As the fishes enumerated above have not all been examined with the same degree of atten tion, we are ignorant of the extent to which they exhibit phenomena exactly resembling one another. But it is well ascertained that they all agree in possessing the power of commu nicating a sudden shock to the hand which touches them. This shock causes a certain degree of temporary numbness not only in the finger which immediately touches the fish, but also in the hand, and sometimes even in the ann. The sensation produced has been com pared by different experimenters to the shock felt on the discharge of a Leyden phial, dif fering from it only in force. Hence the shock caused by an electrical fish is said to be pro duced by a discharge of its electricity. The numerous facts relating to the phenomena which accompany or are connected with this discharge, which have been collected by the industry of the many observers of the last and the present age, who have devoted their atten tion to the subject,* may be conveniently ar ranged under the following heads : I. the circumstances under which the discharge takes place : 2. the motions of the fish in the act of discharging : 3. physiological effects of the discharge : 4. magnetical effects of the discharge : 5. chemical effects of the dis charge : 6. results of experiments on the transmission of the discharge through various conducting bodies : 7. the production of a spark and evolution of heat : 8. results of experiments in which the nerves, electrical organs, and other parts, were mutilated : 9. descriptions of the electrical organs in the several fishes which have been anatomized.
I. Circumstances under which the discharge takes place.—Electrical fishes exert their pecu liar power only occasionally, at irregular inter vals, and chiefly when excited by the approach of some animal, or by the irritation of their surface by some foreign body. The discharge, both with regard to time and intensity, seems to be dependent on an exertion of the will. They discharge both in water and in air. Sometimes the discharge is repeated several times in close succession ; at other times, par ticularly when the fish is languid, only one discharge follows each irritation. The inten sity of the torpedo's discharge is generally greater when the fish is vigorous, becomes gra dually less as its strength fails, and is wholly imperceptible shortly before death takes place ; but Dr. Davy has met with some languid and dying fish which exerted considerable electrical power. No irritation has ever produced a dis charge after death. The intensity of the elec trical power seems to hear no relation to the size of the fish, at least after it has attained mature age; small fish are almost always ac tively electrical.
The torpedo sometimes bears great irritation, even the firm grasp of a hand, without dis charging. In these circumstances it writhes and twists itself about for some time, using strong efforts to escape, before it emits its electricity. In a few instances it has been
found impossible by any means to excite even vigorous torpedos to discharge. Both Lac& pMe and Reaumur handled and irritated the most lively torpedos, even while yet in their native element, without experiencing any shock. But generally the shocks are stronger when the skin of the fish is in any way irritated. All electrical fishes soon become exhausted and die, even in sea-water, when they are excited to give a continued succession of discharges. But fishes much exhausted by frequent dis charges recover their electrical energy after a few hours' rest. The torpedo seems to possess electrical power even in the earliest periods of its existence. Spallanzani relates that he found within a female torpedo two living fetuses, which gave distinct shocks on being removed from their coverings. Dr. Davy, also, once received a sharp although not a strong shock, in extracting fatal fish from the uterine cavities of a dying torpedo.
When the Gymnotus is grasped by the hand, the intensity of the discharge is moderate at first, but is increased if the pressure be conti nued. The torpedo discharges whenever it is taken out of the water ; and Walsh found that a vigorous fish repeats the discharge as often as it is lifted out, and again on being re-im mersed ; also that it gives more violent shocks in air than in water. Spallanzani found the shock to be more severe when the fish was laid on a plate of glass. The following observation, reported by Walsh, seems to prove that the Gymnotus can distinguish at some distance between substances capable of receiving and conducting its discharge, and those which can not conduct ; and that (excepting when it is much irritated) it discharges only when con ducting bodies are presented to it. Two wires were put into the water of the vessel in which a Gymnotus was swimming; these wires were of some length, and stretched; they termi nated in two glasses filled with water placed at a considerable distance from each other. Whilst the apparatus remained in this state, and the circulation was of course interrupted, the animal did not prepare to exercise his power, but whenever any conducting substance filled the interval, and rendered the circle complete, it instantly approached the wires, arranged itself, and gave the shock.
The same fish, according to the observations of Messrs. Humboldt and Bonpland, appears to have the power of transmitting its discharge in any direction it pleases, or towards the point where it is most sharply irritated; and further, it seems to be able to discharge, some times min a single point, at other times from the whole of its surface. Dr. Davy has satis fied himself that the Torpedo also has the power of discharging its electricity in any direction it chooses.