Page:Scientific Monthly, volume 14.djvu/565



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):

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.