( 240 ) 



to i)e fulfilled. It is easy to genemlize also the notion "foix-e" in 

 such a way that this law is satisfied. The only thing that is required 

 for this purpose is that also the time derivative of the electromagnetic 

 momentum is called "force". 



To this the objection has been raised that we must consider the 

 ether to l.)e stagnant, and that il is therefore, meaningless to speak 

 of a force which is exerted upon it. Hut we speak of the momentum 

 of this stagnant ether and 1 do not see why we cannot as well 

 speak of the force which acts on it. Moreover we may a\oid both 

 expressions and attribute tlie momentum not to the ether, but to 

 the electromagnetic cjiergy, and in the same way we may take the 

 force to be exerted on this energy. Thus we also attribute the entropy 

 not to the vacuum or lo the ether, but lo the radiating energy. 



These are after all mere questions of nomenclature. More important 

 is the question, whether the motion of the centre of inertia of an 

 isolated system is really uniform. It is evident that we may assume 

 this to be the case if we conceive the electromagnetic momentum 

 to consist of a mass which is in motion. As an instance we will 

 consider a stationary body with a mass M and a ray of light which 

 is absorbed by it. The ray represents a quantum of momentum which 

 we will denote by iiir, the ra(balion propagating with the velocity c. 

 When the ray is absorbed, the total momentum must remain constant. 

 Now we can make two different assumptions. In the first place that 

 of PoiNCARK M, ^vho assumed that the mass M obtained a velocity /", 

 so that Mr = inc. The uniformity, however, of the centre of inertia 

 required the following rather startling assumption abuut the mass 7/?,: 

 when the radiation is absorbed the mass 1» is stopped, il is howexer 

 not annihilated, Init i)ecümes stationary at that place where the 

 energy has been absorbed. The body which has absorbed the energy 

 however moves in the mean time away from that j)lace. Poincare 

 himself declares that a |)hysical meaning cannot be ascribed to this 

 theory. 



Another possible assumption was pi*oposed by die present \vriter 

 also in 1900 in defending his theses on tlie occasion of his promotion 

 to the degree of doctor. This assiimplion consists in this, that we 

 ima"'ine the mass in to remain in the bodv which has absorbed the 

 energy. This body would llien obtain a velocity ^/, which is determined 

 bv the e(|uation {M -\- m) 1:' :^ inc. Tliis assumpliou involves a hypo- 

 thesis with a very decided physical meaning, namely that the mass 

 of a body depends on its energy. In '1900 however there seemed to 



1) H. P01NGARÉ. Livre Jubilaire dedii'' a H. A. Lorentz p. 252 Anno 1900. 



