The polarization of the molecules of the acid becomes intensified by the presence of the two oppositely electrified metals; and conversely, the two metals have the difference in their electrical states intensified by the action of the acid molecules. When the copper and zinc plates are connected by a wire out of the fluid, their polarity becomes so intensified that the constituents of the molecules of hydrochloric acid are drawn asunder, the negative chlorine being attracted by the positive zinc, and the positive hydrogen by the negative copper plate. Union takes place between the chlorine and zinc, forming chloride of zinc, which dissolves in the water; but the hydrogen which is attracted by the copper plate does not unite with it, but rises in bubbles along its surface to the top of the liquid. This evolution of the hydrogen at the copper plate, and not at the point where the chlorine leaves the acid and unites with the zinc, can only be explained by supposing that when this action takes place there is at the same time an interchange of constituents in the intervening chain of molecules lying between the two plates, such as is represented in the diagram above.When the exciting liquid is dilute sulphuric acid, it was formerly supposed that witter was the electrolyte or body decomposed; its oxygen uniting with the zinc to form oxide of zinc, which subsequently united with sulphuric acid to form sulphate of zinc, while the hydrogen of the water escaped at the surface of the copper plate; thus:

But it is now believed that it is the sulphuric acid, now written H2SO4, which is the subject of decomposition, the action being as follows:

Modern theory regards the zinc as combining directly with the body SO4 or oxysulphion, without preliminary oxidation. In electrolysis where the two plates form the electrodes of a battery, and are composed of metals neither of which combines with the liquid, the case is different, and both water and sulphuric acid are decomposed, as will be subsequently shown.-Voltaic Batteries. A battery consists of a combination of two or more couples for the purpose of increasing the electromotive force, each additional couple adding its force to that of the battery. This idea presented itself to Volta when he invented the pile which bears his name. It may be constructed by laying upon a bottom piece of wood a disk of copper, and upon this a disk of cloth moistened with dilute acid or a solution of some salt, and upon this a disk of zinc, and repeating this order to an indefinite extent, one end of the battery terminating in a copper and the other in a zinc disk; the order of the elements being copper, fluid, zinc, copper, fluid, zinc, in one direction, and zinc, fluid, copper, zinc, fluid, copper, in the other; so that it is immaterial whether the pile is commenced with copper or with zinc, the positive current, when the terminal plates are connected by a wire, always flowing from the copper to the zinc through the wire, and from the zinc to the copper through the layer of cloth.

The copper and zinc plates should be soldered together around the edges, to prevent the acid or exciting liquid from getting between them except where they are separated by the cloth, because the chemical action which would ensue would tend to urge a current in the opposite direction. Soon after constructing the pile Voltamade a different arrangement, which he called the couronne de tasses, or crown of cups. This was formed of a number of cups, each containing dilute acid or a solution of salt, and a plate of copper and one of zinc, a short distance apart; the copper in one cup being connected by a wire with the zinc plate in the next, instead of lying directly upon or against it. This mode of connection answered the same purpose, allowing electrical polarity to be induced with equal facility when the connecting wires were of sufficient sectional area. When the number of cups, which was indefinite, was completed, the circuit was closed, as was the pile, by connecting the terminal plates with a wire.

The cups were joined together in the following manner: Commencing at the positive pole or electrode of the battery, there would be a plate of copper in the first cup, bearing a binding screw at its top, by which it could be connected with a wire; then opposite it, in the same cup, a plate of zinc, connected by a wire or strip of copper with the copper plate in the next cup, and so on; the last plate in the last cup being zinc, connected with a wire, which latter, when joined to the wire connected with the copper plate in the first cup, closed the circuit. In Volta's cup battery, as well as in the pile, the terminal plates were connected with plates of the opposite metal, a method adopted in accordance with the contact theory; but these additional plates have been discarded as useless. Cruikshank in 1802 modified the form of the pile by using a trough and omitting the separating pieces of cloth, which then became unnecessary. In this battery, which is represented in fig. 3, a plate of zinc and one of copper are placed together in pairs and held in vertical grooves, all the zinc plates facing in one and all the copper plates in the other direction.

The connection between the pairs of plates should be impervious to the fluid in the trough, for the same reason that a similar condition must be observed in the construction of the pile. It is plainly observable that Cruikshank's battery is only a horizontal voltaic pile, possessing but little originality, and not the novelty or convenience of Volta's crown of cups. Useful modifications of it, however, have been made. A common form, sometimes still employed, consists of a wooden trough divided into separate compartments containing the exciting fluid, into each of which are suspended a zinc and a copper or a zinc and a platinum plate, from a horizontal wooden beam, the opposite elements in each compartment being connected together. The beam slides in vertical grooves in posts at the end of the trough; by which means the plates may be raised out of or lowered into the liquid. (See fig. 4.) They may also be easily removed from the beam and cleaned or amalgamated with mercury, an operation which it is essential to perform with zinc plates which are not of pure metal; and it not being practical to procure this, the operation of amalgamation is therefore universal.