Conceive a pile raised in the zinc, moisture, and make it communicate with the ground by its copper base. In the state of equilibrium all the pieces of this pile will have an excess of vitreous electricity depending upon the rank which they oc cupy. If we touch the upper piece, the excess which it possesses will run out into the ground, and it will tend to re-supply itself from the inferior pieces across the humid conductors. But these conductors not being perfect, a certain time is necessary for this effect ; and if we repeat the discharge before the re storation has been completely made, the upper piece will take vitreous electricity from the piece of copper which it immediately touches, so that the latter will acquire an excess of resinous electricity ; and the same thing will happen, more or leas, to all the me tallic pairs which compose the pile.
This being established, introduce now round the plates an atmosphere of oxygen. According to Dr Wollaston's experiments, this oxygen will be attract ed by all the pieces of zinc that are in the vi treous state ; it will combine then with their sub, stance in consequence of its affinity for them, and of the electrical influence which produces it. But the oxide of zinc which results will, in its turn, be attract ed towards the surface of the upper piece of copper, which the imperfection of the conductors leaves in a resinous state ; it will carry then to this piece the vitreous electricity of the metallic zinc which it abandons ; and this motion of transport continued from the bottom to the top of the pile, will re-estab lish the transmission of electricity. The same thing would still happen in a pile communicating with the ground by its summit of zinc, because the imperfect state of the conductors will allow, in the same man ner, the metallic elements to acquire opposite states of electricity.
This explanatIon, which we owe to Sir H. Davy, applies equally well to all the other chemical decom positions which go on within the pile. The pro ducts which result, attracted towards the differently electrified surfaces, transport along with them the electricity of these surfaces, and produce directly the same result which would arise from a perfect con ductibility.
All the modifications then which occur in the che mical condition of the humid conductors, must be ex pected to influence the action of the pile, and even the quantity of electricity which it communicates to the condenser by a simple contact ; hence the dif ferences which the same piles present at different pe riods of their action; and this ought also to have an influence on their change of power according to the number of pairs of plates employed.
The progressive and inevitable decline in the power of electromotive machines constructed with humid conductors, has given rise, among philosophers, to a vast number of to discover some con struction of a pile with all its conductors perfectly dry. Hitherto their efforts have been vain, or at
least the piles thus formed have never possessed a conductibility sufficient for the production of che mical decompositions, the principal object for which a permanent apparatus is to be desired.
In this respect, Volta discovered among metallic substances a very remarkable relation, which ren ders the construction of a pile with these substances alone, impossible. This we shall now explain ac cording to the views of Volta, but without having had any opportunity of verifying it ourselves.
If the metals be arranged in the following order,— silver, copper, iron, tin, lead, zinc, each of them will become vitreous by its contact with that which precedes it, and resinous by its contact with that which follows it. The vitreous electricity will then pass from the silver to the copper, from the copper to the iron, from the iron to the tin, and so on.
Now,the abovementioned property consists in this, that the electromotive force of the silver upon the zinc is equal to the sum of the electromotive forces of the metals which are situated between them in the series ; so that, in bringing them into contact in this order, or in any other whatever, the extreme metals will always be in the same state as if they had touched each other directly. Hence, if we suppose any number of ele ments thus arranged, the extremities of which, for example, may be silver and zinc, the same result will be obtained, as if the elements had only been formed of these two metals ; that is, the effect, if any, will be the same as what would be produced by a single element.
The preceding property, so far as we yet know, extends to all solid bodies, which are very good con ductors, but does not subsist between them and liquids ; and it is for this reason that we are en abled to construct the pile by the interposition of the latter. Hence arises the division which Volta made of conductors; into two classes, the first com prising solid bodies, the second liquids, and we can not yet construct the apparatus of the column, with out a due combination of both. With the first alone it is impossible, and with the second, we are not yet sufficiently acquainted with the mutual actions of the bodies that compose it, to decide whether it is possi ble or not. This does not appear, however, to be the case, for nature itself really presents us with liquid piles in the electrical apparatus of certain spe cies of fishes, particularly the torpedo. This ap paratus, situated near the stomach of the animal, is composed of a multitude of tubes, ranged side by side, and filled with a particular liquid. It appears that the animal can put this pile into action at will, and it can then communicate real electric shocks to the living bodies which it touches.