We have seen that there are two types of charged bodies, of which charged glass and charged silk are familiar examples. It was Dufay (1699-1739) who discovered that there were two kinds of electricity, one of which he called _vitreous_ (from glass) and the other _resinous_ (from resin—amber). The terms “positive” and “negative” in relation to electricity were first applied by Benjamin Franklin, in 1756. To the electricity of the glass rod Franklin gave the name “positive” and to that of the sealing-wax (or hard rubber, amber, etc.) the name “negative.” These names are now universally in use—though French physicists still speak of vitreous and resinous electricity. I have spoken also of a positive pole (or electrode) and a negative pole (or electrode). The electrodes constituting the two poles of a current are also called the anode and the cathode, the former being the positive electrode and the latter the negative electrode.[18] When it was learned that electrical charges could be distinguished by two opposing terms—positive and negative—it was natural to suppose that there were two distinct kinds of electricity, or “fluids.” This was the view taken by the French chemist Dufay. But the German electrician Æpinus (1724-1802), in his great pioneer work, “_Tentamen Theoriae Electriciatis et Magnetismi_” (An Attempt at a Theory of Electricity and Magnetism—1759), considered the mathematical consequences of the hypothesis of a single fluid, attracting all matter but repelling itself. It soon became apparent, however, that he must assume either the existence of two electrical fluids or the mutual repulsion of material particles. He chose the latter theory. He explained the phenomena of the opposite poles as results of the excess and deficiency of a “magnetic fluid,” which was dislodged and accumulated in the ends of the body, by the repulsion of its own particles, and by the attraction of iron and steel, as in the case of induced electricity.[19] Æpinus, who was unquestionably one of the greatest physicists of the eighteenth century, devised a method of examining the nature of the electricity at any part of the surface of a body, by which means he was enabled to ascertain its distribution. He found that the distribution was in agreement with the attractions and repulsions which objects exert when they are in the neighborhood—“electrical atmosphere”—of electrified bodies. Today we say that such bodies are electrified by induction.