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Protoxide of Iron

Dry protoxide of iron is a black powder; in its hydrated state it is white, and when exposed to the air rapidly absorbs oxygen, assuming first a greyish-green color, and then a brownish-red, which is much brightened by exposure to a red heat, at the same time that its solubility in acids is considerably lessened. The salts of protoxide of iron have a greenish color, but yield nearly colorless solutions, except when concentrated. The white hydrate is precipitated from solutions of the protosalts of iron by the pure alkalies. (Cooley.)