The Romance of Modern Chemistry

TWO METALS BETTER THAN ONE and moisture must therefore be protected. In the case of large structures, such as bridges, locomotives, and steamers, this is done by painting them, but with smaller and more easily-handled articles the same end is attained by coating them with a layer of another metal which is not easily corroded by the action of moist air. Tin and zinc are metals which fulfil these conditions, and they are further comparatively fusible, so that a sheet of iron may be easily coated with either by simply dipping it into a bath of the fused metal. Iron coated in this way with a layer of tin is known as tinplate; similarly treated with zinc, it is known as galvanised iron. So that things are not always what they seem. Even the common pin is a fraud in this sense, for if we could open it out we should find brass wire in the inside, quite different from the white metal on the outside. Brass, as the reader probably knows, is a yellow alloy containing a lot of copper, and the easiest way of showing that pins contain this metal is to dissolve one in nitric acid. The pin is gradually consumed by the acid, it ultimately disappears, and a blue liquid remains, similar to what is obtained by treating a piece of pure copper in the same way. So we may conclude that there is copper in the pin. The white outside is a coating of tin; this, how- ever, is not put on, as in tinplate, by dipping in a bath of the fused metal, but by another interesting method. The reader may recollect that among the things that lent support to the alchemists1 belief in the transmutation of metals was the observation that a piece of iron im- mersed in a solution containing copper acquires the appearance of copper. This little trick can be performed with other metals also, and is applied in the manufacture of pins. The brass wires which form the substance of the 73