the calx or oxide of a metal, iron, for instance, formed by exposure to the air, or by corroding and dissolving its superficial parts by some menstruum. Water is the great instrument or agent in producing rust; and hence oils and other fatty bodies secure metals from rust, water being no menstruum for oil, and therefore not able to make its way through it. Almost all metals are liable to rust. The rust of iron is not merely an oxide of that metal; it contains, besides, a portion of carbonate.