Sulphurous acid gas (S02; 64) dissolved in water and constituting 9.2 per cent. of the solution.

Properties. - A colourless liquid with a strong sulphurous odour.

Preparation. - Vide p. 567, 2H2SO4 + C = CO2 + 2SO2 + 2H2O.

Reactions. - Unlike sulphuric acid, it gives no precipitate with chloride of barium, but if chlorine be added to it, it becomes converted into sulphuric acid, and then gives a precipitate, SO2 + 2H2O + Cl2 = H2SO4 + 2HC1.

Impurities. - Sulphuric acid, solid impurities, too little sulphurous acid.

Test. - It should give no precipitate, or only a slight one, with chloride of barium (little or no sulphuric acid); but very few specimens answer either to this test or to the officinal volumetric test, on account of the liability of the acid to decompose. It should leave no residue on evaporation. Its strength is determined by its sp. gr. 1.04, and the volumetric test.

Dose. - 1/2-1 fluid drachm diluted with water.

Action. - It is a powerful deoxidising agent. It is extremely destructive to plant life, and so may destroy disease-germs.

Uses. - Gaseous sulphurous acid is used to disinfect rooms. The room should be closely shut up, and a brazier with charcoal placed in it. On this sulphur is thrown, and the fumes are allowed to permeate the room for several hours. Care must be taken that the brazier is so placed that there is no danger of anything in the room catching fire. A solution mixed with glycerine may be applied in skin-diseases depending on parasitic fungi. It is very useful in cases of vomiting, especially when the vomited matters have a frothy or yeasty appearance due to the presence of sarcinae and to the occurrence of fermentation in the stomach. Applied as spray it sometimes gives relief in laryngeal phthisis.