ROTATORY MAQNETIC POLARIZATION. 62! 



Electromagnetic properties, once established in a medium, ex- 

 perience therefore a diffusion analogous to that of heat ; but we 

 must remark that the coefficient of conductivity of the medium k, 

 which would produce the same calorific diffusion, is inversely as c. 

 The diffusion of the electromagnetic effects is then inversely as the 

 electrical conductivity, so that a medium which had a perfect 

 conductivity would offer an absolute obstacle to this diffusion. 



Consider, for instance, the case of a linear conductor surrounded 

 by a conducting medium. The moment the principal current is 

 established, the induced current in the surrounding medium has the 

 same strength, and their action on a distant point is null ; the per- 

 manent state is only established after the induced currents have been 

 nullified by the resistance of the medium. But in the degree in 

 which the induced current is enfeebled, a current in the same 

 direction is produced round it, so that the space occupied in the 

 medium by the induced current increases in proportion as the 

 intensity diminishes. 



If the principal current is kept constant, the induced currents 

 diffuse gradually ; when the permanent regime is set up, the values 

 of AF, AG, and AH are null throughout the medium, and only 

 retain finite values in the portion occupied by the circuit of the 

 current. 



639. ROTATORY MAGNETIC POLARIZATION. The ordinary theory 

 of undulations assumes that luminous phenomena are produced by 

 the vibrations of ether ; but it must be admitted that a formal 

 explanation of this kind is outside the range of experiments. We do 

 not know, in fact, what is the true nature of light. The only thing 

 which may be considered to be proved, is that in a ray of light there 

 is a mechanical effect of the nature of a vector in geometry, that is 

 characterized by a magnitude and a direction; this direction is 

 perpendicular to the ray, and it varies periodically in the same plane 

 when the ray is polarized. This is a conclusion from the phenomena 

 of interference. 



In the case of a circularly polarized ray, the magnitude of this 

 mechanical effect, of this vector, is constant, but its direction turns 

 about the ray, and produces a complete revolution in each period. 

 When such a ray traverses a medium under the action of a magnetic 

 force, its velocity of propagation is modified ; it must be concluded 

 thence that there is, in the medium, some rotatory motion, the 

 axis of which is parallel to the direction of the magnetic forces. 

 This rotation does not apply to any finite portion of the medium 

 taken as a whole, and it must be assumed that it is confined to the 



