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Page:Popular Science Monthly Volume 56.djvu/265

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ELECTRICITY FROM THALES TO FARADAY.

Disjointed observations connected with animal electricity had been accumulating for many centuries. The first chronicled note that refers to the subject dates back to 676 a. d. Whether or not entirely by chance, the Arabians named the electric eel, or torpedo, in a way that impresses us now as singularly felicitous, raad (the lightning). Toward the end of the last century Redi discovered that the shock was sometimes conveyed through the line and rod to the fisherman, and Kampfer compared the effects to those of electrical discharges. It does not appear, however, that the resemblance was actually believed to be more than accidental until Bancroft urged, in the last ten years of the eighteenth century, the view which was shortly proved. Investigation since has shown that several other aquatic animals possess this astonishing manifestation of vitality, notably the Gymnotus electricus (Surinam eel), the Trichiurus electricus, and the Tetraodon electricus. Humboldt gives an account of wonderful battles in South America between gymnoti and wild horses. In fact, the most expeditious method, if not the most humane one, of capturing these alarming creatures appears to be to drive horses into the pond inhabited by them, and to allow the eels to exhaust their strength by repeated electric discharges before endeavoring to bring them to land by other means.

Cavendish was one of the most noted experimental investigators in the electrical field during the latter third of the eighteenth century. His work was remarkably accurate, considering the lack of a proper equipment for taking observations incident to operations in those days. He computed the relative conductivities of iron and water as four hundred million to unity, and found that the addition of but one part of common salt to one hundred of water increased the conductivity of the latter a hundredfold. A twenty-six-per-cent solution of salt he found to possess only seven and one quarter times the conductivity of the extremely weak one mentioned. He also established the law that the capacity of condensers (of which the previously mentioned Leyden jar is an example) varies directly as the active area, and inversely as the distance separating the conducting surfaces. It was reserved for later investigators to make the grand discoveries which relate to electrochemical dissociation, but Cavendish succeeded in accurately determining the ratio of combination of the elements of water in a method which superficially suggests the inverse of electrolytic decomposition—i. e., by inducing the combination of hydrogen and oxygen by the electric spark in the instrument known as the eudiometer.

Hard on the heels of this work came news of Galvani's remarkable discovery (1790) of the fact that freshly amputated frogs' legs, on being touched along the lines of the muscles by dissimilar metals,