The _sulphate_, Th(SO₄)₂, is obtained anhydrous by evaporating the excess of acid from a solution of the dioxide in oil of vitriol, or by heating the hydrates. It resembles the sulphates of the rare earth elements, in that it dissolves in water at 0° to form a highly supersaturated solution, from which the hydrated forms separate out almost quantitatively when the temperature is allowed to rise. The solubility relations of the various hydrates, on account of their commercial importance, are somewhat fully treated in Chapter XVIII. A dihydrate, Th(SO₄)₂,2H₂O, is obtained by keeping the tetrahydrate at 110°. The ennea- and octohydrates are isomorphous with the corresponding thorium selenate hydrates, and the ennea- and tetrahydrates with the analogous uranous sulphate hydrates. The hydrates yield the anhydrous salt when heated to 400°; the anhydrous sulphate has already a considerable dissociation tension (15 mm.) at 575°. By treatment with excess of acid, and subsequent heating to 130° _in vacuo_, the _acid sulphate_, Th(SO₄),H₂SO₄, is obtained. An insoluble _basic salt_, ThOSO₄,2H₂O, is formed by continued boiling of the tetrahydrate in dilute solution, or more quickly by heating the solution in a closed tube to 120°-125°; a monohydrate, ThOSO₄,H₂O, is also known. Halla[483] has recently obtained the hydrate, ThOSO₄,5H₂O, by boiling a solution of the neutral sulphate with magnesium sulphate, and also by treating the anhydrous sulphate with a little water in presence of magnesium carbonate. [483] _Zeitsch. anorg. Chem._ 1912, ~79~, 260. By precipitation with potassium sulphate the _double salt_, Th(SO₄)₂,2K₂SO₄,2H₂O, is formed; this is soluble in water but insoluble in potassium sulphate solution. The analogous sodium and ammonium salts are soluble both in water and excess of the corresponding alkali sulphate. The _sulphite_, Th(SO₃)₂,H₂O, is obtained as a white amorphous precipitate by warming a solution of a thorium salt with sulphurous acid. Basic sulphites and double sulphites are also known; the precipitates obtained by addition of alkali sulphite dissolve readily in excess. The hydroxide is almost insoluble in sulphurous acid, behaviour which distinguishes thorium (and zirconium) from all the trivalent metals. No _thiosulphate_ is known, the hydroxide being thrown down from boiling solution by addition of sodium thiosulphate: this method of precipitation was formerly much used for purposes of estimation, but it is more tedious and less accurate than the modern methods.