Page:Popular Science Monthly Volume 38.djvu/371

Rh coating of silver by the process and will be silver-plated. Substitute a solution of nickel nitrate, and the article would become nickel-plated. By using copper sulphate we are enabled to cover the faces of types and cuts with a coating of copper, which increases their hardness and consequently their endurance.

This electrolytic action can be watched if a solution of tin chloride be used. Tin, instead of being deposited, like most other metals, in fine particles, comes out of the solution in quite large crystals. If the current of electricity be made to enter the solution through two wires, placed symmetrically on opposite sides of the wire through which it makes its exit, and the whole is performed in a vessel with glass sides, then, as the current passes, the crystals will appear, as if by magic, growing out around the central wire. This is but a modification of the "lead tree" which appears in many text-books on physics. The tin crystals, however, are much larger and more beautiful than those of lead.

The simplest storage battery, then, would seem to be one constructed of two copper plates suspended in a solution of some zinc salt. A current of electricity passed into this would deposit zinc upon one of the plates. After disconnecting the charging current, the battery of itself would give off a current until the zinc was redissolved. In fact, a modification of this form of storage battery has recently been placed upon the market. The question arises, however, whether it is cheaper to buy zinc sulphate and transform it by expensive horse-power into metallic zinc or to buy metallic zinc directly. Of course, in neither case is the zinc lost, for it can be recovered by chemical means from the solution. If solutions of zinc were abundant in nature and hence inexpensive, this style of storage battery would, undoubtedly, for economic reasons, prevail. Or, still further, if metallic zinc were inexpensive we would have no need of storage batteries at all, but could use primary batteries directly.

It might be well, right here, to define a primary battery. If any two different metals be dipped in an acidulated liquid, and if their external extremities be connected by a wire, a current of electricity will flow through the wire. Such a combination is called a primary battery. Under the same conditions the amount of electricity obtained depends upon the character of. the metals. If nickel and iron were employed, a small amount of electricity would result. If, however, zinc be used in connection with either silver, gold, platinum, carbon, or copper, a large amount is obtained. The first three of the group are very expensive; hence, in most primary batteries, we find zinc combined with either carbon or copper, the differences between the various forms arising from difference in the liquids employed or in the shape of construction.