Extract of a Letter from Mr. Humboldt to Mr. Blumenbach , containing new Experiments on the Irritation cauſed by the Metals with Reſpect to their different Impreſſions on the Organs of Animals . This Letter forms part of Gren’s (German) Journal of Natural Philoſophy for the month of October laſt. The extract was read to the Inſtitute of France, by Citizen Guyton. MR. 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 the 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 flight 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 Mr. Humboldt, as well as his phyſician Dr. Schallern , who aſſiſted at theſe experiments, entertained ſome apprehenſion for the conſequences. Mr. 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? Mr. 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 then paſſes beneath the epidermis. This fluid produced in ſome circumſtances a very ſenſible acid taſte. The two wounds of Mr. 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 ſluid 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 Mr. Volta that he employed the ſolution of pot-aſh (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’s 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 ſealingwax. 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? Mr. 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 Mr. 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. Mr. Humboldt put into a china cup ſome mercury exactly puriſied; 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 Mr. 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 form. 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, Mr. Humboldt has attempted to include all the caſes in the following formulæ: 1. In the State of increaſed Irritability. Poſitive caſes. Frog—muſcular ſleſh. 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. Mr. Humboldt finiſhes 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 iritability of the muſcle while it extinguiſhes that of the nerves, which does not re-appear even aſter 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 let fall on the part, the irritability is re-eſtabliſhed. 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 pot-aſh. 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. A judgment may be formed from this extract of the number of important facts contained in this letter, and of the intereſt they will excite when they ſhall be collected, arranged, and amplified, in the large work which the author is preparing.