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Popular Science Monthly

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condenser of a new sort; the electrometer employed was of the most sensible sort; and various other contrivances were used, which it will be needless to describe in this place; but we could never obtain the smallest appearance of electricity from those metallic combinations. Yet we can infer to no other conclusion, but that if the mere combination, or contact, of the two metals produces any electricity, the quantity of it in our experiments was too small to he manifested by our instruments.' Later, on page 111 of the same volume, he says: 'After many fruitless attempts, and after having sent to the press the preceding part of this volume, I at last hit upon a method of producing electricity by the action of metallic substances upon one another, and apparently without the interference of electric bodies. I say apparently so, because the air seems to be in a great measure concerned in those experiments, and perhaps the whole effect may be produced by that surrounding medium. But, though the irregular, contradictory, and unaccountable effects observed in these experiments do not as yet furnish any satisfactory theory, and though much is to be attributed to the circumambient air, yet the metallic substances themselves seem to be endowed with properties peculiar to each of them, and it is principally in consequence of those properties that the produced electricity is sometimes positive, at other times negative, and various in its intensity.' Cavallo then proceeds to describe the experiments on contact electrification which were described in the previous paper referred to at the beginning of the article. Cavallo's experiments were evidently made in 1795. In the following year Volta announced the discovery of the electrical current. In a letter written to Gren's Neues Journal der Physik, August, 1796, Volta says: 'The contact of different conductors, particularly the metallic, including pyrites and other minerals as well as charcoal, which I call dry conductors, or of the first class with moist conductors, or conductors of the second class, agitates or disturbs the electric fluid, or gives it a certain impulse. Do not ask in what manner: it is enough that it is a principle and a great principle.' It will be seen that at this stage of his discovery Volta was inclined to attribute tho origin of the current to the contact between the metals and his moist "conductors of the second class," though later in the same article he says it is impossible to tell whether the impulse which sets the current in motion is to be attributed to the contact between the metals themselves or between the two metals and the moist conductor, since either supposition would lead to the same results. Later, as was shown in the previous paper by the present writer Volta came to regard the metallic contact as the cause of the electromotive force. In a letter written to Gren in 1797 and published as a postscript to his letter of August, 1796, Volta says: 'Some new facts, lately discovered, seem to show that the immediate cause which excites the electric fluid, and puts it in motion, whether it be an attraction or a repulsive power, is to be ascribed much rather to the mutual contact of two different metals, than to their contact with moist conductors.' The new facts, "lately discovered," to which Volta attributes his change of view were his repetitions of Bennett's experiments of 1789. Volta apparently thought that the current was not only set up by the contact of the two metals of a pair, but that it was kept up by the mutual action of the metals on each other. He accordingly made no attempt to discover whether any changes took place in his circuit while the current was being generated. The chemical action on his metals and the dissociation in his electrolyte seem to have entirely escaped his attention. At least, he did not attach enough importance to them to mention them anywhere in his description of his apparatus. In the meantime a chemical explanation of the phenomena observed by Galvani had been proposed in 1792 by Fabroni, a physicist of Florence. After discussing the Sulzer phenomenon already mentioned in this paper, Fabroni argues that the peculiar taste caused by bringing the two metals into contact while on the tongue is due to a chemical, rather than to an electrical, action. He then discusses the different chemical behavior of metals when taken singly and when placed in contact with other metals. He says:[2] [2] The following quotations from Fabroni have been translated by the present writer from the German of Ostwald's "Elektrochemie," pp. 103, ff. I have already frequently observed that fluid mercury retains its beautiful metallic luster for a long time when by itself; but as soon as it is amalgamated with any other metal it becomes rapidly dim or oxidized, and in consequence of its continuous oxidation increases in weight. I have preserved pure tin for many years without its changing its silvery luster, while different alloys of this metal which I have prepared for technical purposes have behaved quite otherwise. I have seen in the museum at Cortonne Etrusean inscriptions upon plates of pure lead which are perfectly preserved to this day' although they date from very ancient times; on the other hand, I have found with astonishment in the gallery of Florence that the so-called "piombi" or leaden medallions of different popes, in which tin and possibly some arsenic have been mixed to make them harder and more beautiful, have fallen completely to white powder, or have changed to their oxides, though they were wrapped in paper and preserved in drawers. In the same way I have observed that the alloy which was used for soldering the copper plates upon the movable roof of the observatory at Florence has changed rapidly and in places of contact with the copper plates has gone over into a white oxide. I have heard also in England that the iron nails which were formerly used for fastening the copper plates of the sheathing of ships were attacked on account of contact, and that the holes became enlarged until they would slip over the heads of the nails which held them in position. It seems to me that this is sufficient to show that the metals in these cases exert a mutual influence upon each other, and

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