( 242 ) 



thai -2" Trir» = ^. The iriomentiim has then also the right amount 



— ^. In this paragraph I will assume for simplicity that at a given 



(.•„ 



point all electromagnetic energy has the same velocity. 



It is important to |toint out that c is the maximum value which 

 can be assumed by n>. This ^'alue is reached if ^" and p are equal 

 and perpendicular to one another. In all other cases iv> is smaller 

 than c. 



Finally I point out that the essential |)roperty of mass according 

 to our considerations is that //it» = momentum. That moving mass 

 involves a quantity of kinetic energy is not to be considered as 

 essential ^). It is not even genei-ally true. When a body radiates 

 energy in all directions, the energy (and therefore a pai-t of the 

 mass) which was originally in rest, is set in motion. This motion, 

 however, is not connected with any kinetic energy, for the energy 

 is constant. The fact that the motion of a body is accompanied with 

 kinetic energy must therefore be considered as a secondary pheno- 

 menon. The energy is then not only set in motion, but also augmented. 

 So the energy of an electrically charged conductor is augmented 

 with an amount of magnetic energy when this conductor is moved, 

 and something of the same kind nuisl haj)pen in other cases where 

 kinetic energy occurs^). 



We will now put the question : what are the forces which are 

 exerted on the electromagnetic energy? We will start from the well 

 known equation : 



Here L V is an element of volume, o the density of the material 



Ö 

 mass, i> its velocity and w the Poynting vector. The symbol y 



indicates a partial ditferentiation with respect to the time with 

 constant value of the cooi'dinates, and 



Finally represents 



1) This has alreudy been remarked by Laue. 



2) Far from denying the existence of tb ■ etlier, I should be inclined to account 

 for all the inertia by the action of the medium. Gf. Pbys. Zeitschr. p. 600. 1011. 



3) In accordance with Laue a pressure is represented by a positive, a tension 

 by a negative value of p. 



