THIOSULPHURIC ACID (from Gk. tkiov, Melon, sulphur + Eng. sulphuric), less properly 'hyposulphurous acid.' Au acid pound unknown in the isolated state and but little known in the form of dilute aqueous solu tions, but well known in the form of its salts, called thiosulphates (or hyposulphites), espe cially its sodium salt. The thiosulphates are formed by the action of metals (such as zinc, iron. etc.) on acid sulphites. The thiosulphates of the alkalies are freely soluble in water; those of the alkaline earths are much less soluble; while those of the heavy metals are by them selves insoluble, but generally form soluble double compounds with the thiosulphates of the alkalies. The thiosulphate of sodium may be prepared in the laboratory by boiling a solution of sodium sulphite with sulphur. It may also be made from the thiosulphate of calcium. The latter is produced by the action of sulphur on lime. Hence it is produced (along with calcium
pentasulphide) when lime is used for the puri fication of illuminating gas. Further. it is formed when a solution of calcium pentasulphide is exposed to the oxidizing action of the air—a reaction causing the production of the thiosul phate in the manufacture of soda (q.v.) and its increased production in the purification of il luminating gas. Aqueous sodium thiosulphate has the remarkable property of dissolving the halogen salts of silver. mercurous chloride (calo mel) and iodide, the iodide and the sulphate of lead, the sulphate of calcium, and the oxides of copper. Owing to its capacity for dissolving the halogen salts of silver unaltered by light, it is extensively used in photography. It is further employed in the manufacture of certain coal tar colors and in dyeing.