CH. x NITRE, NITRIC ACID, AND NITROGEN 187 



Assuming then that nitrous air already contained oxygen, 

 what was the nature of the substance with which the oxygen 

 was combined ? and what was the fundamental principle of 

 nitric acid and the nitrates ? There was much vague 

 evidence to show that this principle might be identified 

 with " azote," the inactive residue left after depriving air of 

 its oxygen. Thus Priestley had recorded the fact that : 



" Nitrous air, which had been confined above a year in 

 contact with iron, standing in water, was in all respects like 

 phlogisticated common air : it neither diminished common 

 air, nor was diminished by nitrous air, and extinguished a 

 candle" (Experiments o?i Air, 1775, ti r 77)' 



Cavendish, too, had noticed that charcoal deflagrated with 

 nitre produced an air which " as far as I can perceive . . . 

 differs in no respect from common air phlogisticated " 

 (A.C.R. III. 21). But in neither case was the evidence 

 sufficiently definite to justify the view that nitrous air was 

 an oxide of azote, or nitric acid a compound of azote with 

 oxygen and water. 



This inert gas had, indeed, defied almost all attempts to 

 change it or to ascertain its nature. Priestley, after many 

 experiments on diminishing the volume of gases, could not 

 find any substance which would diminish common air to a 

 larger extent than one-fifth, although nearly every other gas 

 that he had handled could by some means or other be 

 condensed almost entirely, e.g. : 



acid air and alkaline air by water, 

 fixed air by lime-water or by potash, 

 nitrous air by oxygen and water, 



and so forth. It was, however, one of his random observa- 

 tions, on the diminution of air by sparking (Experiments and 

 Observations, 1779, IV. 284-287), that enabled Cavendish 

 to solve the problem of the nature of azote, and of the various 

 nitrous compounds. 



