and 



1896.] and Rontgens X Rays. 53 



The convection current has for components 



pp, qp, rp. 

 Hence 



But 4 TO = ^-^, 



ay dz 



dy_dZ_dY d/3_dX_dZ 

 dt dy dz dt dz dx ' 



Hence we have 



d(dfK v \ t I 1 f_ v d fdX dY dZ\) „, 



with similar equations for F and Z. 



By differentiating the equation for X with respect to x, that 

 for y with respect to y, that for Z with respect to z and adding, we 

 get 



d\<k +p dp + Q dp + r d e\ =0 



dt (dt dx * dy <fej 



This equation merely represents that the changes in the 

 volume of electrification at any point is due to the motion of the 

 charged atoms into or away from the small volume surrounding 

 the point. 



To consider the case of a plane wave put 



2-7T 



X = A sin — - (tot — (Ix -f my + nz)), 



A. 

 a 



Y= B sin — (tot — (Ix + my + nz)), 



A. 



2tt 

 Z = G sin — (tot — (Ix + my + nz)). 

 A, 



And substitute in equations 1, we get 



A {^'fJl) = (M + mB + UC) ("* " uZR) ' 



B ( w2 ~k) = {lA + mB + nG) (** " Pr) ' 



