The analogy between the action of electromotive intensity in producing electric displacement and of ordinary mechanical force in producing the displacement of an elastic body is so obvious that I have ventured to call the ratio of the electromotive intensity to the corresponding electric displacement the coefficient of electric elasticity of the medium. The coefficient is different in different media, and varies inversely as the specific inductive capacity of each medium.
Farther along in his treatise Maxwell argues that this "Elec tric Elasticity" is the elasticity by means of which light waves are propagated through the ether. Thus he says (Vol. II, p 431):
According to the theory of emission, the transmission of energy is effected by the actual transference of light corpuscles from the luminous to the illuminated body, carrying with them their kinetic energy, together with any other kind of energy of which they may be the receptacles.
According to the theory of undulation, there is a material medium which fills the space between the bodies, and it is by the action of contiguous parts of this medium that the energy is passed on from one portion to the next, till it reaches the illuminated body.
The luminiferous medium is therefore, during the passage of light through it, a receptacle of energy.
In the undulatory theory as developed by Huyghens, Fresnel, Young, Green, etc., this energy is supposed to be partly potential and partly kinetic. The potential energy is supposed to be due to the distortion of the elementary portions of the medium. We must therefore regard the medium as elastic. The kinetic energy is supposed to be due to the vibratory motion of the medium. We must therefore regard the medium as having a finite density.
In the theory of electricity and magnetism adopted in this treatise, two forms of energy are recognized, the electrostatic and the electrokinetic (see Arts. 630 and 636), and these are supposed to have their seat, not merely in the electrified or magnetized bodies, but in every part of the surrounding space, where electric or magnetic force is observed to act. Hence our theory agrees with the undulatory theory in assuming the existence of a medium which is capable of becoming the receptacle of two forms of energy.
Let us next determine the conditions of the propagation of an electromagnetic disturbance through a uniform medium, which we shall suppose to be at rest, that is, to have no motion except that which may be involved in electromagnetic disturbances.
Maxwell then proceeds to develop an equation for the velocity of an electromagnetic disturbance in terms of the specific inductive capacity and the magnetic permeability of the medium and which, if the specific inductive capacity be taken as the reciprocal of the elasticity and the magnetic permeability be taken as the density of the medium gives an expression for the velocity of a wave motion in an elastic medium. It also gives an expression for the ratio of the electromagnetic to the electrostatic unit of electricity, or the velocity with which a unit electrostatic charge must move in order to become electromagnetically a unit current. This ratio can be deter mined experimentally, and gives a quantity numerically equal to the velocity of light.
