THE GASES OF THE ATMOSPHERE 143 



of mixtures of mono-atomic elements, these atomic weights 

 would be merely the mean atomic weights of the elements con- 

 tained in such mixtures taken in the proportions in which they 

 are present. "We must ask therefore what evidence there is 

 that they are mixtures. Considering helium first, MM. Runge 

 and Paschen, Mr. Lockyer and others have maintained that 

 spectral analysis shows helium to be in reality a mixture of two 

 elements. It is sufficient to remind the two German savants 

 that they have expressed the view that the evidence of com- 

 plexity of helium applies also to the case of oxygen. No one 

 hitherto has suspected oxygen of being a mixture. Our repeated 

 efforts to effect a separation of helium into two elements, by 

 means of diffusion, have only succeeded in showing that the 

 helium from minerals may contain a small quantity of argon. 

 If there were two elements they must have the same density. 

 As to the hypothesis of Mr. Lockyer, who bases his idea on the 

 fact that the stars do not exhibit all the lines of helium, it must 

 not be forgotten that greater or less pressure, and temperature 

 more or less elevated, produce considerable differences in the 

 spectrum of helium in the relative intensity of the lines and even 

 in their existence. Our knowledge of the conditions prevailing 

 in the stars is so incomplete that it may well happen that certain 

 lines are missing and we can draw no conclusion therefrom. 



I cannot answer for the homogeneity of argon with the same 

 assurance. Diffusion experiments gave two portions of gas, the 

 one having the density 19'93, the other less diffusible 20'01. If, 

 however, there is any foreign gas present, it must be in very 

 minute quantity, and would have no considerable effect on the 

 atomic weight. 



The mono-atomicity of helium is connected with its other 

 physical properties. Having an atomic weight nearly four times 

 that of hydrogen, it ought to have a higher boiling point. How- 

 ever, Professor Olszewski has been unable to liquefy it at a 

 temperature much lower than that at which hydrogen becomes 



