Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/227

 through it: it was called by Faraday the specific inductive capacity of the insulator.

The discovery of this property of insulating substances or dielectrics raised the question as to whether it could be harmonized with the old ideas of electrostatic action. Consider, for example, the force of attraction or repulsion between two small electrically-charged bodies. So long as they are in air, the force is proportional to the inverse square of the distance; but if the medium in which they are immersed be partly changed—e.g., if a globe of sulphur be inserted in the intervening space—this law is no longer valid: the change in the dielectric affects the distribution of electric intensity throughout the entire field.

The problem could be satisfactorily solved only by forming a physical conception of the action of dielectrics: and such a conception Faraday now put forward.

The original idea had been promulgated long before by his master Davy. Davy, it will be remembered, in his explanation of the voltaic pile, had supposed that at first, before chemical decompositions take place, the liquid plays a part analogous to that of the glass in a Leyden jar, and that in this is involved an electric polarization of the liquid molecules. This hypothesis was now developed by Faraday. Referring first to his own work on electrolysis, he asserted that the behaviour of a dielectric is exactly the same as that of an electrolyte, up to the point at which the electrolyte breaks down under the electric stress, a dielectric being, in fact, a body which is capable of sustaining the stress without suffering decomposition.

"For," he argued, "let the electrolyte be water, a plate of ice being coated with platina foil on its two surfaces, and these