"The fundamental experiment which led to the discovery of dynamical electricity [1786] is due to Galvani, professor of anatomy in Bologna. Occupied with investigations on the influence of electricity on the nervous excitability of animals, and especially of the frog, he observed that when the lumbar nerves of a dead frog were connected with the crural muscles by a metallic circuit, the latter became briskly contracted. ... Galvani had some time before observed that the electricity of machines produced in dead frogs analogous contractions, and he attributed the phenomena first described to an electricity inherent in the animal. He assumed that this electricity, which he called vital fluid, passed from the nerves to the muscles by the metallic arc, and was thus the cause of contraction. This theory met with great support, especially among physiologists, but it was not without opponents. The most considerable of these was Alexander Volta, professor of physics in Pavia. Galvani's attention had been exclusively devoted to the nerves and muscles of the frog; Volta's was directed upon the connecting metal. Resting on the observation, which Galvani had also made, that the contraction is more energetic when the connecting arc is composed of two metals than where there is only one, Volta attributed to the metals the active part in the phenomenon of contraction. He assumed that the disengagement of electricity was due to their contact, and that the animal parts only officiated as conductors, and at the same time as a very sensitive electroscope. By means of the then recently invented electroscope, Volta devised several modes of showing the disengagement of electricity on the contact of metals. ... A memorable controversy arose between Galvani and Volta. The latter was led to give greater extension to his contact theory, and propounded the principle that when two heterogeneous substances are placed in contact, one of them always assumes the positive and the other the negative electrical condition. In this form Volta's theory obtained the assent of the principal philosophers of his time." _A. Ganot, Elementary Treatise on Physics; translated by Atkinson, book 10, chapter 1._ Volta's theory, however, though somewhat misleading, did not prevent his making what was probably the greatest step in the science up to this time, in the invention (about 1800) of the Voltaic pile, the first generator of electrical energy by chemical means, and the forerunner of the vast number of types of the modern "battery." {772} ELECTRICITY: A. D. 1810-1890. The Arc light.