of 1782. By means of this apparatus, according to M. Biot, the smallest quantities of electricity, emanating from a source that can reproduce them constantly as they are taken away, were fixed and accumulated in a conductive plate by virtue of the momentary attraction of electricity of a different denomination, from which they were withdrawn when it was desired to make them perceptible and to subject them to observation. During this time Volta was still trying to find signs of electricity during the processes of evaporation and boiling and changes of temperature; and he finally thought he had discovered electrical effects during the evaporation of water—in the phenomena which are now attributed to the friction of the vapor. These results suggested the closer examination of the phenomena of atmospheric electricity, concerning which—Meteorologia Elettrica—he wrote a number of letters to Lichtenberg. Two of his letters related to electrical measurements and the straw electrometer, in which the angle of divergence of two electrified straws was measured. He also, according to Prof. Schuster, constructed the first absolute electrometer, and compared his other instruments with it, so that it would be possible now to refer all his measurements to absolute units. His electrometer consisted of a balance, one pane of which was a flat round disk. Below this disk was placed a large parallel plate, conducted away to earth, while stops were arranged so that the disk could not approach nearer than within two inches of the plate. In the unelectrified state the balance was in a condition of equilibrium. When the disk was electrified, it was attracted toward the plate, but kept at its proper distance by the stops; weights were then added in the other plate of the balance until the disk was pulled away from the stops. The letters also contain discussions on the action of points and flames in discharging electricity. To Volta are further owing the invention of the electric eudiometer and of the inflammable air or hydrogen lamp. Prof. Schuster regards as worthy of mention also Volta's investigations on gas analysis and his paper on the expansion of gases by heat. He showed the causes which had led different experimenters to inconsistent results, and established independently what is now known as the law of Charles.
Volta's crowning discovery of the voltaic pile grew out of researches which were suggested by Galvani's famous experiment with the frog. Galvani attributed the phenomena which he observed in the frog's muscle to a new kind of electricity, which he called animal electricity. Volta, following up his experiments with more accurate instruments and by a more careful method, came to a different conclusion. He noticed that the convulsions of the frog's muscle were very rarely produced when a single metal was used, and then only under conditions of extreme
