778 Prof. J. H. Jeans on the 



If c is the conductivity for currents of frequency p, this 

 expression must be equal to \ cX 2 , and hence we must have 





fffm 2 (10) 



1 + NV 



8. For the same quantity c, Sir J. J. Thomson, by a different 

 method, obtains 



-PIP) 2 - od 



where t is the time of description of a free-path, and is 

 connected with k by the relation 



2 m 



If we eliminate t between this and (11) we get as Thomson's 

 value for c, 



fcpm , 



Sm lsV 



C = K 



The divergence between this and formula (10) can, I think, 

 be traced to the fact that Thomson's system is, so to speak, 

 kinematical and not dynamical. In a dynamical system the 

 time of a free-path must depend on the velocity with which 

 the path is described. 



Propagation of Light. 



9. Let X be the intensity parallel to 0#, measured in 

 electromagnetic units, and i x the corresponding component of 

 convection-current, connected with X by equation (8). The 

 total current parallel to Ox is 



J^^+i (12) 



so that this is equal to ^— ( ^ — ^/- ) . 



For waves of frequency p, we can take both X and i 

 proportional to e tpt , so that from equation (8), 



• _ 1 ^x 



