6 10 GENERAL THEORIES. 



The great problem which the philosophy of science raises is to 

 know the constitution of the single medium by which all physical 

 phenomena may be explained. If calculation shows that electro- 

 magnetic phenomena are propagated not only in air, but in all 

 bodies, with the velocity of the propagation of light, the question 

 would have made a great step; for it would be shown that this 

 medium exists, and that in all probability electrical and luminous 

 phenomena are only different manifestations of the properties with 

 which it is endowed. Such is the conclusion from Maxwell's theory. 

 Faraday's discovery of the action of a magnetic field on the polari- 

 zation of the light which traverses it, would be a natural consequence 

 of the connection which the common medium establishes between 

 the two orders of phenomena. 



628. GENERAL EQUATIONS. In order to determine the con- 

 ditions of the propagation of an electromagnetic disturbance in a 

 medium, we shall suppose this medium at rest that is to say, not 

 subject to any other motion than that resulting from the dis- 

 turbance itself. 



Equations (n), (13), and (14) of (572), give 



From which is deduced 



4?r cV 

 an equation which may be written in the symbolical form 



The medium being fixed, the differentials of the co-ordinates 

 in respect of time are null. Equations (10) of (571) give then 



from this follows 



(i 2 ) 



