After these general observations, which, scanty and incon clusive as they are, appear to be all that our present know ledge upon the subject will warrant, we must proceed to examine more minutely into the nature of the action that is exercised by the galvanic apparatus. From the remarks that have been already made, it will be obvious, that in the operation of the pile, there are both electrical and chemical phenomena produced ; and it has been a point very warm ly contended, which of these is the most essential, or rather of them is the primary effect, and, consequently, is to be considered as the cause of the other, and of the whole train of actions. Volta, and most of the continental philoso phers, support the electrical hypothesis ; while there are several distinguished experimentalists in this country who maintain, that the chemical action is the one which gives rise to all the changes that are produced, and therefore con stitutes the primary action of the instrument.
In all the experiments that were performed with the two metals, previous to the discovery of the pile, with the ex ception of those of Fabroni, which seem to have been but little attended to, the only point in discussion was, whether the effects were to be referred to the electric fluid, or to a new agent inherent in the animal body. Volta strenuously adopted the opinion, that they depended simply upon com mon electricity, and accounted for them by supposing, that the contact of the two metals had the power of altering the quantity of electricity which was natural to them, adding a portion of it to the one, and subtracting it from the other, re spectively. To this power he gave the title of electro-mo tion ; and he spoke of it as a new property, which had not been before noticed, and distinctly claimed to himself the merit of its discos ery. He conceived that he might in crease the power of the instrument, or rather concentrate the effect of a number of separate pairs of metal, by in terposing between each pair a conducting substance, which, without altering the electric state of the metals, might in crease the effect, by transmitting it through a number of successive stages. Whatever we may think of the hypo thesis, the experiment to which it gave rise was most for tunate ; for it led to the construction of the pile ; tin appa ratus, by means of which the most curious and important discoveries have been made in the different departments of natural philosophy.
Although Volta completed the discovery of the pile, and fully ascertained its action on the animal body, yet it is not a little remarkable, that lie limited his inquiries to this ob ject, and seems to have been totally ignorant of the farther powers of the instrument of which he was possessed. This circumstance must appear the more remarkable, when we recollect that upon the very first employment of it by Messrs Nicholson and Carlisle, they perceived its chemical action, and became aware of its importance as an agent in the de composition of bodies. Cruickshanks, Davy, Wollaston, Henry, and the other English philosophers, farther deve loped its powers in this respect, which had so completely escaped the notice of Volta, and they were consequently Jed to form a different idea of the mode of its operation.
Dr Wollaston seems to have been the first who decidedly adopted the opinion, that the chemical action of the pile is the pi imary origin of all the changes which it experiences, and is the cause of the electrical effects ; and the same idea was embraced by Sir II. Davy, although he has since aban doned it for the hypothesis of electric energies.
We must now proceed to examine the two leading theo ries of the galvanic action, as exhibited in the pile, with more minuteness; and we shall begin with that of Volta's, or the one which supposes a change in the electrical con dition of the metals to be the primary cause of its ope:ation. This philosopher has given a statemel.t of his opinions on the subject, in several letters which he wrote to his friends, and which have been published in different scientific jour pals. His first communication was in a letter to Cavallo ; the second to Gren, (Phil. Trans. l793 ; Ann. de Chun. xxiii. 276.) : both written before his discover) 01 the pile. His original account of this apparatus is contained in a let ter to Sir Joseph Banks, in which tie explains his ideas re specting its action ; and he afterwards farther developed them in letters to Delametherie and to Van Mai urn, (Phil. Trans. 1800; Nicholson's Journal. 8vo, i. 13.3 ; Ann. de Chim. xl. 225.) In some of these papers, Volta details his hypothesis at considerable length ; yet, alter an attentive examination of them, it appears to us that they are not al together consistent with each other ; and that, without any intimation of the circumstance, he has, in fact, given to the world two distinct hypotheses.
The letter written to Cavallo, of which we have already given some account, is Volta's first essay on the subject of galvanism, and contains an account of Galvani's original discovery, and of the additional experiments which he had himself performed by the combination or the two metais. He accounts for all the facts on the principle, that when metals are placed in certain circumstances with respect to each other, there is u a destruction of the equilibrium of the electricity. This action is stated to consist essentially in two metals, when placed in contact, giving the one to the other a portion 01 its natural electricity, so that the one be comes positive and the other negative. Some combinations of metals possess this electromotive faculty much more powerfully than others ; those that Galvani and Volta origi nally employed, were zinc and silver ; and in this case the zinc acquired the electricity and became positive, while the silver lost electricity or became negative. In this paper no other principle is referred to, and the action is not spoken of as belonging to any class of bodies except the the metals. Volta speaks of the principle as a new law of electricity, which had net been before noticed, and decided ly claims to himself the discovery of it.