UNIVERSITY 



CH. xx.] DIFFICULTIES 193 



electricities, will depend on their masses ; and if the 

 mass of either the positive or the negative electricity 

 were zero or very small, it would be shifted completely, 

 however short the time. Accordingly a short wave 

 would then be retarded just as much as a long one, 

 that is to say, there would be no discrimination of 

 waves, and therefore no dispersion. The amount of 

 dispersion actually experienced will therefore furnish 

 a measure of the relative inertia of the two kinds of 

 electricity in an atom. 



The only substance to which this theory of refraction 

 and dispersion can apply, will be one whose atoms act 

 individually or in isolated manner ; that is to say, they 

 must be gases, the more perfect the better. Now the 

 dispersive power of hydrogen has been measured, and 

 is not zero ; consequently there must be some mass 

 both in the positive and in the negative electricity 

 which hypothetically constitute an atom of hydrogen, 

 and the measured amount of dispersion will enable us 

 roughly to say what the smaller mass may be. The 

 result is, that if M is the aggregate mass of the 

 carriers of positive electricity, and nm the mass of 

 the carriers of negative electricity, n being their 

 number so that the mass of the whole atom is 

 M + mn we get, with the aid of Ketteler's measure- 

 ments for the refractive index of hydrogen for light 

 of different wave-lengths, 



M 1 



=1 approximately. 



n 



This shows that for a hydrogen atom n is approxi- 

 mately 1 (and it has to be a whole number) ; and it 

 also shows that M is not small compared with nm : in 

 fact that it is much bigger, which is an unexpected 

 and puzzling result. 



