Percussæ gelido trepidant sub pectore fibræ, Paullatim per membra levat; terrâque repulsum est LUCAN. vi. 752. The form which lay before inert and dead, CHAPTER I. DISCOVERY OF VOLTAIC ELECTRICITY. E have given the name of mechanico-chemical to for these sciences are concerned with cases in which mechanical effects, that is, attractions and repulsions, are produced; while the conditions under which these effects occur, depend, as we shall hereafter see, on chemical relations. In that branch of these sciences which we have just treated of, Magnetism, the mechanical phenomena were obvious, but their connexion with chemical causes was by no means apparent, and, indeed, has not yet come under our notice. The subject to which we now proceed, Galvanism, belongs to the same group, but, at first sight, exhibits only the other, the chemical, portion of the features of the class; for the connexion of galvanic phenomena with chemical action was soon made out, but the mechanical effects which accompany them were not examined till the examination was required by a new train of discovery. It is to be observed, that I do not include in the class of mechanical effects the convulsive motions in the limbs of animals which are occasioned by galvanic action; for these movements are produced, not by attraction and repulsion, but by muscular irritability; and though they indicate the existence of a peculiar agency, cannot be used to measure its intensity and law. The various examples of the class of agents which we here consider,-magnetism, electricity, galvanism, electro-magnetism, thermo-electricity, differ from each other principally in the circumstances by which they are called into action; and these differences are in reality of a chemical nature, and will have to be considered when we come to treat of the inductive steps by which the general principles of chemical theory are established. In the present part of our task, therefore, we must take for granted the chemical conditions on which the excitation of these various kinds of action depends, and trace the history of the discovery of their mechanical laws only. This rule will much abridge the account we have here to give of the progress of discovery in the provinces to which I have just referred. The first step in this career of discovery was that made by Galvani, Professor of Anatomy at Bologna. In 1790, electricity, as an experimental science, was nearly stationary. The impulse given to its progress by the splendid phenomena of the Leyden phial had almost died away; Coulomb was employed in systematizing the theory of the electric fluid, as shown by its statical effects; but in all the other parts of the subject, no great principle or new result had for some time been detected. The first announcement of Galvani's discovery in 1791 excited great notice, for it was given forth as a manifestation of electricity under a new and remarkable character; namely, as residing in the muscles of animals.1 The limbs of a dissected frog were observed to move, when touched with pieces of two different metals; the agent which produced these motions was conceived to be identified with electricity, and was termed animal electricity; and Galvani's experiments were repeated, with various modifications, in all parts of Europe, exciting much curiosity, and giving rise to many speculations. appears It is our business to determine the character of each great discovery which in the progress of science. Men are fond of repeating that such discoveries are most commonly the result of accident; and we have seen reason to reject this opinion, since that preparation of thought by which the accident produces discovery is the most important of the conditions on which the successful event depends. Such accidents are like a spark which discharges a gun already loaded and pointed. In the case of Galvani, indeed, the discovery may, with more propriety than usual, be said 1 De Viribus Electricis in Motu Musculari. Comm. Benon. t. vii. 1794, to have been casual; but, in the form in which it was first noted, it exhibited no important novelty. His frog was lying on a table near the conductor of an electrical machine, and the convulsions appeared only when a spark was taken from the machine. If Galvani had been as good a physicist as he was an anatomist, he would probably have seen that the movements so occasioned, proved only that the muscles or nerves, or the two together, formed a very sensitive indicator of electrical action. It was when he produced such motions by contact of metals alone, that he obtained an important and fundamental fact in science. The analysis of this fact into its real and essential conditions was the work of Alexander Volta, another Italian professor. Volta, indeed, possessed that knowledge of the subject of electricity which made a hint like that of Galvani the basis of a new science. Galvani appears never to have acquired much general knowledge of electricity: Volta, on the other hand, had laboured at this branch of knowledge from the age of eighteen, through a period of nearly thirty years; and had invented an electrophorus and an electrical condenser, which showed great experimental skill. When he turned his attention to the experiments made by Galvani, he observed that the author of them had been far more surprized than he needed to be, at those results in which an electrical spark was produced; and that it was only in the cases in which no such apparatus was employed, that the observations could justly be considered as indicating a new law, or a new kind of electricity. He soon satisfied himself3 (about 1794) that the essential conditions of this kind of action depended on the metals;-that it is brought into play most decidedly when two different metals touch each other, and are connected by any moist body;—and that the parts of animals which had been used discharged the office both of such moist bodies, and of very sensitive electrometers. The animal electricity 2 Phil. Trans. 1793, p. 21. 3 See Fischer, viii. 625. of Galvani might, he observed, be with more propriety called metallic electricity. The recognition of this agency as a peculiar kind of electricity, arose in part perhaps, at first, from the confusion made by Galvani between the cases in which his electrical machine was, and those in which it was not, employed. But the identity was confirmed by its being found that the known difference of electrical conductors and non-conductors regulated the conduction of the new influence. The more exact determination of the new facts to those of electricity was a succeeding step of the progress of the subject. The term animal electricity' has been superseded by others, of which galvanism is perhaps the most familiar. I think it will appear from what has been said, that Volta's office in this discovery is of a much higher and more philosophical kind than that of Galvani; and it would, on this account, be more fitting to employ the term voltaic electricity; which, indeed, is very commonly used, especially by our most recent and comprehensive writers. Volta more fully still established his claim as the main originator of this science by his next step. When some of those who repeated the experiments of Galvani had expressed a wish that there was some method of multiplying the effect of this electricity, such as the Leyden phial supplies for common electricity, they probably thought their wishes far from a realization. But the voltaic pile, which Volta described in the Philosophical Transactions for 1800, completely satisfies this aspiration; and was, in fact, a more important step in the history of electricity than the Leyden jar had been. It has since undergone various modifications, of which the most important was that introduced by Cruikshanks, who substituted a trough for a pile. But in all cases the principle of the instrument was the same;-a continued repetition of the triple combination of two metals and a fluid in contact, so as to form a circuit which returns into itself. Fischer, viii. p. 683. |