Sulphuretted hydrogen, prepared by the action of sulphuric acid on sulphuret of iron, is also much used as a test for the metals. Sulphuretted hydrogen gives yellow precipitates in neutral solutions containing arsenious or arsenic acid, which are soluble in ammonia and hydrosulphuret of ammonia, but insoluble in excess of hydrochloric acid; it also yields a yellow precipitate in solutions of the persalts of tin; the precipitate is however soluble in hydrochloric acid. It gives brown or black precipitates in acid solutions of the following metallic bases: - proto- and per-salts of mercury, salts of silver, lead, copper, bismuth, protoxide of tin, gold, and platinum. The precipitate with the persalts of mercury is at first yellow, but becomes black with excess of sulphuretted hydrogen. Salts of the following metals (contained in the Pharmacopoeia) are not precipitated by hydrosulphuric acid in acid solutions: the proto- and per-salts of iron, oxide of zinc, and manganese.