348 POPULAR SCIENCE MONTHLY 



It might be supposed also that the movements of matter proper are 

 exactly compensated by those of the ether; but that would lead us to 

 the same reflections as before now. The principle so understood will 

 explain everything, since, whatever might be the visible movements, we 

 always could imagine hypothetical movements Avhich compensate them. 

 But if it is able to explain everything, this is because it does not enable 

 us to foresee anything; it does not enable us to decide between the 

 different possible hypotheses, since it explains everything beforehand. 

 It therefore becomes useless. 



And then the suppositions that it would be necessary to make on 

 the movements of the ether are not very satisfactory. If the electric 

 charges double, it would be natural to imagine that the velocities of 

 the diverse atoms of ether double also, and for the compensation, it 

 would be necessary that the mean velocity of the ether quadruple. 



This is why I have long thought that these consequences of theory, 

 contrary to Newton's principle, would end some day by being aban- 

 doned, and yet the recent experiments on the movements of the elec- 

 trons issuing from radium seem rather to confirm them. 



Lavoisier's Principle. — I arrive at the principle of Lavoisier on the 

 conservation of mass. Certainly, this is one not to be touched without 

 unsettling all mechanics. And now certain persons think that it seems 

 true to us only because in mechanics merely moderate velocities are 

 considered, but that it would cease to be true for bodies animated by 

 velocities comparable to that of light. Now these velocities, it is be- 

 lieved at present, have been realized; the cathode rays or those of 

 radium may be formed of very minute particles or of electrons which 

 are displaced with velocities smaller no doubt than that of light, but 

 which might be its one tenth or one third. 



These rays can be deflected, whether by an electric field, or by a 

 magnetic field, and we are able, by comparing these deflections, to 

 measure at the same time the velocity of the electrons and their mass 

 (or rather the relation of their mass to their charge). But when it 

 was seen that these velocities approached that of light, it was decided 

 that a correction was necessary. These molecules, being electrified. 

 can not be displaced without 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 molecule and the electrodynamic 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 elec- 

 trons, is of exclusively electrodynamic origin. This is what forces us 



