208 JAMES CLKltK MAXWELL 



such as castor oil, olive oil, sperm oil, neatsfoot oil, 

 and also lor ether, the differences are considerable. 



It seems probable that the reason .for this difference 

 lies in the fact that, in the light waves, we are dealing 

 with the wave velocity of a disturbance of an ex- 

 tremely short period. Now, we know that the sub- 

 stances mentioned shew optical dispersion, and we 

 have at present no completely satisfactory theory from 

 which we can calculate, from experiments on very 

 short waves, what the velocity for very long waves 

 will be. In most cases Caiichy's formula has been 

 used to obtain the numbers given. The value of K, 

 however, as found by experiment, corresponds to these 

 infinitely long waves, and to quote Professor J. J. 

 Thomson's words, "the marvel is not that there 

 should not be substances for which the relation K 

 ~ /i- does not hold, but that there should be any for 

 which it docs." * 



It has been shewn, moreover, both by Professor J. 

 J. Thomson himself and by Hlondlot, that when the 

 value of K is measured under very rapidly varying 

 electrifications, changing at the rate of about 25,000,000 

 to the second, the value of the inductive capacity lor 

 glass is reduced from about (>'S or 7 to about 2*7 ; the 

 square root of this is 1'C, which does not ditler much 

 from its refractive index. The values of the inductive 

 capacity of paraffin and sulphur, which it will bo 

 remembered agree fairly with Maxwell's theory, were 

 found to be not greatly different in the steady and 

 in the rapidly varying field. 



On the other hand, some experiments of Arons 



* In hi8 sentence /A htunls for the rofrwtive index. 



