192 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



He then added some more oxygen to the residue and 

 continued to spark till no further diminution took place. 

 The excess of oxygen was absorbed by means of " liver of 

 sulphur," 



"After which only a small bubble of air remained un- 

 absorbed, which certainly was not more than T Jy- of the 

 bulk of the phlogisticated air l let up into the tube ; so that 

 if there is any part of the phlogisticated air l of our atmos- 

 phere which differs from the rest, and cannot be reduced 

 to [nitric] acid, we may safely conclude, that it is not 

 more than yi^- part of the whole" (A.C.R. III. 50). 



For 1 10 years this bubble of gas was thought to be merely 

 a residue of unabsorbed nitrogen; but in 1895 Rayleigh 

 and Ramsay found that the atmosphere actually contained 

 about 0-93% of a gas, ARGON, even more inert than 

 nitrogen ; there is therefore no doubt that the bubble 

 which Cavendish noticed was actually a part of the 

 azote " which differs from the rest, and cannot be reduced 

 to nitrous acid." His cautious statement has, indeed, proved 

 to be entirely accurate, in contrast with the hasty assump- 

 tion of his successors that the azote consisted entirely of 

 nitrogen. 



B. COMPOSITION OF THE GASEOUS OXIDES OF NITROGEN. 



Oxides of nitrogen. The experiments of Cavendish 

 showed that nitrogen, the chief constituent of azote, could 

 be converted into ordinary nitre by sparking it with oxygen 

 over potash. This gas was therefore the characteristic 

 constituent of nitric acid. The three gases which Priestley 

 had prepared from nitric acid probably contained the same 

 constituent in combination with oxygen. Experiments were 

 therefore made by different workers to ascertain their com- 

 position. 



1 i.e. Azote. 



