VII. Of the absorption of Nitrous Gas, by solution
of pale green Sulphate of Iron.
a. The discovery of the exact difference between the sulphates of iron, is owing to Proust.[114] According to the ingenious researches of this chemist, there exist two varieties of sulphate of iron, the green and the red. The oxide in the green sulphate contains ²⁷/₁₀₀ oxygen. This salt, when pure, is insoluble in spirit of wine; its solution in water is of a pale green color; it is not altered by the gallic acid, and affords a white precipitate with alkaline prussiates.
The red sulphate of iron is soluble in alcohol and uncrystalizable; its oxide contains ⁴⁸/₁₀₀ oxygene. It forms a black precipitate with the gallic acid, and with the alkaline prussiates, a blue one.
The common sulphates of iron generally consist of combinations of these two varieties in different proportions.
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]
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.