ix SULPHUR AND PHOSPHORUS 183 



The volatile acid and its salts were prepared in 1702 by Stahl, 

 who showed that the salts were converted by exposure to air 

 into salts of the fixed oil of vitriol. This action may be 

 represented by the equation : 



2K 2 SO 3 + O 2 -> 2K 2 SO 4 . 



Potassium Potassium 



sulphite sulphate 



The gas was isolated in 1774 by Priestley, who obtained it from 

 oil of vitriol by the reducing action of olive oil or of mercury : 

 Hg + 2H 2 SO 4 -> HgSO 4 + SO 2 + 2H 2 O 



Mercury -f Sulphuric > Mercuric + Sulphurous + Water 

 acid sulphate anhydride 



The gas is freely soluble in water, but could be collected and 

 preserved over mercury. 



Lavoisier (1777) showed that oil of vitriol might be regarded 

 as a compound of sulphurous anhydride with oxygen and water. 

 He attempted to analyse it by heating it with mercury and 

 collecting all the products, including the mercury which was 

 finally recovered from the action : 



Hg + 2H 2 SO 4 -> HgSO 4 + SO 2 + 2H 2 O 

 HgS0 4 -> Hg + 2 + S0 2 . 



Gay-Lussac, in 1807, carried out a successful analysis by 

 heating the sulphate of alumina in the form of alum, whereby he 

 obtained a mixture of sulphurous anhydride with half its volume 

 of oxygen : 



2A1 2 (SO 4 ) 3 -> 2A1 2 O 3 + 6SO 2 + 3O 2 . 



Sulphate of > Alumina + Sulphurous + Oxygen 

 alumina anhydride 



As sulphurous anhydride contains its own volume of oxygen, 

 the proportion in sulphuric anhydride must be 50 per cent, 

 greater. 



Sulphuretted hydrogen (SH 2 ) is set free in the form of a 

 fcetid gas by the action of acids on sulphides, such as the 

 artificial sulphide of iron, which is produced by fusing the metal 

 with sulphur : 



FeS + H 2 SO 4 -> FeSO 4 + H 2 S. 



Sulphide of + Oil of > Ferrous + Sulphuretted 

 iron vitriol sulphate hydrogen 



