By comparing, in this Manner, the charges obtain ed from piles of the same number of plates, con structed with moistened conductors of different kinds, we find, that water, weak acids, the greater number of saline solutions, substances in general whose conductibility is powerful, give, as nearly as can be judged, the same quantities of five electricity, and give it also by a contact to our senses instanta neous. We may even, with most of these conductors, increase or diminish extremely the extent of their surfaces, without producing any sensible variation in the charge of the condenser, owing, no doubt, to the facility, almost infinite, which their surfaces present to the transmission of the electrical currents. But this is sufficient to prove, in every case, agreeably to the opinion of Volta, that they only act the part of con ductors, and that their contact, or their chemical ac tion, is not. the determining cause of the production of the electricity. With some liquids, however, we find the charges unequal with the same number of plates, whether that they weaken too much the con ductibility by their interposition, as will be presently explained ; or that they exert an electromotive action peculiar to themselves, or to the combinations which they form with the other parts of the apparatus; all these varieties presented themselves in the, numerous experiments made by philosophers during the first pe riod of the invention of the electromotive apparatus.
In the preceding discussion, we have always sup posed, that the electromotive apparatus communi cates by its bast with the ground, from which it may draw all the supplies of free electricity necessa ry for the equilibrium of its parts. But, if we can ceive all the pieces which compose it to be placed originally on an insulator, and that the column it self, and the observer who arranges it, were insulat ed during the time of putting them together, then the quantities of free electricity, necessary for the equili briutn, could not be derived from the ground, and the pile would now supply them of itself by the de composition of the natural electricities of its plates. The zinc pole would acquire an excess of free vi treous electricity, balanced by an equal excess of resinous electricity at the pole of copper ; and from these extremes the quantities of free electricity would go on diminishing to the middle of the column, which would be in a state of neutrality. It is ob vious, in fact, that in this manner the conditions which produce the equal differences of one piece from another would still be observed; and the pieces would preserve, in respect to their quantities of electricity, the same rank we have assigned them in the unin sulated apparatus. These considerations are confirm. ed by experiment, at least in their general results ; for all piles, even those which have been at first erected in communication with the ground, pass into the state we have here described, when they are placed on an in sulator of very small dimensions. The air, in fact, which touches them, gradually carrying off, in this case, their free electricity, they cannot be recharged, but at their own expence ; and the result of this de composition of their natural electricities is the only pohion which remains, after their supply of eleCtrici ty drawn from the ground is at length spent by the effect of the air. In this state, the indications of the
electrometer at the two piles are very weak, and even the most powerful condensers are not sensibly charged. Such an effect is the more worthy of re mark, as it does not accord with the theory of the equilibrium by equal differences. This theory shows, indeed, that the charge of the condenser in the insu lated pile must be less than in that which is not in sulated ; but the proportion which it would point out is very far from approaching that extreme degree of weakness, which experience indicates.
By reflecting on this discordance, we are led to think that the electrical action of the electromotive apparatus may probably be owing, not merely to the quantities of free electricity which appear on its ele ments, as Volta supposed, but that there may exist in it at the same time a very great quantity of latent electricity ; and as this consideration would greatly alter the light in which the action of the pile ought to be viewed, we shall here explain it more particularly.
Let us first recollect the fundamental experiments of Volta on the production of electricity, by the simple contact of two insulated metals. What do these show ?—That there is then manifested upon each of them a certain quantity of free electricity, and that it consists of two opposite kinds. But does it follow from this, that these small quantities are the only ones which are really developed ? Un doubtedly not; and the decomposition of the natural electricities of the two plates during the contact might be enormous, without producing any other external indications than those that have been observ ed. Hence, we see very often that the two sides of a thin plate of glass, coated with metal, may be charg ed with a very considerable quantity of electricity, although the portions set at liberty, and exerting their repulsive force on the electrometer, are very trifling.
In this view, two plates of zinc and of copper, brought into contact, would exactly resemble a si milar plate of glass, after we have insulated it, and after the absorbing action of the air has equalized the repulsive powers of its two aides. Only, in place of the insulating stratum of glass which prevents the two electricities from reuniting, there will be in the metals the electromotive forces, which will retain the two electricities on each side of the surface of contact ; the electrometer and the balance will then render sensible only those portions of electricity which are set at liberty from the two sides of this surface ; and the total quantities of disguised electricities will only become manifest at the moment we form a di rect communication between the plates, in the same manner as in the Leyden jar, or electrified plate of glass.