Rust. Most metals, if exposed to the air, become covered with a superficial coating, consisting of the oxide of the metal. This oxide is commonly known as rust, but is used more frequently in regard to the rust of iron than that of other metals. Iron rust was distinguished in classical times and employed medicinally. Until the overthrow of the phlogiston theory [PHLOGISTON] the chemical nature of rusts was unknown, and they were regarded as the element, the metal being considered a compound of the rust and phlogiston. Although most metals rust if exposed to the atmosphere, It is noticeable and interesting that none do so in perfectly dry air or oxygen, the presence of a small quantity of aqueous vapour being essential.