Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/66

 Franklin's own writings. It originated in connexion with the explanation of the Leyden jar, a matter which is discussed in his third letter to Collinson, of date September 1st, 1747. In charging the jar, he says, a quantity of electricity is taken away from one side of the glass, by means of the coating in contact with it, and an equal quantity is communicated to the other side, by means of the other coating. The glass itself he supposes to be impermeable to the electric fluid, so that the deficiency on the one side can permanently coexist with the redundancy on the other, so long as the two sides are not connected with each other; but when a connexion is set up, the distribution of fluid is equalized through the body of the experimenter, who receives a shock.

Compelled by this theory of the jar to regard glass as impenetrable to electric effluvia, Franklin was nevertheless well aware that the interposition of a glass plate between an electrified body and the objects of its attraction does not shield the latter from the attractive influence. He was thus driven to suppose that the surface of the glass which is nearest the excited body is directly affected, and is able to exert an influence through the glass on the opposite surface; the latter surface, which thus receives a kind of secondary or derived excitement, is responsible for the electric effects beyond it.

This idea harmonized admirably with the phenomena of the jar; for it was now possible to hold that the excess of electricity on the inner face exercises a repellent action through the substance of the glass, and so causes a deficiency on the outer faces by driving away the electricity from it.

Franklin had thus arrived at what was really a theory of action at a distance between the particles of the electric fluid; and this he was able to support by other experiments. “Thus," he writes, "the stream of a fountain, naturally dense and continual, when electrified, will separate and spread in the form of a brush, every drop endeavouring to recede from every other