ARGON AND HELIUM. 363 



ARGON AND HELIUM. 



The true atomic weights of these remarkable gases are still in doul)t. 

 and so far can only be inferred from their specific gi-avities. 



For argon, the discoverers, Ra_yleigh and Ramsay * give various deter- 

 minations of density, ranging, with hydrogen taken as unity, from 19.48 

 to 20.6. In an addendum to the same paper, Ramsay alone gives for 

 the density of argon prepared by the magnesium method the mean value 

 of 19.941. In a later communication f Rayleigh gives determinations 

 made with argon prepared by the oxygen method, and puts the density 

 at 19.940. 



For the density of helium, Ramsay | gets 2.18, while Langlet§ finds 

 the somewhat lower value 2.00. 



From one set of pliysical data both gases appear to be monatomic, but 

 from other considerations they are supposably diatomic. Upon this 

 question controversy has been most active, and no final settlement has 

 yet been reached. If diatomic, argon and helium have apj^roximately 

 the atomic weights two and twent}^ respectively; if monatomic, tliese 

 values must be doubled. In either case helium is an element lying be- 

 tween hydrogen and lithium, but argon is most diflicult to classify. A\'ith 

 the atomic weight 20, argon falls in the eighth column of the periodic 

 system between fluorine and sodium, but if it is 40 the position of the gas 

 is anomalous. A slightly lower value would place it between chlorine 

 and potassium, and again in the eighth column of Mendelejeff' s table ; 

 but for the number 40 no opening can be found. 



It must be noted that ?ieither gas, so far, has been proved to be abso- 

 lutely homogeneous, and it is quite possible that both may contain ad- 

 mixtures of other things. This consideration has been repeatedly urged 

 by various writers. If argon is monatomic. a small imj^urity of greater 

 density, sa}' of an unknown element falling between bromine and rubid- 

 ium, would account for the abnormality of its atomic weight, and tend 

 towards the reduction of the latter. If the element is diatomic, its classi- 

 fication is easy enough on the basis of existing data. Its resemblances 

 to nitrogen, as regards density, boiling point, difficult}'- of liquefaction, 

 etc., lead me personally to favor the lower figure for its atomic Aveight, 

 and the same considerations may apply to helium also. Until further 

 evidence is furnished, therefore, I shall assume the values two and twenty 

 as approximately true for the atomic weights of helium and argon. 



* Phil. Trans., i86, pp. 220 to 223, and 238. 1S95. 



tChem. News, 73, 75. 1896. 



J Journ. Chem. Soc, 1895, p. 6S4. 



g Zeitsch. Anorg. Chem., 10, 2S9. 1895. 



