Symmer advanced the theory of two fluids coexisting in all matter (not independently of each other, as had been previously supposed), which by mutual counteractions produced all electrical phenomena. His conception was that a body, positively electrified, did not exist in that condition because of the possession of a charge of a positive (as distinct from a negative) electric fluid which it had not held before, and did not hold in a normal state; nor that it possessed a greater share of a single electric fluid than it did in an unelectrified condition, as had been believed by Franklin and Watson, and by Dufay respectively; but that such a body contained both positive and negative electricities which, when the body behaved as "unelectrified," entirely counteracted each other, but which, on the other hand, caused a positive or negative charge to be evinced should either positive or negative electricity respectively preponderate. Æpinus was the author of another notable theory, of which we must omit further mention for want of space. 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.