All acids are compounds of hydrogen and something else, and their biting action is due to the readiness with which the "something else" evicts the hydrogen and takes in a metal in its place. Thus hydrochloric acid, given the opportunity, gets rid of its hydrogen and takes in sodium, thereby forming chloride of soda or common salt. Another example is the gold chloride familiar to photographers. This is the result of the action of certain acids upon gold, wherein the acids throw out their hydrogen and take in gold instead. To sum up, then, a salt is just the same sort of thing as an acid, like the sulphuric acid which we used in our "experiment," except that some metal has taken the place of the hydrogen. It is not surprising, then, to find that if we put a salt in the electrolyte instead of an acid we get a similar result. In the one case hydrogen is deposited upon the out-electrode, in the other the metal. In the former case, since hydrogen is a gas, it forms bubbles and floats away, but in the latter the solid metal remains a thin, even coating upon the electrode. That is the principle of electro-plating. The electrolyte consists of a suitable solution containing a salt of the metal to be deposited, and it is placed in an insulating vessel or vat. The articles to be plated form the out-electrode, so that they have to be suspended in some convenient way from a metal conductor by conducting wires. Of course they are entirely immersed in the liquid. The in-electrode is sometimes a plate of platinum (the reason that expensive metal is used being that it is unaffected by the chemicals) or else a plate of the metal being deposited. In the former case, the solution becomes weaker as the work proceeds, and more salt has to be added. In the latter, however, the strength of the solution remains unchanged, for by an interesting interchange the in-electrode adds to it just what it loses by deposition upon the other one. The effect is therefore just as if the current tore off particles from the one and placed them upon the other. This is believed to be due to the agency of the oxygen which in the case of the electrolysis of water becomes free, but which in this case forms with the metal electrode a layer of oxide upon its surface, this oxide being then dissolved away by the liquid. Thus as fast as the metal is deposited upon the out-electrode its place is taken by more metal from the in-electrode.