1 84 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



The gas is a very weak acid, but is able to precipitate insoluble 

 sulphides from solutions of many metallic salts, thus : 



Silver 2AgNO 3 + H 2 S -> Ag 2 S + 2HNO 3 

 Lead Pb(NO 3 ) 2 + H,S - PbS + 2HNO 3 

 Copper CuSO 4 + H 2 S -> CuS + H 2 SO 4 . 



In other cases, however, the action proceeds in the opposite 

 direction, the sulphide being dissolved by the mineral acid and 

 sulphuretted hydrogen set free, thus : 



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

 Zinc ZnS + H 2 SO 4 -> ZnSO 4 + H 2 S. 



Liver of Sulphur is prepared by dissolving sulphur in an 

 alkali or alkaline earth. The primary action is probably the 

 formation of sulphite and sulphide, thus : 



3Ca(9H) 2 + 38 -> CaSO 3 + 2CaS + 3H 2 O. 



Calcium Calcium Calcium 



hydroxide sulphite sulphide 



By the addition of acid, sulphur dioxide and sulphuretted 

 hydrogen are produced : 



CaSO 3 + 2CaS + 6HC1 -> 3CaCl 2 + SO 2 + 2H 2 S + H 2 O ; 



these then interact in the solution to produce sulphur and 

 water : 



SO 2 + 2H 2 S - 38 + 2H 2 O. 



In the formation of liver of sulphur the converse action takes 

 place, sulphur and water being converted into sulphur dioxide 

 and sulphuretted hydrogen ; this reversal of the usual action may 

 be attributed to the affinity of the alkali for the two gases, both 

 of which possess marked acid qualities. 



Further quantities of sulphur may be dissolved owing to the 

 production of thiosulphate and poly sulphides, thus : 



CaSO 3 + S-> CaS 2 O 3 



Calcium Calcium 



sulphite thiosulphate 



CaS + 8* -> CaS^.,. 



Calcium Calcium 



sulphide polysulphide 



