SULPHUR. 209 



stance, manufactured by chemical action which is so largely used in 

 chemical operations and in the manufacture of so many of the most 

 important articles, as is sulphuric acid. 



Impure sulphuric acid was known in the eighth century ; in the 

 fifteenth a purer acid was obtained by heating ferrous sulphate (green 

 vitriol) in a retort. To the liquid distilling over the name of oil of 

 vitriol was given, in allusion to its tl.ick or oily appearance and the 

 green vitriol from which it was obtained. The change is shown in 

 the following reaction : 



4FeSO 4 + H 2 O = 2Fe 2 O 3 + 2SO., + H 2 SO 4 .S0 3 . 



Sulphuric acid is found in nature in combination with metals as 

 sulphates. Thus calcium sulphate (gypsum), barium sulphate (heavy- 

 spar), magnesium sulphate (Epsom salt), and others occur in nature. 



Manufacture of sulphuric acid. Sulphuric acid is manufactured 

 on a very large scale by passing into large leaden chambers simul- 

 taneously, the vapors of sulphur dioxide (obtained by burning sulphur 

 or pyrites in furnaces), nitric acid, and steam, a supply of atmospheric 

 air also being provided for. The most simple explanation that can 

 be given for the manufacture of sulphuric acid is the fact that sul- 

 phur dioxide when treated with an oxidizing agent, in the presence 

 of water, is converted into sulphuric acid : 



S0 2 + O + H 2 O = H 2 SO 4 . 



Only a portion of the oxygen necessary for oxidation is derived 

 from the nitric acid directly ; the larger quantity is obtained from 

 the atmospheric air, the oxides of nitrogen serving as agents for the 

 transfer of the atmospheric oxygen. 



By the action of nitric acid on sulphur dioxide and steam are formed sul- 

 phuric acid and nitrogen trioxide : 



2SO 2 + H 2 O + 2HNO 3 = 2H 2 SO 4 + N 2 O 3 . 



Nitrogen trioxide next takes up sulphur dioxide, water, and oxygen, forming 

 a compound called nitrosyl-sulphuric acid : 



2SO 2 + N 2 O 3 + 2O + H 2 O = 2(SO 2 .OH.NO 2 ). 



This complex compound is readily decomposed by steam into sulphuric acid 

 and nitrogen trioxide : 



2(SO 2 .OH.NO 2 ) + H 2 O = 2H 2 S0 4 + N 2 O 3 



The nitrogen trioxide again forms nitrosyl-sulphuric acid, which again suffers 



decomposition, and so on indefinitely, as long as the constituents necessary for 



the changes are supplied. These facts show that a given quantity of nitric acid 



will convert an unlimited amount of sulphurous acid into sulphuric acid. 



14 



