SULPHUROUS ACID. 321 



gas through a tube surrounded by a freezing mixture of ice and 

 salt, and forms a colourless and very mobile liquid, of sp. gr. 1.45, 

 which boils at 14. The volatility of this liquid is small at con- 

 siderably lower temperatures, and it is not applicable with ad- 

 vantage to produce intense cold by its evaporation (Mr. Kemp). 

 With a little water, it forms a crystalline hydrate, which con- 

 tains 20 per cent of acid, and perhaps therefore 1 4 equivalents 

 of water. 



Sulphurous acid is not decomposed by a high temperature ; 

 but several substances such as carbon, hydrogen and potassium, 

 which have a strong affinity for oxygen, decompose it at a red 

 heat. This acid blanches many vegetable and animal colours, 

 and the vapours of burning sulphur are therefore employed to 

 whiten straw, and to bleach silk, to which they also impart a 

 peculiar gloss. The colours are not destroyed, and may in 

 general be restored by the application of a stronger acid or an 

 alkali. Dry sulphurous acid exhibits no affinity for oxygen, 

 but in contact with a little water, these gases slowly combine 

 and sulphuric acid is formed. From the same affinity for oxy- 

 gen, sulphurous acid deprives the solution of chameleon mineral 

 of its red colour, and throws down iodine from iodic acid. It 

 decomposes the solutions of those metals which have a weak 

 affinity for oxygen, such as gold, silver and mercury (with heat), 

 and throws down these bodies in the metallic state. Sulphu- 

 rous acid is conveniently withdrawn from a gaseous mixture by 

 means of peroxide of lead, which is converted by absorbing this 

 gas into the white sulphate of lead. By nitric acid, sulphurous 

 acid is immediately converted into sulphuric acid. 



Sulphites. The alkaline sulphites have a considerable resem- 

 blance to the corresponding sulphates. Their acid is precipi- 

 tated by the chloride of barium, but the sulphite of barytes is 

 dissolved by hydrochloric acid. Sulphurous acid is a weak acid 

 and its salts are decomposed by most other acids. 



Uses. Besides the application of which sulphurous acid is 

 susceptible in bleaching, it is likewise employed in French 

 hospitals, in the treatment of diseases of the skin. The gas is 

 then applied in the form of a bath. (Dumas, Traite de Chimie 

 appliquee aux Arts, t. 1, p. J51). 



This oxide of sulphur, besides acting as an acid, appears to 

 play the part of a radical, like carbonic oxide, and to pervade a 



