That the prussic acid had not been decomposed, was evident from the bright green produced, when less than a grain of dilute nitric acid was admitted into the solution.
f. From these experiments it was evident, that no red sulphate of iron, or nitric acid, and consequently no ammoniac, had been produced after the absorption of nitrous gas by green sulphate of iron. And when I compared them with the observations of Priestley, who had expelled by heat a minute quantity of nitrous gas from an impregnated solution of common sulphate of iron, and who found common air phlogisticated by standing in contact with it, I began to suspect that nitrous gas was simply dissolved in the solution, without undergoing decomposition.
g. To determine more accurately the nature of the process, I introduced into a mercurial cylinder 410 grains of solution of green sulphate of iron, occupying a space nearly equal to a cubic inch and quarter; it was saturated with nitrous gas, by absorbing 8 cubic inches. This saturated solution exhibited the same appearance as the last; and after remaining near an hour untouched, had evidently deposited no oxide of iron, nor gained any acid properties.
Into a small mattrass filled with mercury, having a tight stopper with a curved tube adapted to it, the greater part of this solution was introduced; judging from the capacity of the mattrass, about 50 grains of it might have been lost. To prevent common air from coming in contact with the solution, the stopper was introduced into the mattrass under the mercury; the curved tube connected with a graduated cylinder filled with that substance; and the mattrass brought over the side of the mercurial trough. But in spite of these precautions a large globule of common air got into the top of the mattrass, from the curvature of the tube. When the heat of a spirit lamp was applied to the solution, it gave out gas with great rapidity, and gradually lost its color. When 5 cubic inches were collected it became perfectly pale green, whilst a yellow red precipitate was deposited on the bottom of the mattrass.
On pouring a little of the clear solution into prussiate of potash, it gave only white prussiate of iron.
But on introducing a particle of sulphuric acid into the solution, sufficient to dissolve some of the red precipitate, and then pouring a little of it into a solution of prussiate of potash, it gave a fine blue prussiate of iron.
Hence the red precipitate was evidently red yellow oxide of iron.
I now examined the gas, suspecting that it was nitrous oxide. On mingling a little of it with atmospheric air, it gave red vapor, and diminished. Solution of sulphate of iron introduced to the remainder, almost wholly absorbed it: the small residual globule of nitrogene could not equal one thirtieth of a cubic inch.
Consequently it was nitrous gas, nearly pure.
Caustic potash was now introduced into the solution, till all the oxide of iron was precipitated. The solution, when heated, gave a strong smell of ammoniac, and dense white fumes when held over muriatic acid. It was kept at the heat of ebullition till the evaporation had been nearly compleated. Sulphuric acid poured upon the residuum gave no yellow fumes, or nitric acid vapor in any way perceptible; even when heated and made to boil, there was no indication of the production of any vapor, except that of the sulphuric acid.