IRON, OXIDE OF. There are four definite oxides of iron viz : The Protoxide . . . Fe. 0 Sesquioxide. . . Black oxide . . Ferric acid . . . The salts of the sesquioxide of iron are in general converted into salts of the protoxide by the action of light ; and, conversely, the salts of the protoxide act as developers by acquiring oxygen, and becoming converted into persalts. The solutions of these salts are generally unstable in the air, and become oxidized. The most important of them is the proto-sulphate.
Hydrated protoxide of iron is a white precipitate formed by adding caustic potass to a solution of .a pure proto salt. When washed and dried it assumes a greenish colour, and is very readily oxidized, becoming dark-green or brown.
Sesquioxide, or peroxide, occurs native as haematite, (a hydrate,) and also in beautiful crystals, as specular iron ore, in the island of Elba. It may be produced by adding ammonia to perchlmide of iron and washing, drying, and igniting the precipitated hydrate. This
oxide is red, and forms the pigment called " Venetian Red." Iron rust is the sesquioxide of the metal. It forms salts which have an acid reaction, and are decomposed by light. They are generally of a brown colour.
Black oxide of iron, or magnetic oxide, is a compound of the protoxide and sesquioxide. It is a natural product, occurring in regular octohedral crystals, and is incapable of forming proper salts. The black scales of iron picked up in a blacksmith's forge approach black oxide in composition. The loadstone is black oxide of iron.
Ferric acid is produced by heating to full redness, for an hour, in a covered crucible, a mixture of one part of sesquioxide of iron, and four parts of dry nitre. This forms an unstable amethystine coloured salt, which is ferrate of potass. When added to a salt of baryta, permanent ferrate of baryta is produced, which is of a crimson colour, and insoluble.