Nitrate of iron made in this way, gives a bright blue mingled with prussiate of potash, and decomposed by the alkalies, a red precipitate. Its solution has little or no affinity for nitrous gas. _b._ When very dilute nitric acid, that is, such as of specific gravity 1,16, is made to oxydate iron, without the assistance of heat, the solution gives out no gas for some time, and becomes dark olive brown: when neutralised it gives, decomposed by the alkalies, a light green precipitate; and mingled with prussiate of potash, pale green prussiate of iron. It owes its color to the nitrous gas it holds in solution. By exposure to the atmosphere it becomes pale, the nitrous gas combined with it being converted into nitric acid. It is then capable of absorbing nitrous gas, and consists of pale nitrate of iron, mingled with red nitrate. I have not yet obtained a nitrate of iron giving only a white precipitate with prussiate of potash, that is, such as contains _only_ oxide of iron at its minimum of oxydation; for when pure green oxide of iron is dissolved by very dilute nitric acid, a small quantity of the acid is generally decomposed, which is likewise the case in the decomposition of nitre by green sulphate of iron. The solutions of nitrate of iron, however, procured in both of these modes, absorb nitrous gas with rapidity, and by sulphurated hydrogene might probably be converted into pale nitrate. As it is impossible to obtain concentrated solutions of pale nitrate of iron, chiefly containing green oxide, its powers of absorbing nitrous gas cannot be compared with the muriatic and sulphuric solutions, unless they are made of nearly the same specific gravity. Nitrous gas is disengaged by heat from the impregnated solution of nitrate of iron, at the same time that much red oxide of iron is precipitated. Whether any nitrous gas is decomposed, I have not yet ascertained; for when unimpregnated pale nitrate of iron is heated, a part of the acid, and of the water of the solution, is decomposed by the green oxide of iron;[128] and in consequence ammoniac, and red nitrate of iron formed, whilst red oxide is precipitated. [128] In this process nitrous oxide is sometimes given out, as will be seen hereafter. X. _Absorption of Nitrous Gas by other Metallic Solutions._ _a._ White prussiate of iron in contact with water absorbs nitrous gas to a great extent, and becomes dark chocolate.[129] _b._ Concentrated solution of sulphate of tin, _probably_ at its minimum of oxydation, absorbs one eighth of its bulk of nitrous gas, and becomes brown, without deposition. _c._ Solution of sulphate of zinc absorbs about one tenth of its volume of nitrous gas, and becomes green. _d._ Solution of muriate of zinc[130] absorbs nearly the same quantity, and becomes orange brown. [129] Hence we learn why no nitrous gas is disengaged when impregnated solution of sulphate of iron is decomposed by prussiate of potash, as in Div. IV. Sec. vii.