Now there is great doubt whether anybody really knows how rust comes to be. The chemist long ago analyzed it and pronounced it a hydrated oxide of iron. So, since the atmosphere contains an abundance of free oxygen and water, what more natural and reasonable than to suppose that these components of the air combine directly with iron. Growing chemical knowledge, however, has discredited the explanation of simple combination; iron, in fact, cannot unite directly with oxygen except at a very high temperature.

Two or three theories of importance have been proposed to account for the phenomenon of rusting.

In the order in which they have been suggested and defended they are the carbonic-acid theory, the hydrogen-peroxide theory, and the electrolytic theory.