It consists in applying metallic mercury to the cleared surface of the zinc plates, by which the pure zinc becomes dissolved and brought to the surface where the action of the acid is confined. In impure unamalgamated zinc, local polarization takes places, forming local currents which greatly diminish or annul the electromotive force. A modification devised by Wollaston consisted in having a sheet of copper brought around one end of a zinc plate and separated from it by pieces of cork. Any number of couples can be united by using a trough divided into compartments, or by employing a number of glass or earthen cups such as are represented in fig. 5. Smee's battery is formed of couples which are the reverse of Wollaston's, there being a middle plate of platinum, or silver covered with finely divided platinum (the latter form increasing the surface and giving an element of strength), with a plate of zinc on each side, not bent, however, around the end of the middle plate as in Wollaston's. It is found that this arrangement is better than to have the positive element in the middle.

A powerful form of battery for heating purposes, in consequence of the immense quantity of electricity it generates, was constructed by Prof. Hare of Philadelphia, and consists of one, or only a few simple couples, having a great metallic surface. A large sheet of zinc, of several hundred square feet of surface, and a similar one of copper, are separated by a piece of felt or cloth saturated with acidulated water and then rolled together in the form of a cylinder. (See fig. 6.) On account of its extraordinary heating power, it is called Hare's calorimotor or de-flagrator. All these forms of batteries, which employ two metallic elements and one fluid, when used for any considerable length of time, are found to be defective on account of the enfee-blement of the current, which is duo to several causes, the principal being: 1, decrease in chemical action in consequence of the gradual separation of the acid by the zinc or positive element, and the accumulation of the salt which is thereby formed; 2, the formation of local currents in the positive plate, in consequence of impurities contained in it, and interfering with the general current; 3, the production of secondary currents which flow in a contrary direction to the general current, and which after a length of time became equal to it, rendering its action null.

The nascent hydrogen which is evolved on the surface of the negative plate has a remarkable power, under the circumstances, of decomposing the salt in the fluid. When copper and zinc plates and dilute sulphuric acid are used, for instance, after a time sulphate of zinc, Zn2 SO4 accumulates, and the hydrogen tends to deprive the oxysulphion, SO4, of a part of its oxygen, thus loosening its hold upon the zinc constituent, which being attracted by the copper plate is deposited in a metallic state upon it; and this action may continue until the surface of the copper is covered, when of course there will be, in place of the original arrangement of two opposite metals immersed in an exciting liquid, two plates of the same kind, and having therefore little or no electromotive force. Electricians have therefore devised several different forms of batteries with the intention of obviating these difficulties, which, because of their more continuous and equable action, have been called constant batteries. The first of these was constructed by Becquerel, and consisted of a sheet of copper in the form of a cylinder enveloped in a bladder containing a saturated solution of sulphate of copper.

The bladder is surrounded by a cylindrical sheet of zinc, and the whole is placed in a vessel containing dilute sulphuric acid or common salt. The battery of Daniell, constructed in 1836, is a modification of Bec-querel's in which a porous earthenware cup is substituted for the animal membrane. It is still regarded as the most constant of batteries, although it is not the most powerful. Fig. 7 represents one cell of a Daniell's battery. A glass jar contains dilute sulphuric acid, in which is placed a zinc plate cast in the form of a cyl-inder; within this is a porous earthenware cylindrical cup; and within this, again, a copper plate in the form also of a cylinder. The porous cell contains a saturated solution of sulphate of copper, and crystals of this salt are placed in the liquid upon shelves, or in a copper-wire basket. Sulphuric acid permeates all the cells, but the porous partition offers a great hindrance to the transfusion of the saline solutions. The nascent hydrogen which is evolved at the surface of the copper plate decomposes the sulphate of copper in the same way that it does the sulphate of zinc in the ordinary zinc and copper couple.

The action of a Daniell's battery may be explained as follows: In the diagram a zinc and a copper plate are represented, having a porous earthenware partition between them; solution of sulphate of copper (C112SO4) being the active fluid in the presence of the copper, and sulphuric acid (II2SO4) the active fluid in the presence of the zinc plate. The polarization of the molecules in both fluids which is effected by connect-ing the terminal plate with a wire is represented in the following diagram:

Fig. 2. Voltaic Pile.

Fig. 2.-Voltaic Pile.

Fig. 3. Cruikshank's Battery.

Fig. 3.-Cruikshank's Battery.

Fig. 4. Modern Trough Battery.

Fig. 4.-Modern Trough Battery.

Fig. 5. Cell of Wollas ton's Battery.

Fig. 5.-Cell of Wollas-ton's Battery.

Fig. 6. Hare's Calorimotor.