We have hitherto only considered the agency of the pile in the decomposition of bodies. It produces, however, most remarkable effects of a different kind. If we form the communication, for example, between the two poles, by very fine metallic wires, and make them gently approach until they touch each other, an attraction arises between them which keeps them united in spite of the force of their elasticity. If these wires, are of iron, a visible spark is excited between them, which, as we shall see immediately, produces a real combustion of the iron. This experiment succeeds with greater certainty when the extremity of one of the iron wires is coated with a thin gold leaf, the latter being consumed at the place where the spark issues. With this spark we may inflame any explosive gas, • and even phosphorus and sul. phur, in the same manner as with the sparks from electrical machines.
We are here merely alluding to the effects pro duced by the most common piles, the plates of which are nearly of the size of a crown piece. But it is easy to conceive that they must become much more considerable if we employ plates of a greater extent of surface, and collected in the same number. For in piles where the number of the elements and the nature of the humid conductors are the same, the thickness of the free electrical stratum upon each plate of the same rank is also the same, as is indicated by theory, and also, as we have already seen, by experiment. Hence it follows, that the total quantities of electricity, which these piles pos. seas in a state of equilibrium, or which they corn municate in a state of motion, are exactly and con stantly proportional to the surface of the plates, what ever be in other respects the modifications that may arise in-the course of the experiment, in consequence of the action of the pile itself. Messrs Gay-Lussac and Thenard have accordingly found, that the quan tities of gas disengaged in a given time, are propor tional to the surfaces of the plates which are employ ed ; or, what comes to the same thing, to the total quantities of electricity. The same increase is ob served in all the other chemical effects. A pile with large plates, though composed of a small number of pairs, is capable of burning several inches of iron wire ; and if to the extent of the plates be joined also the augmentation of force which arises from their number, then the power becomes extreme. These phe nomena of large plates were first observed by Messrs Hachette and Thenard. The action of the Voltaic apparatus, in the heating of bodies, is attended with this remarkable circumstance, that it produces the evolution of heat by its own energy, without the aid of any chemical combination. In this manner, as
Sir Humphrey Davy has shown, the temperature of some bodies, plumbago for example, may be raised •even to ignition, in the most perfect vacuum that can be produced ; and not only raised, but preserved in this state for whole hours together, without losing any part of their weight. Whence then comes the heat which is thus continually disengaged ? or from what inexhaustible source springs the torrent of light which is thus renewed as it flies off? These questions seem to be connected with the most re condite views of the nature of heat and light ; and deserve all the attention of philosophers. Perhaps the electrical current, or rather the two opposite elec trical currents which meet together, and neutralize each other's effect in the substance submitted to ex periment, could be conceived to act on its particles by a compression or percussion, and to extract heat in the same manner as in the boring, or flattening of metals, or in the action of the hammer. But, in deed, we know as little of the nature of these latter phenomena as of the former ; and when we consi der the continued disengagement of heat which such processes occasion, and which has been produced in several experiments made by Count Rumford, we find as much difficulty in imagining the source from which the heat may arise in these circumstances, as in the opposite currents of the Voltaic apparatus.
On this subject an opinion of some boldness has been advanced, which, however, in our absolute ignorance concerning the nature of heat and light, ought not to be passed over in silence. It has been imagined, that the two electrical principles carry within themselves the principles of heat, and that the latter is disengaged at the moment of their reunion. This idea would, in fact, explain, in a very simple manner, all the phenomena of heat which the electro motive apparatus produces; and the progressive dimi nution of these appearances, when it is slow enough to be observed, seems to agree with it. For, if a trough apparatus, during the first moments in which it is charged, can bring to a red heat a certain length of iron wire, it will be found a few instants afterwards to be only able to redden a shorter length, and so on, until at last it becomes incapable of producing igni tion in the shortest wire of the same diameter; and we then observe, that the portion of wire, which is reddened to whatever length it may be reduced, is always situated in the middle of the whole length, so that at one time the ignition is confined to a single spot in the middle of the wire, after which it ceases altogether in the middle.