1. When evaporated, it should leave no residue. 2. Barium chloride gives a copious white precipitate (of barium sulphate) if sulphuric acid be present. 3. Sulphurous acid decolorises a solution of iodine with formation of hydriodic and sulphuric acids, thusSO2 + 2H1O + I2 = H1SO4 + 2HI. 3.2 gr. sulphurous acid neutralize 12.7 iodine.

Of the gas, 1 part in 100,000 of air is perceptible to the nose; 9 parts are disagreeable and provoke cough; 20 parts are irritating, and 43 parts (=4 parts in 10,000) are irrespirable: much less than this will kill plants (Letheby, loc. cit.). In generating it by burning sulphur in air, 32 parts by weight combine with an equal quantity of oxygen to produce 64 parts of sulphurous anhydride, which occupy precisely the same bulk as the oxygen consumed. Its density is high, the sp. gr. being 2.247; a cubic foot weighs about 1,206 gr., and to produce it 603 gr. of sulphur and a cubic foot of oxygen (=5 cubic feet of atmospheric air) are requisite (Letheby: Medical Times, ii., 1873).

Alkaline sulphites and hyposulphites owe their properties to the sulphurous acid liberated from them, and although not officinal are often prescribed.