irritability; but that they occurred with certainty and continued for some time when he had a circuit of two heterogeneous metals. From this he concluded that the exciting principle resided in the metals; and as this principle was evidently electric, since its transmission was interrupted by all the substances that intercept the electric current, that the mere contact of heterogeneous metals would develop a quantity of electricity which, though weak, would be competent when transmitted through the organs of the frog, completing the chain, to produce the convulsions. He demonstrated the verity of his induction by positive and direct experiments through which this weak electricity was accumulated in his condenser and made perceptible. He further found that this mode of development of electricity by simple contact was applicable, not to metallic bodies only, but to all heterogeneous bodies, although with different degrees of intensity according to their several natures; and having discovered the general principle, which had not been suspected before, he applied it to the construction of a new apparatus which was capable of producing infinitely augmented effects. In order to increase the intensity of his contact electricity, he enlarged the number of the metallic disks or plates he employed to produce it. His efforts were for some time unfruitful. He remarked that when he placed a disk of copper between two disks of zinc, or a disk of zinc between two disks of copper, the electrization was neutralized. He then thought to separate the disks by a conducting body, and found that by placing moistened paper between two double metallic disks the electric intensity was doubled. It was after that easy, by increasing the number of disks and separating them by moistened cloth, to obtain an electric intensity corresponding with the number of pairs. Concerning this series of experiments, he wrote in a letter to a French philosopher, M. La Metherie, which was published in the Journal de Physique in 1801: "Having found what degree of electricity I obtained with one of these metallic couples, by the aid of the condenser I use, I proceed to show that with two, three, or four couples, properly arranged that is, all turned in the same direction and communicating with one another by as many moist layers (which are required, as I have shown, to prevent actions in the contrary direction) we have double, triple, quadruple, etc.; so that if with a single couple we succeed in electrifying the condenser to the point of its causing the electrometer to indicate, for example, three degrees, with two couples we will get six, with three nine, and with four twelve degrees, if not exactly, nearly so."
Although opinions may differ as to the interpretation of some of the experiments, Prof. Schuster remarks, in the Encyclopædia Britannica, that there is not much in Volta's writings on the sub-
