The sesquisulphide, Ir 2 S 3, is obtained as a brown precipitate when sulphuretted hydrogen is passed into a solution of one of the sesqui-salts.
When a concentrated solution of auric chloride is treated with caustic potash, a brown precipitate of auric hydrate, Au(OH) 3, is obtained, which, on heating, loses water to form auryl hydrate, AuO(OH), and auric oxide, Au 2 0 3.
Bismuth dioxide, BiO or Bi 2 O 2, is said to be formed by the limited oxidation of the metal, and as a brown precipitate by adding mixed solutions of bismuth and stannous chlorides to a solution of caustic potash.
It may be prepared by heating cuprous sulphide with sulphur, or triturating cuprous sulphide with cold strong nitric acid, or as a dark brown precipitate by treating a copper solution with sulphuretted hydrogen.
Bismuth trisulphide, B12S3, constitutes the mineral bismuthite, and may be prepared by direct union of its constituents, or as a brown precipitate by passing sulphuretted hydrogen into a solution of a bismuth salt.
Stannous sulphide, SnS, is obtained as a lead-grey mass by heating tin with sulphur, and as a brown precipitate by adding sulphuretted hydrogen to a stannous solution; this is soluble in ammonium polysulphide, and dries to a black powder.
On the addition of potassium cyanide they give a brown precipitate of cobalt cyanide, Co(CN) 2, which dissolves in excess of potassium cyanide to a green solution.