120 THE POPULAR SCIENCE MONTHLY. 



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 trans- 

 mission 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 accumu- 

 lated 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 heterogene- 

 ous bodies, although with different degrees of intensity according 

 to their several natures ; and having discovered the general prin- 

 ciple, 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 moist- 

 ened 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 elec- 

 trifying 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 ex- 

 actly, nearly so." 



Although opinions may differ as to the interpretation of some 

 of the experiments, Prof. Schuster remarks, in the Encyclopaedia 

 Britannica, that there is not much in Volta's writings on the sub- 



