1896.] and Rontgens X Rays. 55 



We see from equation (2) that if we neglect the square of the 

 ratio of the velocity of the atoms to that of light we have 



Z = F = Z 



I m n' 



so that to this approximation the direction of propagation of the 

 wave coincides with the direction of the electric intensity ; in this 

 case there is no magnetic force propagated with the wave. 



An interesting case of these convective waves is afforded by 

 the problem of a column of air moving like a wind and carrying 

 along with it both positively and negatively electrified atoms. 

 Some experiments by Hertz (Wied. Ann. 19, p. 78, 1883) seem to 

 indicate that something of this nature exists in the neighbourhood 

 of the positive electrode. 



Let us consider the simple case when the velocity (u) of the 

 column of gas as well as the electric intensity X is parallel to the 

 axis of x. 



In this case the dielectric current is equal to 



if dX 



4nr dt 



The convection current is equal to vp where p is the density of 

 the electrification ; we have however 



^ P -^(KX) = 0. 



So that the total current, i.e. the sum of the dielectric and 

 convection currents is equal to 



K {dX_ u dX 

 4<ir \ at dx 



if K is constant. The total current however in this case vanishes, 

 so that 



dx , dX 



w + "s =0 (1) - 



Now let us consider the case when u depends to some extent 

 on the intensity of the electric field, let for example 



u =s p + olX, 



where p is independent of X. 



Then equation (1) becomes 



dX . f r . dX _ 



