( 241 ) 



l)c lilde reason (o doiil)! the eoiislaiicv of the iiuiss of the Ixtdies. 

 1 iherctore telt üblit^cd to reject this hv[)otiiesis aiul with it the law 

 of the iiiiitoniiity of the iiiotioii of the centre of inertia, and the law 

 action = reaction. 



The electron theory, however, has since that time given rise to 

 donbts as to the constancy of the mass of the bodies, and Kinstein ') 

 has moreover shown that I.okknt/;s theory of relativity reqnires in 

 some cases that we ascribe to the bodies a mass varying with tjjeir 

 energy. It seemed therefore desirable to return to the idea rejected 

 in 1900 and to ascribe a mass to the energy, and that as well with 

 PoiNCARÉ for the case that the energy moves in the electromagnetic 

 tield') as with Einstkin for the case that it occurs in ponderable 

 bodies. Laue') has given a general theory for this latter case. Although 

 my results partially coincide with those of Laue the following 

 considerations will perhaps not be superfluous. 



§ 2. In the tirst [>lace we may deduce from the formulae: 



The current of energy = 5 



1 

 The momentum per iniit volume = — to 



1 

 that the mass of a (piantily of energy f is equal to g. The velocity 



of this mass in the electromagnetic field may be assumed to be 



m = -- . ]F= ^ ((£- -F Jp") representing the density of the energy. I 



say it tnnn be assumed to have that value, for we may also introduce 

 another supposition namely that at a point several [)arts of the energy 

 have different velocities. And in connection with § 5 this assumption 

 has some advantages. 



So if a ray of light moves in an electrostatic field, the abo\e xaliie 

 of IV is differeiit from c. We may however also assume that the 

 light energy moves with the velocity c, and liie other (piantilies of 

 energy move with different velocities or are stationary. It is however 

 immaterial which supposition is introdiu-ed, if only care be taken 



1) A. Einstein. Ann. d. I'hys XVttl p. 639, 19Ü5, and XXIll p. 871, 1007. 

 Compare also G. Nordstrom and M. Abraham I^liys. Zeitsclir. X and XI Anno 

 (1909 and 1910) and H. A. Lorentz. Versl. Kon. Alcad. Amsl. Juni 1911, p. 87. 

 (Still to be published in lliese Proceedings), 



-) H. PoiNCARÉ I.e. Compare also A. Einstkin. Ann. d. Phys. XX p. 627.1906 

 and M. Planck. Ann. d. Phys. XXVI p. 1. 1908 and Phys. Zeitsdir. iX p. 828. 

 1908. 



S) M. Laue. ""Das Rolativitatsprinzi])." Vif.wec nnd SoiiN. braunsc liwoip; 1!)11. 

 Also Ann. d. Pliys. XXXV p. 524. 1911. 



17* 



