VELOCITY OF THE-, PROPAGATION OF LIGHT. 615 



Suppose that the plane wave in question is polarized, and that 

 the electrical disturbance is directed along the x axis. We shall 

 then have X = Z = 0; z/ = a/ = 0; ^=^=0; Q = R = 0; G = H = 0. 



The electrical energy for unit volume is expressed by 



2 STT 8a-\V 



and the electromagnetic energy 



These two expressions are equal, for if we multiply the two 

 members of the first of the equations (21) by the equal factors 



<>F , 3F is 



and - , and integrate with respect to /, we get 



The total energy of the medium in which the waves are propa- 

 gated is therefore half in the form of electrostatic energy, and 

 half in the form of electromagnetic energy. 



Let / denote each of these energies for unit volume. In virtue 



of its electrical state (104) the medium is subject to a tension - 



parallel to the x axis, and a pressure of the same value parallel 

 to the axis of x and z. In virtue of its electromagnetic condition, 

 the medium is subject to the same actions, except that the x axis 

 must be replaced by that of y t and conversely. These actions 

 destroy themselves in the plane of the wave, and there remains a 

 pressure p parallel to this plane equal to half the total energy 

 for unit volume. 



A ray of light produces therefore in the medium a pressure 

 parallel to the direction of the motion, and would exert a repulsion 

 on a plate of metal which it encountered. It is possible that this 

 effect may have some part in the motion of the radiometer. 



633. VELOCITY OF THE PROPAGATION OF LIGHT. The true 

 control of this theory is, then, that in all media the velocity of 



