198 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



"the nitrous acid." The name NITRIC ACID, now used to 

 distinguish the colourless acid prepared by Glauber's 

 method, was not introduced until 1787. 



Nitric and nitrous acids. It was recognised both by 

 Priestley and by Scheele that pure nitric acid is colourless, 

 but becomes red when reduced or deoxidised by contact 

 with inflammable substances. Thus Scheele in his Treatise 

 on air and fire (1777) asserts that : 



" The colours of the acid of nitre are accidental. When 

 a few ounces of fuming acid of nitre are distilled by a very 

 gentle heat, the yellow separates itself from it and goes into 

 the receiver, and the residuum in the retort becomes white 

 and colourless like water. This acid has all the chief 

 properties of acid of nitre, except that the yellow colour is 

 wanting. This I call the pure acid of nitre ; as soon, 

 however, as it comes into contact with an inflammable 

 substance, it becomes more or less red. This red acid 

 is more volatile than the pure, hence heat alone can separate 

 them from one another ; and, for exactly the same reason 

 the volatile spirit must go over first in the distillation of 

 Glauber's spirit of nitre. When this has gone over, the 

 colourless acid follows" (A.C.R. VIII. 20-21). 



The gradual reduction of the acid is described (A.C.R. 

 VIII. 21) as producing: 



(1) Red nitrous vapours (nitrogen peroxide). 



(2) A weak, volatile PHLOGISTICATED ACID OF NITRE 

 (nitrous acid). 



(3) Colourless nitrous air (nitric oxide). 



(4) Complete destruction of the acid (production of 

 nitrogen). 



Scheele examined in detail the properties of the phlog- 

 isticated acid of nitre, prepared by the action of inflam- 

 mable substances upon the colourless acid. He made 

 the important discovery that a salt of the phlogisticated 

 acid was produced by heating common nitre or saltpetre 

 until it ceased to "boil." In addition to collecting the gas 



