In 1827, before Dr. Davy performed his ex periments, s knit ar magnetic effects were observed by means of the multiplier, by MM. De Blainville and Fleuriau, at La Rochelle. They thrust into the electrical organ of a torpedo the two needles which terminate the wires of Schweigger's multiplier, and immediately saw the magnetic needle describe more than half a revolution.* V. Chemical effects of the discharge.—It does not appear that any observer before Sir II. Davy attempted to ascertain what chemical effects the discharge from electrical fishes is capable of producing. But Sir Ilumphry obtained only negative results. Ile passed the shocks of the torpedo through the unterrupted circuit made by the silver wire through water, without being able to perceive the slightest de composition of the Dr. John Davy, however, has obtained decisive evidence of chemical agency being exerted by animal elec tricity. The fishes which he made use of in his experiments were more recently taken from the sea, and were, consequently, more vigorous than those which were the subjects of Sir Ilumphry's observations; and it was, probably, owing to this circumstance that the results which he obtained were different from those of his brother's experiments.
By means of golden wires, one of which was applied to the upper surface of the fish, and the other to its under surface, Dr. Davy passed the discharge from a torpedo through solutions of nitrate of silver, common salt, and superacetate of lead, and found that all were decomposed. The decomposition of the superacetate of lead was effected only when the fish seemed to put forth all its energy, after being much irritated4 From the solution of nitrate of silver, the metal was precipitated only on the wire connected with the ventral surface of the fish. When platina wires were used, and plunged into nitric acid, gas was given off only from that in con nexion with the dorsal surface. A solution of iodide of potassium and starch having been subjected to the discharge conveyed along the platina wires, had the iodine in combination with the starch precipitated from it on the wire from the upper surface.§. By the same dis charges which produced these chemical effects, the needle in the galvanometer was moved, the spirit in the air-thermometer was raised, and needles in the spiral were magnetized.
VI. Results of experiments on the transmis sion cf the discharge through various conduct ing bodies.—Almost all bodies which are con ductors of common and voltaic electricity con duct also the discharge of electrical fishes; and those which are non-conductors with regard to the former are the same with regard to the latter.
But the discharge of the torpedo, when feeble, does not pass along even good conductors; and this circumstance has given rise to some dis crepancy between the statements of different observers. Walsh received the torpedo's dis charge through iron bolts and wet hempen cords. The French fishermen declare that they sometimes receive shocks through nets, while the fish is twelve feet distant from their hands. But Humboldt and Gay Lussac state that they received no shock when they touched the fish with a key or any other conducting body ;* further, that when they placed the fish upon a metallic plate, so that the inferior surface of its electric organ touched the metal, the hand which supported it felt no shock : and they concluded from their experiments that the torpedo could not transmit its discharge through even a thin layer of water ; although they found that when two persons applied each one hand to the fish, and completed a circuit through their own bodies by means of a pointed piece of metal held in the other hand, and plunged into a little water placed upon an insulating body, both felt the shock. In one instance Dr. Davy received the torpedo's shock through water, but his hand was within a very short distance of the fish. Walsh transmitted the torpedo's discharge through a chain of eight persons, who com municated with one another only by water con tained in basins, in which their hands were immersed. And the same observer also found that when a torpedo was touched with a single finger of one hand, while the other hand was held in the water at some distance, shocks were distinctly felt in both hands. Numerous observations made on the Gyinnotus leave no doubt with regard to the passage of its discharge through water. If a person hold his finger in the water several inches (some say even ten feet) distant from the fish, and another person touch it, both receive shocks equally severe. Dr. Williamson found that a person holding his finger in a stream of water, running from a hole made in the bottom of a wooden vessel in which a Gymnotus was swimming, very dis tinctly felt all the discharges given by the fish. The discharge from the Gymnotus passes through a chain of ten persons, so that they all seem to feel the shock jli the same degree. It is con ducted by iron rods several feet in length. It does not pass through air, interposed between metallic conductors, until these are brought within about one-hundredth of an inch of each other.
So far as they have been examined, the phe nomena presented by the discharge of the Silurus have been found to be nearly the same as those just detailed.