£04 Dr. G. Rudorf on the Molecular and 



(d) From the refractive index. 

 From the relation K = (yu, A=ao ) 2 , we get 



fjb 2 + 2 6 



whence 



p 



l=^N<x 3 . 

 tt 



The refractivities (fjL — 1) of the inactive gases compared 

 with air=l have been determined by Ramsay and Travers 

 (Phil. Trans. I. c.) and from these the refractive indices can be 

 calculated, using /^ a ir = 1*000292. This gives the following 

 values : — 





He. 



Ne. 



A. 



Kr. 



X. 



/jL—1 for air=l... 



0-1238 

 1-0000361 



0-2345 

 1-0000685 



0-9665 

 1-000282 



1-450 

 1-000424 



2-364 

 100069 





For Neon, there are no better data at our disposal, so the 

 value above given must stand. But recent determinations 

 with the other gases show that the values obtained by 

 Ramsay and T ravers are a little too high. 



Besides the determinations of Burton and of Guthbertson 

 and Metcalfe for He given above, we have also those of Scheel 

 and Schmidt (Ber. d. d. pliys. Ges. vi. p. 207 (1908)), and 

 of Herrmann (ib. pp. 211, 467); but as Herrmann's results 

 point to a dispersion much greater than that of air, and 

 Scheel and Schmidt's results indicate anomalous dispersion, 

 they are probably less trustworthy than those of the other 

 mentioned observers. 



For Kr and X, the dispersion has been measured by 

 Mr. and Mrs. Guthbertson (Proc. Roy. Soc. lxxxi. p. 440 

 (1908 j), using large quantities of gas. They found for 



Kr, /*-! = 0-0004189(1 + 



/-, 6-97 \ 



and for 



1=0-0006823 (l+j^ 



> 



We can now calculate cr, using for He, A, Kr, and X the 

 /^-values for \=co, and for Xeon the value obtained by 

 Ramsay and Travers for white light. The results are 

 tabulated in the appended table. 



