ARGON. 523 



many chemical compounds, from some of which it can readily be 

 prepared. The identification of atmospheric nitrogen with that 

 contained in niter and nitric acid is due to Henry Cavendish, 

 whose exact and skillful work not only established this fact, but 

 led to an observation of great interest in connection with the dis- 

 covery of argon. In a paper which appeared in 1785 Cavendish 

 says : " As far as experiments hitherto published extend, we 

 scarcely know more of the phlogisticated part of our atmosphere 

 [nitrogen] than that it is not diminished by lime water, caustic 

 alkalies, or nitrous air ; that it is unfit to support fire, or maintain 

 life in animals; and that its specific gravity is not much less than 

 that of common air " ; and raises the question " whether there are 

 not in reality many different substances compounded by us under 

 the name of phlogisticated air." He then describes an experiment 

 for the purpose of deciding this point. By passage of electric 

 sparks through a mixture of air and oxygen, the nitrogen was 

 converted into a compound absorbed by the dilute alkali over 

 which the gases were confined. The sparking was continued 

 until no further diminution of volume took place, when, on re- 

 moving the excess of oxygen by absorption in " liver of sulphur/' 

 " only a small bubble of air remained unabsorbed." From this 

 he concludes that " if there is any part of the phlogisticated air of 

 our atmosphere which differs from the rest, and can not be re- 

 duced to nitrous acid, we may safely conclude that it is not more 

 than a hundred and twentieth part of the whole." Cavendish 

 was apparently satisfied with this as a proof of the simple char- 

 acter of atmospheric nitrogen, and his work has been accepted as 

 conclusive for more than a century ; but we now know that this 

 " small bubble of air " which survived his experiment must have 

 been argon. 



It seems strange that a substance present in the air all about 

 us, and whose actual quantity is enormous, should have defied 

 detection through so many years of exact and searching chemical 

 work; but the explanation lies largely in the fact that argon 

 forms no compounds, so far as is known, and thus fails to assert 

 itself in the presence of the almost equally indifferent nitrogen 

 with which it is mixed. 



Indeed, the hint which led to its discovery was obtained in the 

 course of a purely physical investigation. For some years Lord 

 Rayleigh has been engaged in the exact determination of the 

 densities of some of the more permanent gases. In dealing with 

 nitrogen, it was found that this gas, when prepared from chem- 

 ical compounds, was about one half per cent lighter than the 

 nitrogen obtained from air. This discrepancy at once suggested 

 contamination with some known impurities. A careful search 

 proved, however, that this was not the case. The possible ex- 



