SULPHUR. 89 



Xitrogen dioxide is capable of readily absorbing oxygen from 

 the atmospheric air, forming nitrogen tetroxide : 



NO + O = N0 2 . 



The nitrogen tetroxide again transfers one atom of its oxygen 

 to the sulphurous acid, and so on indefinitely, as long as sul- 

 phurous acid and oxygen are present: 



m H 2 SO S + N0 2 == H 2 SO 4 -f NO. 



The liquid sulphuric acid thus 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 1.6), 

 and is not pure, but contains an excess of water, and frequently 

 either some sulphurous or nitric acid. By evaporationm shal- 

 low leaden pans it is further concentrated, until it shows a 

 specific gravity of 1.72. "When this point is reached the acid 

 acts upon the lead, wherefore the further concentration is con- 

 ducted in vessels of glass or platinum, until a specific gravity of 

 1.84 is obtained. This acid contains about 96 per cent, of sul- 

 phuric acid; the remaining 4 per cent, of water cannot be ex- 

 pelled by heat. 



Properties of sulphuric acid. Pure acid has a specific gravity 

 of 1.848; it is a colorless liquid, of oily consistence, boiling at 

 338. It has a great tendency to combine with water, absorb- 

 ing it readily from the atmospheric air. Upon mixing sulphuric 

 acid and water, heat is generated in consequence of the com- 

 bination taking place between the two substances. To the same 

 tendency of sulphuric acid to combine with water, must be 

 ascribed its property of destroying and blackening organic 

 matter. Organic substances generally contain the elements 

 carbon, hydrogen, and oxygen. Sulphuric acid added to such 

 organic substances removes the elements hydrogen and oxygen 

 (or at least a portion of them), combines them into water with 

 which it unites, leaving behind compounds so rich in carbon 

 that the black color predominates. It is due to this decompos- 

 ing action of sulphuric acid upon organic matter that traces of 

 the latter color sulphuric acid dark yellow, brown, and when 

 present in larger quantities almost black. The poisonous, 

 caustic properties are due to the same action. 



