VI I. The production of a spark, and evolu tion of heat.—No observer has hitherto seen light emitted from the body of any electrical fish at the time of the discharge; but, by artificial arrangements, some have succeeded in pro ducing sparks in the course of the circuit de scribed by the discharge. In 1792, Gardini saw a spark from a torpedo's discharge, in the course of his repeating sonic of Walsh's experiments. And in 1797, Galvani obtained a small spark, visible only with the aid of a lens, from a torpedo ; but it does not appear that any other observer has been equally successful with regard to this fish. Very recently, Dr. Davy has directed his attention particularly to this point, and, although he used active fish, and took every possible precaution, he could neither, in the light, detect the slightest indications of the passage of electricity through even very small intervals of air, nor observe a spark in the dark. Ile was equally unsuccessful in using an elec troscope formed on the principle of Coulomb's, which displayed sparks when touched either with a small rod of glass slightly excited, or of sealing-wax. Ile varied the trials, using highly rarefied air at ordinary temperatures, and also condensed air deprived of moisture, with the same negative result. Ile insulated the fish on a plate of glass, wiped its margin dry, and besmeared it with oil, but no spark could be procured.
Dr. Davy was more successful in obtaining indications of the evolution of heat during the torpedo's discharge. Ile used Harris's electro meter, and saw proof of an elevation of tem perature in the motions of the fluid in the air thermometer; thus corroborating the prediction of Dr. Faraday, who was previously convinced that, by means of this instrument, the evolution of heat by animal electricity would be made evident. Dr. Davy made several experiments with the view of ascertaining whether very fine platina wire might not be ignited in the passage of the electricity of the torpedo, but never witnessed the expected effect. Upon this he remarks, "This want of ignition may, at first view, seem contrary to the effect on the ther mometer; but perhaps it ought not to be con sidered so, taking into account the rapid wan ner in which the heat evolved in the tine pla tina wire must be carried off by the adjoining compound wire of platina and silver."* From the discharge of the Gymnotus, Walsh, Fahlberg, Guisan, and other observers of the last century, obtained sparks. NValsh attached a thin sheet of pewter to a plate of glass, cut a very fine slit in it, and then passed the discharge along die metallic sheet, the fish being at the time out of the water. A spark was very dis tinctly seen at the margins of the slit. Fahlberg of Stockholm used the same kind of apparatus, but with gold leaves instead of pewter, and placed the margins of these about a line apart. Dr. Williamson fixed two brass rods in a frame, and brought their points to within one-hundredth of an inch of each other, but, although the dis charge of the gyinnotus passed from one rod to the other through the intervening air, there was no spark. Humboldt watched an active Gym notus for a long time during the night, and irritated it so as to obtain from it many sharp discharges, but he saw no spark.
VIII. Results of experiments in which the Viii. Results of experiments in which the nerves, electrical organs, and other parts were mutilated.—The general result of these experi ments is, that destruction of the communications between the electrical organs and the nervous centres is followed by annihilation of the power of discharging.
According to Mr. Todd, (whose experiments were made on the torpedo at the. Cape of Good hope,) it is necessary to cut through all the nerves going to the electrical organs to destroy their peculiar powers. !le cut through all on one side, and some on the other, but still shocks were given. lie also lacerated the organs themselves extensively, without destroy ing the discharging power. Mr. Todd found that fishes in which all the electrical nerves had been cut appeared more vivacious after the operation than before it, and actually lived longer than others not so injured, but which were excited to discharge frequently.* In repeating Mr. Todd's experiments, Dr. Davy obtained very similar results; but he mentions that "when a small portion of brain was accidentally left, contiguous to the elec trical nerves of one side, and with which they were connected, the fish, on being irritated, gave a shock to an assistant, who grasped the corresponding, electrical organ."t Spallanzani found that the torpedo loses its power of giving shocks after the aponeurotic covering of the electrical organs is removed ; but that the cutting out of die heart does not lessen this power until the animal life begins to suffer from the loss of blood.
Humboldt cut a Gymnotus through the mid dle of the body transversely, and found that the anterior portion alone continued to give shocks.
Experiments of this kind have not yet been performed on the Silurus ; but, judging from the structure of the organs in this fish, we have every reason to expect that the results of such experiments on it would be the same. While we would not be understood to sanction the wanton repetition of experiments such as these, which cannot but be productive of much suffer ing to the subjects of them, we must yet repeat here the suggestion recently made by l'rofessor Miller of Berlin with regard to future experi ments on the Gymnotus and Silurus. He points out how very desirable it is to ascertain whether the double organs of these fishes act as opposite electromotors, which might be determined by cutting out one organ from either side, and then exciting the fish to discharge. The same dis tinguished physiologist remarks that if he had an opportunity of experimenting on the torpedo, his first experiment should be, after having cut through the nerves going to the electrical organs, to irritate their cut extremities, still in connexion with the organs, with mechanical and galvanic stimulants, with the view of discovering whether these would excite the organs to discharge their eleetrieity.r IX. Anatomy of the electrical organs.—The experiments referred to in the former section sufficiently demonstrate that the manifestation of the peculiar power possessed by electrical fishes depends on the integrity of the connexion between their nervous centres and certain organs of a peculiar structure, which have been named the electrical organs. These have been particularly examined in the Torpedo, Gymnotus, and Silurus, by several anatomists, and no doubt is entertained that they, togs they with their large nerves, are the sole means employed in bringing this mysterious agent under the control of the animal's volition. They are therefore well worthy of an attentive examination.