The experiments described by one of us in another paper have established that the necessary condition for the chemical reactions between nitric acid and the metals copper, mercury, and bismuth, is the presence and continuous formation of nitrous acid. These results confirm the previous observations of Russell in the case of the metal silver. Further, it has been shown that when these metals are introduced into purified nitric acid, no apparent change takes place at first, but after some time gas bubbles are seen to rise from the surface of the metal, and the liquid then contains an amount of nitrous acid which can be detected by the more delicate reagents. The amount of this acid increases up to a maximum and constant point, while concomitantly the amount of metal dissolved per unit of time equally increases to a maximum and constant value. It seemed, therefore, desirable to ascertain if these phenomena determined by the balance are correlated with any alteration of electromotive force. To avoid any misunderstanding, however, it would be advisable at the outset to state that this paper is separated from that of one of us “On the Condition of Chemical Change between certain Metals and Nitric Acid,” rather on account of division of labour than of difference of subject. The idea of measuring the electromotive force of cells consisting of platinum, certain metals, and nitric acid occurred to the latter of us, while the apparatus and method of measurement finally adopted are due solely to the former.
Previous Investigations
. Many years ago Faraday pointed out that neither nitrogen peroxide nor nitric acid are conductors when pure, but that the addition of water to the former, or of nitrous acid to the latter, produces the property of conductivity “in a very high degree among electrolytes.” Thus the presence of nitrous acid is the necessary condition that nitric acid should become an electrolyte. Again, Faraday proved that the electric position of certain pairs of metals in nitric acid is varied according to the conditions of concentration and temperature; in fact, experiments on this point were considered by him to be of especial importance in confirming the chemical and disproving the contact theory of the voltaic cell.
IV. The conditions of chemical change between nitric acid and certain metals
