6l2 GENERAL THEORIES. 



When the medium is not a conductor, ^=0, and the value of A^, 

 which is proportional to the density of the free electricity, is 



c) 2 

 independent of the time. There remains then = ; that is to 



say that is a linear function of the time, or a constant. These 

 two functions 6 and ^ play no part therefore in the phenomena 

 due to periodical disturbances. 



629. PROPAGATION OF UNDULATIONS IN A DIELECTRIC. In 

 the case of a dielectric, equations (15) may be reduced to 



_- . 



c) 2 G 

 (17) K/i -AG = 0, 



<) 2 H 

 K, -AH = 0. 



These equations define the manner in which the functions F, G, 

 and H vary with the time, and therefore the propagation of electro- 

 magnetic disturbances ; they are of the same form as that of vibratory 

 motion in a solid elastic body. 



The velocity V of the propagation of a disturbance is given by 

 the expression 



(18) V 



630, PLANE WAVES. Suppose, in fact, that at an instant the 

 electromagnetic disturbances form a plane wave perpendicular to 

 the axis of z. The medium will be traversed by plane waves parallel 

 to the first, and all the quantities, whose variations determine these 

 waves, are simply functions of z and of /, independent of x and 

 y. Equations (2) of (567) become then 



