The corrosion of copper by oxidation on exposure to the air takes place very slowly, the metal becoming soon coated with a skin of carbonate commonly called "verdigris," though that name is correctly applied to a basic acetate of copper. This familiar film on the surface of exposed copper constitutes a protection against further oxidation. The action of salt water on copper which is also accessible to the air is rapid, but may be in some degree modified by alloying a small proportion of phosphorus with the metal. Dr. Percy has made the remark that for more than a century European metallurgists have been familiar with small thin bars of cast copper, of Japanese manufacture, which present a beautiful rose-coloured tint, due to an extremely thin and pertinaciously adherent film of red oxide of copper, or cuprous oxide. This tint is not in the least degree affected by free exposure of the bars to the atmosphere. He has had such bars in his possession for more than 30 years, and although they have been freely exposed to the atmosphere during the whole of that period, yet they hare not undergone the least change in appearance. Every one knows that when a piece of ordinary copper is exposed to the atmosphere, it speedily acquires a dark-coloured tarnish.
Hence the conclusion that there is some peculiarity on* the surface of the Japanese copper, which protects the underlying metal from atmospheric action, and that peculiarity, it may be demonstrated, is the presence of a film of cuprous oxide, in a particular physical state, which acts like varnish. The bars of Japanese copper are actually cast under water, the metal and the water, previously heated to a certain degree, being poured at a high temperature. When copper is so cast, under suitable conditions of temperature, it acquires a coating of cuprous oxide, which acts in the manner described. The temperature is such that the so-called spheroidal action of water comes into play, and the metal flows, tranquilly under the water. The superficial oxidation is probably due to the action of a film of steam, which there is reason to believe surrounds the copper under these conditions; and when copper is heated to a high temperature in steam, the latter, as shown by Regnault's experiments, is decomposed, with the evolution of hydrogen, and the formation of cuprous oxide.