800 Dr. G. Rudorf on the Molecular and 



(c) From the dielectric constant (K). 



The Clausius-Mosotti theory of dielectrics gives 



For He, K has recently been determined by Hochheim ( Verh. 

 d. d. phys. Ges. x. p. 446 (1908)), who found K = ±'000074. 

 Using this value we get 



V = 0'0000247 and <r = 0-597 x 10" 8 cm. 



The dielectric constant of argon is unknown. 



(d) From the refractive index (/*). 



The electromagnetic theory of light gives for \=oo y 

 K=/a 2 ; hence 



/* 2 + 2 



and as /j, does not differ greatly from unity, we get 

 y_/* 2 -l_ 2(>-l) 



z* 2 +2 a * 



Burton (Proc. Eoy. Soc. lxxx. p. 390 (1908)) has deter 

 mined the dispersion of helium and argon. He found for 



„=V; 



Helium, /*= 1-00003478+ - fi 



r 



Argon, /*= 1-0002792 + 



16 



X 2 ' 

 and for 



1-6 X IP" 15 

 X 2 



The value for He agrees well with that obtained by 

 Cuthbertson and Metcalfe (Proc. Eoy. Soc. lxxx. p. 411 

 (1908)):— 



/ ,_ 1 = -0000347(l+-g rr ). 



We have, therefore, 



/**=„ for Helium = 1*0000347, and 

 /x A=00 for Argon =1-0002792. 



Using these values we get 



<r for Helium = 0-559 x 10" 8 cm., and 

 <r for Argon = 1*59 x 10~ 8 cm. 



