Some other acids of sulphur remain to be noticed. Among these is thiosulphuric acid, H₂S₂O₃, of which the sodium salt is produced by digesting together sodium sulphite with sulphur, just as, with oxygen, sodium sulphate is formed. In the latter case it may be supposed that the atom of oxygen inserts itself between the sodium atom and the sulphur atom with which it is in combination, thus :

; hence the name

" thio" sulphate, the " thion," or sulphur, replacing the oxygen of sulphuric acid. The sodium salt forms large transparent crystals of the formula Na₂S₂O_s.5H₂O ; the barium salt is sparingly soluble, and forms a crystalline precipitate on adding a solution of the sodium salt to one of barium chloride ; the lead salt is insoluble, and the silver salt is a white precipitate, which rapidly turns dark on application of heat, being converted into silver sulphide : Ag₂S₂O₃ + H₂O.Aq = Ag₂S + H₂SO₄.Aq. On acidify-ing any one of the soluble salts, the acid is momentarily liberated ; but it immediately decomposes into sulphurous acid and sulphur, H₂S₂O₃.Aq = H₂SO₃.Aq + S, the latter rendering the liquid milky. The sodium salt, when a solution of iodine in one of potassium iodide is added to it, undergoes the reaction : 2Na₂S₂O₃. Aq + I₂.Aq = 2NaI.Aq + Na₂S₄O₆.Aq. The salt formed is named tetrathionate of sodium. It will be considered shortly.