[130] In both of these solutions the metal is at its minimum of oxydation. The absorption of a small quantity of nitrous gas by white vitriol was observed by Priestley. _e._ These are all the metallic substances on which I have experimented. It is more than probable that there exist others possessing similar powers of absorbing nitrous gas. Whenever the metals capable of decomposing water exist in solutions at their minimum of oxydation, the affinities exerted by them on nitrous gas and water, will be such as to produce combination. The powers of metallic solutions to combine with nitrous gas at common temperatures, as well as to decompose it at higher temperatures, will probably be in the ratio of the affinity of the metallic oxides they contain, for oxygene. XI. _The action of Sulphurated Hydrogene on solution of Green Sulphate of Iron, impregnated with Nitrous Gas._ _a._ In an experiment on the absorption of nitrous gas by solution of green sulphate of iron, I introduced an unboiled solution of common sulphate, deprived of red oxide of iron by sulphurated hydrogene, into a jar filled with nitrous gas; the absorption took place as usual, and nearly six of gas entered into combination, the volume of the solution being unity. On applying heat to a part of this impregnated solution, the whole of the nitrous gas it contained (as nearly as I could guess), was expelled undecompounded, and no yellow precipitate produced. Prussiate of potash poured into it gave only white prussiate of iron; and when it was heated with lime, no ammoniacal smell was perceptible. I could refer this phænomenon to no other cause than to the existence of a small quantity of sulphurated hydrogene in the solution. That this was the real cause I found from the following experiment. _b._ One part of a solution of green sulphate of iron, formed by the agitation of common sulphate of iron in contact with sulphurated hydrogene, was boiled for some minutes to expel the small quantity of gas retained by it undecompounded. It had then no peculiar smell, and gave a white prussiate of iron with prussiate of potash; the other part had a faint odor of sulphurated hydrogene, and gave a dirty white precipitate with prussiate of potash. Nearly equal quantities of each were saturated with nitrous gas, and heated. The unboiled impregnated solution gave out all its nitrous gas undecompounded; whilst in the boiled solution it was partly decomposed, yellow precipitate and ammoniac being formed. _c._ This singular phænomenon of the power of a minute quantity of sulphurated hydrogene, in preventing the decomposition of nitrous gas and water, by green oxide of iron, will most probably take place in other impregnated solutions. It seems to depend on the strong affinity of the hydrogene of sulphurated hydrogene for oxygene. XII. _Additional Observations._