Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/303

 Maxwell was impressed, as Kirchhoff had been before him, by the close agreement between the electric ratio c and the velocity of light ; and having demonstrated that the propagation of electric disturbance resembles that of light, he did not hesitate to assert the identity of the two phenomena. "We can scarcely avoid the inference," he said, "that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena." Thus was answered the question which Priestley had asked almost exactly a hundred years before: "Is there any electric fluid sui generis at all, distinct from the aether?"

The presence of the dielectric constant ε in the expression cε-$1⁄2$, which Maxwell had obtained for the velocity of propagation of electromagnetic disturbances, suggested a further test of the identity of these disturbances with light: for the velocity of light in a medium is known to be inversely proportional to the refractive index of the medium, and therefore the refractive index should be, according to the theory, proportional to the square root of the specific inductive capacity. At the time, however, Maxwell did not examine whether this relation was confirmed by experiment.

In what has preceded, the magnetic permeability μ has been supposed to have the value unity. If this is not the case, the