PRINCIPLES OF MATHEMATICAL PHYSICS 615 



These molecules, being electrified, could not be displaced with- 

 out agitating the ether; to put them in motion it is necessary to 

 overcome a double inertia, that of the molecule itself and that of the 

 ether. The total or apparent mass that one measures is composed, 

 therefore, of two parts: the real or mechanical mass of the mole- 

 cule and the electro-dynamic mass representing the inertia of the 

 ether. 



The calculations of Abraham and the experiments of Kaufmann 

 have then shown that the mechanical mass, properly so called, is 

 null, and that the mass of the electrons, or, at least, of the negative 

 electrons, is of exclusively electro-dynamic origin. This forces us to 

 change the definition of mass; we cannot any longer distinguish 

 mechanical mass and electro-dynamic mass, since then the first would 

 vanish; there is no mass other than electro-dynamic inertia. But 

 in this case the mass can no longer be constant, it augments with the 

 velocity, and it even depends on the direction, and a body animated 

 by a notable velocity will not oppose the same inertia to the forces 

 which tend to deflect it from its route, as to those which tend to 

 accelerate or to retard its progress. 



There is still a resource; the ultimate elements of bodies are 

 electrons, some charged negatively, the others charged positively. 

 The negative electrons have no mass, this is understood; but the 

 positive electrons, from the little we know of them, seem much 

 greater. Perhaps they have, besides their electro-dynamic mass, 

 a true mechanical mass. The veritable mass of a body would, then, 

 be the sum of the mechanical masses of its positive electrons, the 

 negative electrons not counting; mass so defined could still be con- 

 stant. 



Alas, this resource also evades us. Recall what we have said 

 of the principle of relativity and of the efforts made to save it. And 

 it is not merely a principle which it is a question of saving, such 

 are the indubitable results of the experiments of Michelson. 



Lorentz has been obliged to suppose that all the forces, what- 

 ever be their origin, were affected with a coefficient in a medium 

 animated by a uniform translation; this is not sufficient; it is still 

 necessary, says he, that the masses of all the particles be influenced 

 by a translation to the same degree as the electro-magnetic masses 

 of the electrons. 



So the mechanical masses will vary in accordance with the same 

 laws as the electro-dynamic masses; they cannot, therefore, be con- 

 stant. 



Need I point out that the fall of the principle of Lavoisier in- 

 volves that of the principle of Newton? This latter signifies that 

 the centre of gravity of an isolated system moves in a straight line; 

 but if there is no longer a constant mass, there is no longer a centre 



