LECTURE XVI.] HISTORY OF CHEMISTRY. 345 



lower temperatures by the aid of liquid hydrogen boiling under 

 low pressure, in order to approach as nearly as possible to the 

 absolute zero. 02 



It is also noteworthy that ozone, which was obtained in 

 the liquid state by Hautefeuille and Chappuis in 1882 by the 

 aid of liquid ethylene, 63 can easily be prepared in an approxi- 

 mately pure condition by the use of liquid air, so that Troost 

 was able to determine its boiling point 64 and Ladenburg its 

 density.' 55 The latter determination is of especial importance, 

 since the molecular formula O 3 , deduced from it, constitutes 

 one of the most emphatic arguments in favour of the whole 

 molecular theory ; and this formula, which till then had only 

 been supported by Soret's experiments, 66 could not be regarded 

 as finally settled. 



But the results that have been furnished by this agency 

 with respect to the discovery of new elements are almost of 

 greater consequence. 



When Lord Rayleigh compared the relative density of 

 atmospheric nitrogen with that of nitrogen prepared from 

 ammonia and other nitrogen compounds, he found a differ- 

 ence (in the third decimal place) which could not possibly be 

 ascribed to an experimental error. 67 He therefore resolved 

 upon a minute investigation in order to find out the substance 

 that was mixed with atmospheric nitrogen. This investigation 

 he then carried out along with Ramsay, and it led to the 

 discovery of argon, an element of which it is very difficult 

 to obtain any compounds. 68 The molecular weight, deduced 

 from the density, gave the number 39-92, 69 and since by 

 Kundt's method (compare p. 300) the monatomic character of 

 the gaseous molecules was indicated, its atomic weight would 

 be represented by the same number. The question as to 

 the position of this element in the periodic system is thereby 



62 Proc. Roy. Soc. 64, 227 ; Ann. Chim. [7] 17, 5. w Comptes 

 Rendus. 94, 1249. 64 Ibid. 126, 1751. 6r> Berichte. 31, 2508, 2830; 

 32, 221. fic Annalen. 138, 45 ; Supplementband 5, 148. 67 Nature, 

 46, 512. {58 Rayleigh and Ramsay, Proc. Roy. Soc. 57, 265 ; Z. physik. 

 Chem. 16, 344; Phil. Trans. 1895 (A), 187. 69 Berichte. 31, 3121. 



