GALVANISM.

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tlie integuments and muscles ot' the abdomen, and remove the en-trails, by which means you will lay bare the crural nerves. Thenpass one bla le of the scissars under the nerve, and cut off the spinewith the flesh close to the thighs, by which means the legs w ill re-main attached to the spine by the nerves alone. This done, leavea small bit only of the spine attached to the crural nerves, and cutoff all the rest. Tims you will have the lower limbs G, 11, fig. 1,Plate LX XIX, of the frog adhering to the bite of spine A B, bymeans of the crurr.l nerves C, D. T hese legs must be flayed inorder to lay bare the muscles; and a bit of tin-foil should be wrap-ped round the spine ATI. With this preparation the experimentmay be performed in various ways, but the two which follow arethe best. Hold the preparation by the extremity of one leg, theother leg hanging down, with the armed bundle of nerves andspine lying upon it. In this situation interpose a piece of silver,as a half-crown, between the lower thigh and the nerves, so that itmay touch the former with one surface, and the metallic coatingof the latter with the other surface, or with its edge; and you willfind that the hanging leg will vibrate very powerfully, sometimesso far as to strike against the hand of the operator, which holds theother leg. Otherwise, place twm wine-glasses, both full of water,contiguous to each other, but not actually' touching. Put thethighs and legs of the preparation in the water of one glass, andlaying the nerves over the edges of the two glasses, let the bit ofspine with its armour (viz. tin-foil) touch the water of the otherglass. T ilings being thus prepared, if vou form the communica-tion between the-waters of the two glasses, by means of silver, orput the fingers of one hand into the water of the glass that containsthe legs,and bolding a piece of silver in the other,you touch the coat-ing of the nerves with it, you w ill find that the prepared legs moveso powerfully as sometimes to jump fairly out of the glass. Differentmetals(properly disposed were soon found to alfect several of thesenses in a remarkable manner. Let a man lay a piece of metal,as zinc, upon his tongue, and a piece of some other metal, as silver,under the tongue ; on forming the communication between thosetwo metals, either’Ey bringing their outer edges in contact, or byth° interposition of some other piece of metal, he will perceive apeculiar sensation, a kind of irritation, accompanied with a sort ofcool and subacid taste, not exactly like, and yet not much dif-ferent from, that which is produced by artificial electricity. Thesensation seems to be more distinct when the metals are of theusual temperature of the tongue. The silver or gold may be ap-plied to any other part of the mouth, to the nostrils, to the ear, orto other sensible parts of the body, whilst the zinc is applied to thetongue; and on making the communication between the two metals,the taste will be perceived upon the tongue. The effect is rathermore remarkable when the tongue touches the zinc in a small part,and the silver in a great portion of its surface, than vice versa. In-stead of the tongue, the two metals mav also be placed in contactwith the roof of the mouth, as far back as possible ; and on com-pleting the communication, the taste or irritation will be perceived.Dilferent persons are variously affected by this application ofmetals; with some the sensation or taste is so slight as to behardly perceived, whilst with others it is very strong, and even dis-agreeable. Some persons feel merely a pungency, and not properlya taste. In order to alfect the sense of sight by means of metals,let a man in a dark place put a slip of tin-foil upon the bulb of oneof bis eyes, and let him put a piece of silver, as a spoon or the like,in his mouth. On completing the communication between thespoon and the tin-foil, a faint flesh of while light will appear be-fore bis eyes. This experiment may be performed in a more con-venient manner, b v placing a piece of zinc between the upper lipand the gums, as high up as possible, and a silver piece of moneyupon the tongue; or else by putting a piece of silver high up inone of the nostrils, and a piece of zinc in contact with the upperpart of the tongue; for in either case the Hash of light will appearwhenever the two metals are made to communicate, either by theimmediate contact of their edges, or by the interposition of othergood conductors. By continuing the contact of the two metals,the appearance of light is n it continued, it being only visible atthe moment of making the contact, and sometimes, though rarely,at the instant of separation : it may therefore be repeated at plea-sure, by disjoining, ami again connecting, the two metals. Whenthe eves are in a state of inflammation, the appearance of light ismuch stronger. When the science was advanced no farther than

