ELECTRICITY FROM THALES TO FARADAY. 251 



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 tor- 

 pedo, 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 w^hich 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 elec- 

 tricus, 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 caiDturing 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 cen- 

 tury. 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 solu- 

 tion of salt he found to possess only seven and one quarter times the 

 conductivity of the extremely weak one mentioned. He also estab- 

 lished the law that the capacity of condensers (of which the pre- 

 viously mentioned Leyden jar is an example) varies directly as the 

 active area, and iuA'ersely as the distance separating the conducting 

 surfaces. It was reserved for later investigators to make the grand 

 discoveries which relate to electrochemical dissociation, but Caven- 

 dish 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 remark- 

 able discovery (1790) of the fact that freshly amputated frogs' legs, 

 on being touched along the lines of the muscles by dissimilar metals, 



