210 NON-METALS AND THP:iR COMBINATIONS. 



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 oif. 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 

 point is reached the acid acts upon the lead, wherefore the further 

 concentration is conducted in vessels of glass or platinum, until a 

 specific gravity of 1.84 is obtained. This acid contains about 95 

 per cent, of sulphuric acid ; the remaining 5 per cent, of water can- 

 not be expelled 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 330 C. 

 (626 F.). When cooled it forms crystals, which melt at 10 C. 

 (50 F.). When heated to about 160 C. (320 F.), the acid begins 

 to fume and gives off sulphur trioxide. The 100 per cent, acid 

 gives off sulphur trioxide, and diluted acid gives off water when 

 heated, until an acid of 98.33 per cent, is reached, which boils 

 at the constant temperature of 330 C. It has a great tend- 

 ency to combine with water, absorbing it readily from atmo- 

 spheric air. Upon mixing sulphuric acid and water, heat is gen- 

 erated in consequence of the combination taking place between the 

 two substances. To the same tendency of sulphuric acid to com- 

 bine with water must be ascribed its property of destroying 

 and blackening organic matter. It is due to this decomposing 

 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. (See Experiments, page 179.) 



Sulphuric acid is a very strong dibasic acid, which expels or dis- 

 places most other acids ; its salts are known- as sulphates. 



The sulphuric acid of the U. S. P. should contain not less than 

 92.5 per cent, of H 2 SO 4 , corresponding to a specific gravity of not 

 V>ss than 1.826 at 25 C. 



The diluted sulphuric acid, Acidum mlphuricum dilutum, is a mix- 

 ture of 100 parts by weight of acid and 825 parts of water, or of 



