Extract of a Letter from M. Humboldt to M. Blumenbach, containing new Experiments on the Irritation caused by the Metals with respect to their different Impressions on the Organs of Animals. From Gren’s “Journal of Natural Philosophy.” M. Humboldt is one of the philosophers who has made the most numerous observations on the phenomenon discovered by Galvani, concerning the irritability produced by the contact of different metals with the parts of animals in which the principle of life is apparently extinguished. As long ago as the year 1795, he observed that the animal irritability was augmented by the oxygenated muriatic acid. Not having discontinued his attention to this object, the perusal of the physiological writings of Reil, and his correspondence with Scarpa and Volta, afforded him indications for new enquiries, of which he has occasionally had the courage to make himself the subject. “In a conversation,” says he, “with M. Scarpa, at Pavia, on the effects which galvanism produced upon myself, nothing surprised him more than the appearance of a lymphatic and serous humour on my back. ‘What can be the nature,’ said he, ‘of this stimulant, which in a few instants changes the nature of the vessels to such a degree as to cause them to prepare humours, which, the instant they touch epidermis, excite inflammation, and mark their course by a redness which lasts for whole hours?’” M. Humboldt promised to repeat the experiment; and the account he gives of the facts constitutes one of the most interesting articles in his letter. For this purpose he caused two blistering plasters to be applied on the deltoid muscle of both shoulders. When the left blister was opened, a liquor flowed out which left no other appearance on the skin than a slight varnish, which disappeared by washing. The wound was afterwards left to dry up: this precaution was necessary, in order that the acrid humour which the galvanic irritation would produce might not be attributed to the idiosyncrisis of the vessels. This painful operation was scarcely commenced on the wound, by the application of zinc and silver, before the serous humour was discharged in abundance: its colour became visibly dark in a few seconds, and left on the parts of the skin where it passed, traces of a brown inflamed red. This humour having descended towards the pit of the stomach, and stopped there, caused a redness of more than an inch in surface. The humour, when traced along the epidermis, left stains, which after having been washed, appeared of a blueish red. The inflamed places having been imprudently washed with cold water, increased so much in colour and extent, that M. Humboldt, as well as his physician Dr. Schallern , who assisted at these experiments, entertained some apprehension for the consequences. M. Humboldt has not undertaken to determine the nature of the fluid which produces such astonishing effects; but he applies himself to circumscribe the phenomena in the real circumstances which produce them. He judiciously varies the preparations, and carefully notes all the results; being persuaded that the cause of galvanism cannot be explored with success, but by observing the proportions in which the chain of metals either irritates or has no effect: and to extend still more this vast field of observation, he employs various means to raise or diminish the irritable capacity of the animal organs. What is the sensation which the galvanic irritation produces? M. Humboldt has discussed this question. “No one,” says he, can speak more decidedly on this subject than myself, having made several experiments on my own person, the seat of which, in some instances, was the socket of a tooth which I had caused to be extracted; in others, certain wounds which I made in my hand; and in others, the excoriations produced by four blistering plasters. The following is his answer:— The galvanic irritation is always painful, and the more so in proportion as the irritated part is more injured, and the time of irritation more prolonged. The first strokes are felt but slightly; the five or six following are much more sensible, and even scarcely to be endured, until the irritated nerve becomes insensible from continued stimulus. The sensation does not at all resemble that which is caused by the electric commotion and the electric bath; it is a peculiar kind of pain, which is neither sharp, pungent, penetrating, nor by intermissions, like that which is caused by the electric fluid. We may distinguish a violent stroke, a regular pressure, accompanied by an unintermitting glow, which is incomparably more active when the wound is covered with a plate of silver, and irritated by a rod of zinc, than when the plate of zinc is placed on the wound, and the silver pincers are used to establish the communication. When the communication is made by the contact of the epidermis, it produces no effect; it appears to insulate like glass, when interposed between the wound and the metal; but if the skin be removed, by making two wounds at eight inches distance, and a plate of zinc be placed on one of them, and on the other a leg of a frog prepared, this last is seen to contract itself when it communicates with the zinc by the silver wire, which proves that the galvanic fluid there passes beneath the epidermis. This fluid produced in some circumstances a very sensible acid taste. The two wounds of M. Humboldt having been covered, one with silver, the other with zinc, an iron wire of several feet in length, attached to the zinc, was conveyed between his upper lip and the spongy substance of the teeth, and thence to the tongue of another person. When the iron wire was made to touch the silver, a strong contraction of the scapular muscle took place, and at the same instant the person whose tongue formed part of the chain of communication perceived the sensation of acidity. There are also cases in which the fluid acts on the organs of taste without producing any sensible effect on the organs of motion: such is that where the epidermis serves as the conductor from zinc to the frog; for there is not then any contraction, but merely an acid taste on the tongue. The author, having learned from M. Volta that he employed the solution of potash (oleum tartari per deliquium), in order to augment the conducting power, availed himself successfully of this means to raise the capacity of the animal organs. He moistened one of his wounds with this liquor, which produced little pain; but the galvanic irritation was more violent, and accompanied with more heat; sparks appeared and disappeared before his eyes; the