[38] When rapidly boiled, an alkaline solution of stannous oxide deposits tin and forms stannic oxide, 2SnO = Sn + SnO_{4}, which remains in the alkaline solution. [39] Weber (1882) by precipitating a solution of stannous chloride with sodium sulphite (this salt as a reducing agent prevents the oxidation of the stannous compound) and dissolving the washed precipitate in nitric acid, obtained crystals of _stannous nitrate_, Sn(NO_{3})_{2},20H_{2}O, on refrigerating the solution. This crystallo-hydrate easily melts, and is deliquescent. Besides this, a more stable anhydrous basic salt, Sn(NO_{3})_{2},SnO, is easily formed. In general, stannous oxide as a feeble base easily forms basic salts, just as cupric and lead oxides do. For the same reason SnX_{2} easily forms double salts. Thus a potassium salt, SnK_{2}Cl_{4},H_{2}O, and especially an ammonium salt, Sn(NH_{4})_{2}Cl_{4},H_{2}O, called _pink salt_, are known. Some of these salts are used in the arts, owing to their being more stable than tin salts alone. Stannous bromide and iodide, SnBr_{2} and SnI_{2}, resemble the chloride in many respects. Among other stannous salts a sulphate, SnSO_{4}, is known. It is formed as a crystalline powder when a solution of stannous oxide in sulphuric acid is evaporated under the receiver of an air-pump. The feeble basic character of the stannous oxide is clearly seen in this salt. It decomposes with extreme facility, when heated, into stannic oxide and sulphurous anhydride, but it easily forms double salts with the salts of the alkali metals. In gaseous hydrochloric acid, stannous chloride, SnCl_{2},2H_{2}O, forms a liquid having the composition SnCl_{2},HCl,3H_{2}O (sp gr. 2·2, freezes at -27°), and a solid salt, SnCl_{2},H_{2}O (Engel).