THE STORAGE OF ELECTRICITY. 357 



coating of silver by the process and will be silver-plated. Substi- 

 tute 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 in- 

 creases their hardness and consequently their endurance. 



This electrolytic action can be watched if a solution of tin chlo- 

 ride 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 con- 

 structed 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 cur- 

 rent, 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 inexpen- 

 sive, this style of storage battery would, undoubtedly, for eco- 

 nomic 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 ob- 

 tained. 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 aris- 

 ing from difference in the liquids employed or in the shape of 

 construction. 



