620 GENERAL THEORIES. 



and the coefficients ought to satisfy the conditions 



This expression represents a wave which is propagated parallel to 



the z axis, with a velocity V equal to -, and the amplitude of which 



? 

 rapidly diminishes. 



The value of the coefficient of absorption p is 2wrV. The 

 absorption of light ought therefore to increase with the electrical 

 conductivity. Experiment does, in fact, show that most transparent 

 substances are dielectrics, and that all good conductors are very 

 opaque. 



This ratio, however, is not absolute, for certain metateare trans- 

 parent in very slight thicknesses, and many dielectrics are opaque. 

 We ought to exclude electrolytes, which are almost all transparent, 

 for the decomposition which accompanies the passage of electricity 

 completely changes the nature of the phenomenon, and we are no 

 longer dealing with a mere effect of conductivity. 



638. CONDUCTING BODIES. Let us consider, finally, an isotropic 

 conducting medium, or at any rate a medium in which the phe- 

 nomena of conduction predominate over those of electrical displace- 

 ment. If we neglect then the terms containing the factor K, 

 in equations (15) as well as the functions and ^, we get 



^F 

 W = AF, 



<)G 

 (29) 4,1* = AG. 



Each of these equations has the same form as that which gives 

 the diffusion of heat in Fourier's theory. 



For, if k is the coefficient of thermal conductivity of an isotropic 

 medium (70), and if the function F be considered as giving the 

 temperature at each point, the expression AF represents the flow 



<)F 

 of heat which in unit time penetrates unit volume ; is the 



corresponding rise of temperature, so that the coefficient qirck is 

 the calorific capacity for unit volume. 



