Fabroni mentions among other facts, that mercury and tin when pure, and kept. distinct from other metals, will re main a long time without tarnishing, but when alloyed, or kept in contact with other metals, they soon begin to. exhibit signs of oxidation. Ire remarked, that coins composed of a pure metal were more durable than such as were com posed of a mixture of metals. lie mentions the corrositn w hien takes place, when copper I ours are soldered with another metal, and in the copper sheathing of ships when fastened with iron nails. These phenomena are supposed to depend upon a chemical affinity between the metals, by which their particles are individually attracted towards each other, while the separation of the particles of the solid metal, which is caused by their tendency to unite, permits the oxygen' to act upon them. " These facts," he says," as veil as many others of the same nature, no less common titan well known, ought to have proved to philosophers, that the metals, by exercising their mutual attractive force, must by the same energy diminish their respective powers of aggregation; that though neither of them separately may be able to attract oxygen from the atmosphere, or from water, they ma) acquire that power by simple mechanical touch as they pass to new combinations." He then relates sonic experiments which he performed, in order to observe the comparative effect produced by placing metals first in separate vessels of water, then in the same vessels, Lut not in contact; and lastly, in the same vessels, and also in contact. In the two former cases there was no change produced, while in the latter there was a considerable degree of oxidation. He afterwards entered upon some speculations on the source of the oxygen, and he found that, by covering the water with a stratum of oil, and thus excluding the atmosphere, the process of oxida tion was retarded. This fact he adduces as a powerful ar gument in favour of the chemical hypothesis. lie concludes, that the oxygen in these cases cannot be derived from the water, because where the water decomposed there would be a liberation of hydrogen. The effect that is produced on the senses of vision and of taste, he attributes to the formation of new chemical compounds, which act in a pe culiar manner upon the organs; an opinion which may be true, to a certain extent, so far as respects the tongue, but it is not easy to conceive how it could cause the flash of light which is excited in the eye by the contact of the me tals. See Journal de Physique, xlix. 348.
Fabreni's paper did not appear to excite much attention at the time when it was published, as it directly opposed the current of popular opinion. But many of the statements have been since verified by succeeding philosophers ; and when we consider that it was written before the discovery of the pile, it must be regarded as displaying much sagacity and cite observation.
In this state the science of galvanism remained until the year 1800. It was generally admitted, that the immediate agent in producing the phenomena was the electric fluid, and that the actions exhibited by the animal body depended merely upon its extreme sensibility to small quantities of this fluid. The experiments principally consisted in dif ferent combinations of conducting substances with parts of the animal body, composing what was called the galvanic arc or circle. The chemical effects had been little attended to, except by Fabroni; and his remarks, although truly in genious, must be regarded rather as the first glance of a series of important facts, than as constituting their complete developement.
The second period of the history of galvanism commen ces with the important discovery made by Volta, of the instrument which has been called the galvanic or -voltaic pile. Volta, as we have seen above, adopted the idea, that the action excited by the two metals, depended upon an alteration in their respective states of electricty, a destruction of its equilibrium. The effect produced by one pair of plates could only be comparatively small; but he supposed that by interposing a conductor between several pairs of plates, it might be multiplied and concentrated in an indefinite degree. Ile accordingly provided a number of silver coins, and pieces of zinc of similar dimensions; these were disposed in pairs, and between each pair was placed a piece of card soaked in water ; and thus a pile or column was formed, in which the three substances, silver, zinc, and water, existed in regular rotation. (Plate CCIA Fig. 1.) The effect of the combination fully justified the expectations of the discoverer. All the phenomena that had been excited by a single pair of metals were far ex ceeded by those of the pile, while by touching the two ends of it at the same time, it was found that a distinct shock was felt in the arms. This fully established the opinion that had been formed, and was generally adopted, of the identi ty of electricity and galvanism ; although there were still some circumstances connected with the latter, which ap peared not to be completely analogous to the usual opera tions of the electric fluid.
Ile found, that 40 pairs of the metallic discs, with the proper number of pieces of moistened card interposed, were sufficient to produce a shock, which was very dis tinctly felt in the hands and arms, and that by increasing the number of pairs, the power of the pile was proportionally augmented. In order to produce the lull effect, it was found necessary that two pieces of metal, either composing the extremities of the instrument, or in contact with them, should be firmly grasped_hy the two hands ; and the shock might, in this case, be repeated for any number of times, as long as the pasteboard between the two metals remained sufficiently moist. Volta conceived, that the newly invent ed apparatus was analogous in its action to the electrical organ of the torpedo. The experiments that he perform ed with it were almost exclusively confined to the animal body; an he appears to have entertained no idea of the important use which would afterwards be made of the pile, as an instillment of chemical analysis. It is indeed a little remarkable, that after making so curious a discovery, he should have rested there, and not have proceeded with the farther prosecution of the subject. It would be unjust not to acknowledge, in the warmest terms, the obligation un der which the scientific world was laid by the discovery of Volta ; but, at the same time, it must be admitted, on the other hand, that the benefit of the discovery has been obtain ed by others. An account of the pile was written by Volta, in the form of two letters to Sir Joseph Banks, which were published in the Philosophical Transactions of London for the year 1800.