SULPHUROUS ACID, an acid having (probably) the formula H2S0s, and prepared by dissolving sulphur dioxide gas (see SULPHUR) in water to saturation; the acid being formed by. the union of one moleculi of the dioxide with one molcule of water, according to the equation SO2 1120=---H2S0s. Sulphurous acid has never been isolated, and is known only in its aqueous solution, in combination with bases in the form of the salts known as °sulphites,* and as a solid hydrate. At 70° F. water dissolves about 35 times its own volume of sulphur dioxide; the solubility being greater at lower temperatures, and less at higher ones. When an aqueous solution of sulphurous acid is cooled below 41° F., a crystalline hy drate of the acid separates out, the composition of which is not definitely known. Sulphurous acid has the taste and smell of sulphur dioxide gas, and is strongly acid. It readily gives off sulphur dioxide gas, and upon standing in con tact with the air it gradually absorbs oxygen and becomes converted into sulphuric acid. Its composition is also modified by the action of light, probably by the formation of a more complex oxy-acid of sulphur. It acts as a dibasic acid, combining with the oxides, hy drates and carbonates of many of the metals to form salts (that is, sulphites) which are readily decomposed by the addition of stronger acids, with the liberation of sulphur dioxide.
It is used in the bleaching of silk and wool but not so much as formerly, having been largely displaced by hydrogen peroxide. When the hydrogen atoms of the acid are both re placed by a metallic base, the resulting salt is called a °normal sulphite); and when only half of the hydrogen of the acid is so replaced, the salt is called an °acid sulphite,* or a abisul phite? Both of the sulphites of sodium 'are extensively used in photography; the normal sulphite having the formula Na2S0s7H.0, and the acid sulphite the formula HNaS0s. (See PHOTOGRAPHY). Industrially, the bisulphites of calcium and of magnesium are of exceeding importance, since the aqueous solutions of these substances possess the power of dissolving the gummy matters by which the fibres of certain kinds of wood are cemented together. Upon this property, the °sulphite process) for the manufacture of wood pulp is based. (See PAPER). Calcium sulphite is also used in the brewing industry.