GALVANISM.
article GALVANISM, in the Encyclopedia, con tains a detailed exposition of a great number of facts, relative to this new and interesting branch of physics. Since the time it appears to have been written, va rious remarkable phenomena have been brought to light by means of the Voltaic apparatus; and it ap pears to us, that we are now enabled, as well by the extension of knowledge thence resulting, as by the more profound discussion of the Galvanic action it self, to place its theory in a clearer and more de terminate point of view, and to combine, in a more philosophical manner, a number of facts, of which a too confined examination had led to inconsistent and contradictory conclusions. Such is the princi pal object of the supplementary article which we now offer to our readers.
It seems, first of all, indispensable to establish, with a little more precision some historical details regard ing the order in which the discoveries of Galvani and of Volta were made. Most authors who have written on this subject mention the origin of the discovery of Galvani, and the accident of his ob serving convulsions in the muscles of frogs ex posed at some distance from an electrical machine, from which he was drawing sparks. This appear ance was, in fact, the first which he observed; and it was the astonishment which it excited that induced him to vary the circumstances of the experiment in ment, this action ceases ; the electricity of the same kind which had been repelled into the ground, returns instantly to unite with the opposite kind ; and its quick passage excites contractions in the muscular organs of the animal. But, since the sus pension of the animal by a hook of copper to a rail ing of iron also produces contractions in these muscles at the moment of contact, is it not natural to imagine that these convulsions are produced by the accidental developement of some electrical cur rent, which this contact occasions? This, however, was not Galvani's conclusion. He chose, rather, to view these motions as the unexpected effect of a par ticular source of electricity, having its seat in the nerves and muscles ; and the action of which he vainly attempted to assimilate with that of the Ley den phial. But, in reading the work where Galvani has explained this .hypothesis, entitled, De viribus ekaricilatis in mote musculari commentaries, we easily perceive, that he had no idea of the true theory of the electrical influences ; and, as he has allowed him self to be carried away by hypothetical ideas, we are the more led to admire the rare sagacity by which he has been able to detect, and to vary with so much art, the extraordinary phenomenon of convulsions apparently spontaneous, and which chance had pre sented to his view.
The connection of these phenomena with those which an electric current produces in passing through our organs, could not escape so able an electrician as Volta • and it may be said, that chance itself, in making them succeed to the sensible effects of the influences of artificial electricity, had as it were taken care to indicate their true source by this re semblance. Volta, accordingly, did not hesitate con cerning their nature ; but, conceiving that the ex citing cause of these movements, whatever it was, must be extremely subtile, since it had been pro duced independently of the will, even of the observer, he began to examine what precise quantity of elec tricity was necessary to excite convulsions in the or gans of a frog, by sending discharges through them. He thus found that this quantity was so extremely inconsiderable, that it was scarcely sufficient to make the leaves or threads of the delicate electrometer he made use of to diverge sensibly. This result being well established, he compared it with the other fact, established by the experiments of Galvani, that the contact of two or more metals, of different kinds, was, or at least at that time seemed to be, necessary for the excitation of convulsions; and he drew this conclusion, that the contact of these different kinds of metals was the real circumstance, though previously which determined the sudden develope ment of electricity. Following out this truly funda mental idea, Volta collected under one point of view all the experiments made by Galvani ; pointed out the methods of repeating them with certain effect, and with the highest degree of energy of which they were susceptible ; and, lastly, joined to them several phenomena of animal sensation, to which sufficient attention had not as yet been 'Said, undoubtedly on account of their being quite separated from the other facts already known ; but which, rightly examined, also bore the most evident relation to the irritating action excited in the living organs by the mutual contact of several metals.