84 PHYSICAL PROPERTIES AND COMPOSITION 



Substance Refractive index Dielectric constant 



Hydrogen .... 1-00014 1-000264 = i-oooi3 2 



Carbon dioxide . . 1-00045 1-00097 = i-ooc48 3 



Hexane .... 1-3683 1-8536 = 1-3608^ 



Octane .... 1-3902 i 9382 = i^Sgg 2 



Decane .... 1-4001 1-9641 = 1.4015''* 



Benzene .... 1-4816 2-2221 = 1-4777^ 



Since the calculation involves an uncertain extrapolation 

 to infinite wave-length by Cauchy's formula, which is not 

 applicable to the purpose in the case of bodies with anoma- 

 lous dispersion, the direct measurement of refractive index 

 for Hertzian waves 1 of very long wave-length (6 m.) affords 

 a very useful completion ; the highest refractive indices 

 so observed were 



castor oil .A 7 " = 3-05 K 4-67 = 3-1 6 2 . 



water 2 #=8-91(17) K = 80 = (8.99)2. 



On the other hand Clausius and Mossotti found a relation 

 between dielectric constant and space occupied, on the 

 assumption of spherical molecules : 



j d 



in which d is the fraction of the total volume actually 

 occupied by matter. The expression represents the diminu- 

 tion of dielectric constant down to unity for indefinite 

 dilution, and on the other hand its increase with in- 

 creasing density by condensation or cooling 3 , as for ben- 

 zine, e. g. 



Liquid at 19-6, K 2-218. Vapour at 100, K =. 1-0027. 



Since d is proportional to the density D we have for 

 a given body: 



K-i 



-^ = const., 



1 Arons and Rubens, Wiecl. Ann. 42. 581 ; 44. 206. 



3 Cohn, Zeeman, Wied. Ann. 57. 15 (1896). 



3 Abegg and Seitz, Zeitschr. f. Phys. Chem. 29. 242. 



