104 NON-METALS AND THEIR COMBINATIONS. 



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 : 



SO 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 S0 4 -f 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.N0 2 ). 



This complex compound is readily decomposed by steam into sulphuric acid 

 and nitrogen trioxide : 



2(SO 2 .OH.N0 2 ) + H 2 O = 2H 2 SO 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. 

 There is, however, an unavoidable loss of small portions of nitric acid, or 

 oxides of nitrogen, for which reason some nitric acid has to be supplied daily. 



It is likely that other chemical changes than the ones mentioned take place 

 in the acid chamber, but according to modern investigations these are the 

 principal ones. 



The liquid sulphuric acid formed in the lead-chamber collects at 

 the bottom of the chamber, whence it is drawn off. In this state it 

 is known as chamber acid (specific gravity 1.50), and is not pure, but 

 contains about 36 per cent, of water, and frequently either sulphurous 

 or nitric acid. By evaporation in shallow leaden pans it is further 

 concentrated, until it shows a specific gravity of 1.72. When this 



