Pure sulphate of iron occurs in the form of large, transparent prismatic crystals, of a delicate green color: by exposure to the air they gradually absorb oxygen and become rusty on the surface. Solution of sulphate of iron, colorless at first, afterwards changes to a red tint, and deposits a brown powder; this powder is a _basic_ persulphate of iron, that is, a persulphate containing an excess of the oxide or _base_. By the addition of sulphuric or acetic acid to the solution, the formation of a _deposit_ is prevented, the brown powder being soluble in acid liquids. The crystals of sulphate of iron include a large quantity of water of crystallization, a part of which they lose by exposure to dry air. By a higher temperature, the salt may be rendered perfectly _anhydrous_, in which state it forms a white powder. Aqueous solution of sulphate of iron absorbs the _binoxide of nitrogen_, acquiring a deep olive-brown color: as this gaseous binoxide is itself a reducing agent, the liquid so formed has been proposed as a more energetic developer than the sulphate of iron alone. Iron, Protonitrate of. Symbol, FeO NO{5} + 7 HO. Atomic weight, 153. This salt, by careful evaporation _in vacuo_ over sulphuric acid, forms transparent crystals, of a light green color, and containing 7 atoms of water, like the protosulphate. It is exceedingly unstable, and soon becomes red from decomposition, unless preserved from contact with air. The following process is commonly followed for preparing protonitrate of iron:-- Take of nitrate of baryta 300 grains; powder and dissolve by the aid of heat in three ounces of water; then throw in, by degrees, with constant stirring, crystallized sulphate of iron, _powdered_, 320 grains. Continue to stir for about five or ten minutes. Allow to cool, and filter from the white deposit, which is the insoluble sulphate of baryta. In place of nitrate of baryta, the nitrate of lead may be used (sulphate of lead being an insoluble salt), but the quantity required will be different. The atomic weights of nitrate of baryta and nitrate of lead are as 131 to 166; consequently 300 grains of the former are equivalent to 380 grains of the latter. Iron, Perchloride of. Symbol, Fe{2}Cl{3}. Atomic weight, 164. There are two chlorides of iron, corresponding in composition to the protoxide and the sesquioxide respectively. The protochloride is very soluble in water, forming a green solution, which precipitates a dirty white protoxide on the addition of an alkali. The perchloride, on the other hand, is dark brown, and gives a foxy-red precipitate with alkalies.