity must be wholly due to diminution of the effective elasticity of the medium. 
The hexagonal structure imagined for quartz molecules by J. and P. Curie, and 
independently by Lord Kelvin,§ in order to explain piezo-electricity, or any other 
symmetrical grouping, exactly satisfies this condition ; the molecule in the state of 
equilibrium has no resultant electric moment ; but under the influence of pressure or 
of change of temperature a deformation of the molecule occurs, which just introduces 
the observed piezo-electric or pyro-electric polarity. 
[(Added June 14.) On the present view however there is absolutely no room for 
VON Helmholtz’s more general theory of non-circuital currents. The disjJacenient 
of an electric charge constitutes a rotation in the medium round the line of the 
displacement, but the electric field which causes the displacement is here also itself a 
rotation round an axis in the same direction ; whereas in von Helmholtz’s theory 
the inducing electric force is not considered to have any intrinsic electric displacement 
of its own. When both parts are taken into account, the electric displacement becomes 
circuital throughout the field. There is thus nothing in the postulate of circuital 
currents that would require us to make the electric moment of a molecule indefinitely 
small; so that specific inductive capacity might still, if necessary, be explained or 
illustrated in the manner of Faraday and Mossotti.] 
* Emplo^'ecl by Maxwell, “Dynamical Theory,” § II, ‘Phil. Trans.,’ 1864. 
t “On the theory of Electrodynamics,” ‘ Roy. Soc. Proc.,’ 1890. 
t The term electric moment is employed, after Lord Kelvin, as the precise analogue of magnetic 
moment. 
§ Lord Kelvin, “ On the piezo-electric quality of Quartz,” ‘ Phil. Mag.,’ Oct., 1893, Nov., 1893. 
