The green sulphate may be converted into the red by oxygenated muriatic acid or nitric acid. The common sulphate may be converted into green sulphate, by agitation in contact with sulphurated hydrogene. The green sulphate has a strong affinity for oxygene, it attracts it from the atmosphere, from oxygenated marine acid, and nitric acid. The alkalies precipitate from it a pale green oxide, which if exposed to the atmosphere, rapidly becomes yellow red. The red sulphate of iron has no affinity for oxygene, and when decomposed by the alkalies, gives a red precipitate, which undergoes no alteration when exposed to the atmosphere.[115] [115] I have been able to make these observations on the sulphates of iron, most of them after Proust. _b._ The absorption of nitrous gas by a solution of sulphate of iron, was long ago discovered by Priestley. During this absorption, he remarked a change of color in the solution, analogous to that produced by the mixture of it with nitric acid. This chemical fact has been lately applied by Humbolt, to the discovery of the nitrogene generally mingled with nitrous gas. Vauquelin and Humbolt have published a memoir, on the causes of the absorption[116] of nitrous gas by solution of sulphate of iron. They saturated an ounce and half of sulphate of iron in solution, with 180 cubic inches of nitrous gas. [116] Annales de Chimie, vol. xxviii. pag. 182. Thus impregnated it strongly reddened tincture of turnsoyle; when mingled with sulphuric acid, gave nitric acid vapor; and saturated with potash, ammoniacal vapor. By analysis, it produced as much ammoniac as that contained in 4 grains of ammoniacal muriate, and a quantity of nitric acid equal to that existing in 17 grains of nitre. Hence they concluded, that the nitrous gas and a portion of the water of the solution, had mutually decomposed each other; the oxygene of the water combining with the oxygene and a portion of the nitrogene of nitrous gas to form nitric acid; and its hydrogene uniting with the remaining nitrogene, to generate ammoniac. They have taken no notice of the nature of the sulphate of iron employed, which was most probably the common or mixed sulphate; nor of the attraction of the oxide of iron in this substance for oxygene. _c._ Before I was acquainted with the observations of Proust, the common facts relating to the oxygenation of vitriol of iron induced me to suppose, that the attraction of this substance for oxygene was in some way connected with the process of absorption. The comparison of the experiments of Humbolt and Vauquelin, with the observations of Proust, enabled me to discover the true nature of the process.