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the knowledge of the above-mentioned farts, it was doubtfulwhether the convulsions of prepared animal limbs, and the sen-sations which are produced by the application of metallic sub-stances, were owing to some electrical propeitv peculiar to theanimal parts, which might perhaps be conducted through the metalsfrom one part to the other; or to a small quantity ot electricity,which might be supplied by the metals themselves. The lattersupposition however was soon verified by the result of various ex-periments, which prove in the most convincing manner that elec-tricity is produced by the mere contact, not only of metallic sub-stances, blit likewise of other bodies. The electricity thus pro-duced by the mere contact of two bodies is so very small as notto be prreeived without great care, and without using some ofthose artifices for discovering small quantities of electricity, whichhave been mentioned above. But the discoveries of the ingeniousMr. Volta have shewn a method of increasing that electricity to amost extraordinary degree. The action of metallic substancesupon the organs of living, or of recently dead animals, has beenfully manifested by the above-mentioned discoveries of Galvaniand others: hut, previously to those discoveries, a variety of facts,frequently asserted, imperfectly known, and often disbelieved, in-dicated a peculiar action arising from a combination of differentmetallic bodies in certain cases. It had been long asserted, thatwhen porter (and some other liquors also) is drunk out of a pewter-pot, it has a taste dilferent from what it has when drunk out of glassor earthenware. It has been observed, that pure mercury retainsits metallic splendour (luring a long time; hut its amalgam withany other metal is soon tarnished or oxidated. The Etruscan in-scriptions, engraved upon pure lead, are preserved to this day!whereas some medals of lead and tin, of no great antiquity, aremuch corroded. Works of metal, whose parts are soldered to-gether by the interposition of other metals, soon tarnish about theplaces where the different metals are joined. When the copper-sheeting of ships is fastened on by means of iron nails, those nails,but particularly the copper, are readily corroded about the placeof contact. Jt tiad been observed that’a piece of zinc might hekept in water for a considerable time, with hardly oxidating at all;hut that the oxidation would soon take place if a piece of silver hap-pened to touch the zinc, whilst standing in water. The discove-ries of Galvani induced many able philosophers to repeat, the ex-periments on this subject; among whom Volta, Aldini , Valli, andDr’s. Monro, Fowler, and Creve, particularly distinguished them-selves, and fully confirmed the truth of the effects. Galvani hadsupposed that these phamomena were produced by the eh ctiieityof the animal conveyed between the nerves and muscles throughthe conductor applied. This hypothesis was successfully opposedby Volta, who made it evident, that the electricity is excited bymeans of the contact of the different sulistances, and that they donot act merely as conductors. 'This view ot the subject led the in*genious Volta to the discovery of the pile, called the Voltaic pil e >by which the Galvanic influence may he extended to almost anydegree. Volta invented two forms of this instrument represented mfig. 16, and 17, and to he described in this article. Volta trans-mitted an account of this truly admirable-discovery, to the RoyalSociety London, soon after it was made. Several experimentswere soon made with this apparatus, by the philosophers ot thiscountry; the first of any note were performed by Messrs. Nicolsonand Carlisle. Mr. Cruickshauk of Woolwich, paid considerableattention to this subject, and to him we are indebted for the disco-very of the Galvanic battery or trough, fig. 1«J, being a much moreconvenient form of the Voltaic pile. It was soon found from re-pealed experiments that the pile or battery performed the parts ota powerful agent in chemistry, and water, sulphuric acid,were decomposed by it. Mr. Davy, professor of chemistry at theRoyal Institution, discovered that the electricity is produced bya chemical action of the different substances on each other, at'<that the effect is greater or less as this action is so, ami that there-fore two dissimilar metals are not essential to the Galvanic pro*cess. This ingenious and truly indefatigable philosopher has alength, by the application of his principle, succeeded in the dejcomposition of the fixed alkalies and some of the earths, which hj l<before been ranked among the simple bodies This brilliant d lS "covery appears to open an extensive field for further invesligation->and acquisitions to our stork of knowledge. The identity, howevt j»of the Galvanic and electric lluid had been disputed by many P _

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