92. It is not our object here to give a complete description, either historical or scientific, of the voltaic battery, but rather to give such an account as will enable our readers to understand what the arrangement is, and what sort of effect it produces; and with this object we shall at once proceed to describe the battery of Grove, which is perhaps the most efficacious of all the various arrangements for the purpose of producing an electric current. In this battery we have a number of cells connected together, as in Fig. 8, which shows a battery of three cells. Each cell consists of two vessels, an outer and an inner one; the outer vessel being made of glass or ordinary stone ware, while the inner one is made of unglazed porcelain, and is therefore porous. The outer vessel is filled with dilute sulphuric acid, and a plate of amalgamated zinc--that is to say, of metallic zinc having its outer surface brightened with mercury,--is immersed in this acid. Again, in the inner or porous vessel we have strong nitric acid, in which a plate of platinum foil is immersed, this being at the same time electrically connected with the zinc plate of the next outer vessel, by means of a clamp, as in the figure. Both metals must be clean where they are pressed together, that is to say, the true metallic surfaces of both must be in contact. Finally, a wire is metallically connected with the platinum of the left-hand cell, and a similar wire with the zinc of the right-hand cell, and these connecting wires ought, except at their extremities, to be covered over with gutta-percha or thread. The loose extremities of these wires are called the _poles_ of the battery. 93. Let us now suppose that we have a battery containing a good many cells of this description, and let the whole arrangement be insulated, by being set upon glass supports, or otherwise separated from the earth. If now we test, by appropriate methods, the extremity of the wire connected with the left-hand platinum plate, it will be found to be charged with positive electricity, while the extremity of the other wire will be found charged with negative electricity. 94. In the next place, if we connect these poles of the battery with one another, the two electricities will rush together and unite, or, in other words, there will be an electric current; but it will not be a momentary but a continuous one, and for some time, provided these poles are kept together, a current of electricity will pass through the wires, and indeed through the whole arrangement, including the cells. The direction of the current will be such that _positive electricity may be supposed to pass from the zinc to the platinum, through the liquid; and back again through the wire, from the platinum at the left hand, to the zinc at the right_; in fact, to go in the direction indicated by the arrow-head.