“Many experiments were made in all parts of Europe,” says Doctor Roget, and “an opinion had been very prevalent that the real source of the power developed existed in the muscle and nerve which formed part of the circuit, and that the metals which composed the other part acted merely as the conductors by which that agency was transferred from the one to the other of these animal structures. But the discoveries of Volta dispelled the error, by proving that the sources of power were derived from the galvanic properties of the metals themselves when combined with certain fluids,” and decided that this principle was electricity. From this the “general fact” was deduced—viz., “that when a certain portion of a nerve which is distributed to any muscle is made part of a galvanic circuit, convulsions, generally of a violent and convulsive kind, are produced in that muscle.” Volta at length made the discovery that when two metals were brought together electricity was developed, and by uniting a disc of copper and one of zinc, and subjecting them to the test of an Electroscope, he found positive and negative electricity developed in the zinc and copper respectively; so Volta came to the conclusion that each metal parted with electricity, and one became all “positive” and the other all “negative.” But when he came further to consider the possibility of building up a “pile” of these metal discs sufficiently strong to produce electric effects, he found that if his theory were correct he would lose from one side of the metal all he would gain from the other, and therefore he could never obtain more than the slight effect he had originally produced. This was at first a difficulty apparently impossible to remove. It was so self-evident that the discs of metal, if placed in a pile in a series of pairs, would continually exercise like effects to the first pair of discs, that Volta was puzzled, and for some time he could not arrive at any reasonable solution. At last it struck him that if he placed between the discs some slow-conducting substance, the electricity would not pass from disc to disc, and the force developed or set in motion would be more powerful. He made the experiment. The result was the Voltaic pile made in 1800, of which we give an illustration (fig. 219). A communication on the subject of Electricity by contact, written by M. Volta, is to be found in the _Philosophical Proceedings_ for the year 1800.