296 M. L. Lorenz on the Identity of the 



be considered approximately correct, especially as a perfectly 

 constant conductivity is presupposed, a homogeneity which does 

 not in fact exist. The chief result, however, that all good con* 

 ductors of electricity absorb light to a great extent , is in marked 

 accordance with experiment. 



When the electrical conductivity is very small, equations (7) 

 give 



A=8tt-. 

 a 



Now in the case of copper, whose conductivity has been given 



Tc 

 above, - has been found equal to 283433 ; but for all transpa- 

 rent bodies the conductivity is millions of times as small as that 

 of copper ; and liquids form a marked exception, where the che- 

 mical activity and the mobility of the particles exert so great an 

 influence on the determination of the conductivity in the proper 

 sense that it becomes in fact impossible ; we thus find that the 

 conductivity of all other transparent media is so many millions of 

 times as small as that of the metals that the coefficient of ab- 

 sorption A, as well as the last member of equations (B), will dis- 

 appear, by which the latter become quite identical with the 

 equations of light. Just as we can infer their opacity from the 

 good conductivity of the metals, so from the very small trans- 

 parency of a body we may conclude that, as compared with metals, 

 it is an extremely bad conductor of the electrical current, a result 

 which experiment has also fully confirmed. 



The vibrations arising from equations (B) are transversal ; and 

 even if the member containing II be retained, longitudinal vibra- 

 tions will not be possible. By differentiating the three equations 

 (8) in reference to x, y> and z, and addition, we obtain 



if 



at 



du dv dw _„ 

 dx dy dz 



It is manfest from this that 6 cannot be a periodical function 

 of the time, from which it follows that longitudinal vibrations 

 cannot take place. As, moreover, this equation shows that the 

 value of 6 diminishes as the time increases, and is independent 

 of the components of all surrounding points, we are compelled 

 to assume generally that = 0; from which it follows, since 



, lde 



that in the interior of a body with constant conducting-power no 



