In the letter to Delametherie, written after the discovery of the pile, Volta still farther developer his hypothesis, but without altering the ground on which it rests. He describes each pair of metals as the efficient part of the apparatus, and speaks of the fluid that is interposed between them, as merely carrying the electricity from one pair to another, without producing any change in it. In his letter to Van Marum, he relates the following fundamental experiment, as it is called : A plate of copper and a plate of zinc are placed in contact with each other, but so that a part of each plate projects beyond the other; and he finds, that of the parts which thus project, one becomes positive and the other negative. So far all these opinions appear to be con sistent with each other ; but in the letter written to Gren, an idea is brought forward, which is not noticed in the other essays, and which seems to be essentially different from them. All conductors of electricity are divided into two classes, the dry and the moist ; and electricity is sup posed to be always excited, when two conductors of either of these classes are placed in contact with one conductor of the other class. In this way one metal only would ap pear to be sufficient for a galvanic combination, provided there be two moist conductors in contact with it. How the fluids act in this case, or what relation they bear to each other and to the metal, we are not exactly informed ; but we may conclude, that it is not from any chemical operation, because in the letter to Delametherie, written four years after that to Cren, it is expressly said, that the fluids have no effect but in transferring the electricity from one metal to another.
Upon the whole we may conclude, that Volta conceives the electricity to be excited by the metals producing a de gree of electro-motion, or by destroying the natural equili brium of the electricity ; one metal thus becoming positive and the other negative, they each of them exhibit signs of electricity to an electrometer or other similar instrument. The only use of the fluid is to transfer the electricity which is excited by one pair of metals to the next pair ; and al though a chemical action may take place between the fluid and the metal, this action is merely incidental, and is not essential to the productPon of the galvanic effects.
The objections to Volta's hypothesis are very forcible ; in the first place, it does not appear that the chemical el fects of the pile are, as he supposes, merely incidental. They seem, indeed, absolutely essential to its action, for when perfectly pure water is interposed between the me tals, or when the apparatus is placed in any situation, where it is excluded from obtaining a supply of oxygen, it ceases to act. The same thing happens when the acid, or other oxidating fluid, is all expended ; and in short, it may be stated, that whatever promotes the action of the fluid upon one of the metals, increases the energy of the instrument, and whatever tends to prevent or destroy this action sus pends the energy. It has been urged as an objection to
Volta's hypothesis, that it does not provide for any absolute increase of electric power. The two metals, by their con tact, become one positive and the other negative, and this is equally the ease with each pair ; but the fluid that is in terposed between the metals is conceived to restore the equilibrium of the electricity, which has been disturbed by the metals. This is the whole effect of the apparatus, and we are not informed how any electricity can be actually produced or generated, as it would appear that the nature of the instrument is to cause an electric action in one part, which must be immediately counteracted by another part. Whatever deficiency of electricity there was in any copper plate would be instantly supplied by the water communi cating the superabundant electricity of the opposite zinc plate, so that the effect of the whole would be reduced sim ply to the difference between the two extreme plates of copper and zinc. A third, and perhaps a still stronger ar gument against the electric hypothesis, is, that the funda mental position on which it rests, is itself objectionable. Volta supposes that two metals, as for example, a plate of zinc and one of copper, when placed in extensive contact with each other, may become respectively positive and ne gative. This he endeavours to prove by direct experiment ; but it will be found that in none of the cases is the experi ment precisely in point. He adduces some facts, where metals were found respectively positive and negative, that had been in contact, but were afterwards separated : in one of his experiments the metals never actually touched, but were connected by a moist conductor, and in the experi ment which we have related above, it was only the project ing parts of the plates that could be made to exhibit the opposite electric states. And here we may be allowed to entertain some doubt respecting the accuracy of the fact ; it is evidently an experiment of a most delicate nature, and Mr Cuthbertson, who attempted to repeat it, obtained results contrary to those stated by Volta. (Nicholson's Journ. Svo. ii. 281.) In the experiments of Bennet and Cavallo, where electricity was induced upon metallic bodies by contact, it is to be observed, that they were no longer in contact when they manifested signs of electricity, and it appears not easy to conceive how two metals can be in extensive contact, without communicating their electricity to each other, so as to acquire precisely the same state. The experiments of De Luc, on the dissection of the pile, seem to be strong ly adverse to the electric hypothesis. In the second dis tribution of the ternai y groups, the two metals are in con tact, and therefore any electrical effect might be produced, which would arise from this circumstance ; there was also the fluid between them, which wluld serve as a conductor of electricity, yet, because the aOaratus was so arranged that this fluid could not act upon the zinc and oxidate it, no proper galvanic effect ensued.