When sulphur, selenium, or tellurium is heated in air, the element takes fire and burns ; the chief product in each case is the dioxide. That of sulphur is a colourless gas, possessing the well-known odour of burning sulphur; it is condensable to a liquid at -8°, and it freezes at -79°. The gas is soluble in water, uniting with it to form the acid, H₂SO₃ ; on cooling the solution to -6° and saturating it with the gas, crystals of the formula H₂SO₃.8H₂O separate; this is a hydrate of sulphurous acid. Selenious anhydride, a white solid, also dissolves in water, and from the solution selenious acid crystallises out, with the formula H₂SeO₃. It would naturally be imagined that these acids should have the structural formulas OS=(OH)₂ and 0=Se(OH)₂, inasmuch as the oxides are 0=S=0 and 0=Se=0 ; moreover, the chlorides 0=S=C1₂ and 0=Se=Cl₂, thionyl and selenosyl chlorides, are known; and these react at once with water, forming the acids. It is to be presumed that there is exchange of chlorine for hydroxyl, as usual: 0=S=C1₂ + 2HOH = 0=S=(0H)₂ + 2HC1. But there is evidence, similar in kind to that adduced in- the case of nitrous and phosphorous acids, to show that while sodium sulphite has the formula 0=S=(ONa)₂, silver sulphite is better expressed by, sulphur being a hexad.

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