1 88 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Cavendish (1781) finds- nitric acid in the water produced 

 by the explosion of hydrogen and oxygen. By burning 

 hydrogen in air, Cavendish, in 1781, obtained 135 grains of 

 " pure water " " which had no taste nor smell " ; but when he 

 exploded hydrogen with an excess of oxygen the 30 grains 

 of water which he obtained " was sensibly acid to the taste, 

 and by saturation with fixed alkali and evaporation, yielded 

 near two grains of nitre ; so that it consisted of water united 

 to a small quantity of [nitric] acid" ^A.C.R. III. 16). 



The explosion of hydrogen with an excess of oxygen always 

 led to the production of nitric acid, even when the oxygen 

 was prepared without using a nitrate, e.g., from red lead and 

 oil of vitriol, or from the leaves of plants by exposing them 

 to sunlight in a vessel filled with water. But when the 

 proportion of oxygen was reduced, so that only about 2% of 

 it remained unburnt, " the condensed liquor was then not 

 acid, but seemed pure water." Cavendish also found that 

 " when inflammable air was exploded with common air ... 

 the condensed liquor was not in the least acid" (A.C.R. III. 

 1 8), although a large excess of oxygen was provided. The 

 production of nitric acid was therefore dependent (i) on 

 the presence of an excess of oxygen, and (2) on the high 

 temperature reached in the explosion of hydrogen with 

 oxygen. 



As it seemed improbable that the nitric acid could be 

 derived from the oxygen, Cavendish attributed its formation 

 to the oxidation of azote (present as an impurity) during 

 the explosion. 



Cavendish (1784) prepares nitric acid by sparking air. 

 The high temperature which Cavendish thought to be 



1 Priestley, in 1788, discovered nitrate of copper in the moisture pro- 

 duced by exploding inflammable air with common air in a copper vessel ; 

 in one experiment 442 grains of the green liquor contained enough nitric, 

 acid (free and combined) to yield 23 grains of nitre (Experiments and 

 Observations on Acidity, Water ; and Phlogiston, Phil. Trans., 1788, 

 78, 147-157, 3 1 3~330)- 



