6l6 GENERAL THEORIES. 



the propagation of the magnetic disturbances is the same as the 

 velocity of light. 



Let us suppose, in the first case, that the medium in question is 

 air. The coefficient K would be equal to unity if the electrostatic 

 units had been adopted. In the electromagnetic system, the value 



of this coefficient (608) is . It follows from this that 



a? 



. 



N/K 



Hence the velocity of the propagation of an electromagnetic 

 disturbance in air is equal to the ratio of the units ; this ratio 

 ought then to be equal to the velocity of the propagation of light. 

 Now experiment gives values for these two velocities which differ 

 extremely little from 300,000 kilometres per second, and the most 

 recent researches agree in giving numbers which are the nearer 

 each other, the more exact the measurements have been. Such a 

 coincidence cannot be due to accident, and Maxwell's theory finds 

 thus a most striking experimental confirmation. 



634. SPECIFIC INDUCTIVE CAPACITY. Let us now consider a 

 dielectrical medium, the refractive index of which is n, and its 

 specific inductive capacity greater than that of air. If V is the 

 velocity of the propagation of light in air, and V its velocity in 

 the medium in question, we have 



72V' = V, or 



On the other hand the velocity V" of electromagnetic disturbance 

 will be obtained if we replace K by K', which gives 



K'V" 2 = KV 2 =i. 



TC' 



In order that V" and V shall be equal, we must have n 2 = . 



K 



It follows then from this theory that the specific inductive 

 capacity of a dielectric with respect to air is equal to the square 

 of its refractive index. 



A difficulty here presents itself in the experimental verification 

 of this conclusion, which arises from the dispersion of refracting 

 media. As the refractive index varies with the wave-length the 

 most natural idea would be to take the limiting value of the index : 