tongue moistened with the same, distinctly perceived the acid sensation, although the communication was established only between zinc and zinc. The thigh of the frog, moistened with the alkaline solution and laid upon a plate of glass, without touching either metal or carbonic matter, fell of itself into violent convulsions, the antagonist muscles of the legs and toes being incessantly agitated. Irritability has been re-established by this application in the animal parts, where it had been extinguished by warm solutions of the oxide of arsenic. Lastly, the irritation (which does not commonly take place when the nerve and the muscle are armed with the same metal, the different metals being between the coatings), becomes manifest after this preparation; which seems to indicate that the alkali not only irritates the nerve, but likewise adds to its irritability. The author applied this method to amphibious animals, which he roused from their winter sleep, and in which he perceived a peculiar symptom of irritability. These observations led him to distinguish two states of the animal organ. The first, of irritability naturally or artificially raised or excited; the second, irritability in a less degree. These two states, which he calls positive and negative, are merely, as he remarks, different degrees, and not phenomena absolutely distinct from each other. In individuals naturally sensible, the effects produced by alkaline solutions, by the oxygenated muriatic acid, by the solution of oxide of arsenic, are very rarely of the same intensity. In the case of increased irritability, muscular motions are observed without metal or carbonic matter. They may be obtained with metals, though without communication between the nerve and the muscle; that is to say, without the regular connection or chain. They may be also obtained by forming the chain of similar metals. Let the crural nerve of an animal naturally tenacious of life be placed upon glass. Let a small piece of fresh muscular flesh be fixed on a stick of sealing-wax, and then brought into contact with the crural muscle. The result will be a violent convulsion at the instant when the chain of communication is completed. The same thing happens if, instead of the small piece of muscular flesh, a detached piece of the crural nerve be fixed on the stick of sealing-wax. The connection is therefore formed of two things, nerve and muscular fibre. How in this simple case can the fluid which passes from the nerve into the muscle cause it to be contracted? M. Humboldt thinks that it becomes stimulant, merely because it returns from the nerve into the nerve by a foreign animal matter; that is to say, not organically connected with the nerve. The disparity of the metals forming the chain has hitherto appeared as a necessary condition to produce galvanic irritation. This hypothesis, however, is overturned by the experiments of M. Humboldt. If it be true that, in the state of less irritability, there is very rarely contraction with similar metals (as Volta affirms, contrary to Aldini), this circumstance becomes indifferent in the case of increased irritability. M. Humboldt put into a china cup some mercury exactly purified; he placed the whole near a warm stove, in order that the entire mass might assume an equal temperature: the surface was clear, without the appearance of oxidation, humidity, or dust. A thigh of a frog, prepared in such a manner that a crural nerve and a bundle of muscular fibres of the same length hung down separately, was suspended by two silken threads above the mercury. When the nerve alone touched the surface of the metal, no irritation was manifested; but as soon as the muscular bundle and the nerve touched the mercury together, they fell into convulsions so brisk that the skin was extended as in an attack of tetanus. We ought not to be surprised at the precaution here taken by M. Humboldt to heat the mercury. This is required in consequence of the opinion which he announces, that the parity of the metals does not depend on the homogeneity of their chemical constituent parts, but of their heat, polish, hardness, and force. Gold, placed between two armatures of zinc, produces irritation only when the gold is moistened by some volatile fluid, or by the moisture of respiration. Lastly, M. Humboldt has attempted to include all the cases in the following formula:— 1. In the State of Increased Irritability. Positive cases. Frog—muscular flesh. Frog—zinc—zinc. Frog—zinc—muscular flesh—silver. Frog—zinc—silver—zinc. Frog—muscular flesh—silver—zinc. Frog—zinc—muscular flesh—silver—muscular flesh—zinc. 2. In the State of Diminished Irritability. Positive cases. Frog—zinc—silver. Frog—zinc—muscular flesh—silver—zinc. Frog—zinc—muscular flesh—silver—muscular flesh—silver—zinc. Negative cases. Frog—zinc—zinc. Frog—zinc—muscular flesh—silver. Frog—zinc—muscular flesh—silver—zinc. M. Humboldt finished this letter by some observations which he has collected in the course of his experiments on the sthenic or asthenic virtue of chemical agents; that is to say, their energy or their inefficacy to produce irritation. Alkalis appear to be to the sensible fibres what acids are to muscular groups. The muriatic acid augments the irritability of the muscle while it extinguishes that of the nerves, which does not re-appear even after the acid has been saturated with alkali. By continuing to bathe the nerve with an alkaline solution, an entire atony is at length produced by excess of irritation; but if a few drops of muriatic acid be left on the part, the irritability is reestablished. A thigh of a frog, irritated even to total relaxation by a warm solution of oxide of arsenic, has exhibited new convulsions after having been immersed for two minutes in a solution of potash. The sthenic virtue of the oxygenated muriatic acid is not less remarkable. Thighs of frogs naturally flaccid, and weakened still more by the galvanic process for seven hours, which afforded no sign of motion when silver served as a conductor between zinc and the nerve, exhibited violent contractions when the nerve was moistened with oxygenated muriatic acid. The author refers to this subject the experiment which he published in 1793, in his Flora Fribergensis, by which it is ascertained that ordinary muriatic acid retards the germination of plants, but that oxygenated muriatic acid had caused a plant to germinate in seven hours, which required thirty-eight in pure water in order to arrive at the same development. This fact appears to him to indicate some relation between the vegetable and animal organization.