MECHANICS AND RADIUM. 205 



that this fictitious mass varies with the velocity (when 

 this is comparable with the velocity of light), and that 

 the force of the inertia of self-induction is not the 

 same when the velocity of the projectile is increased 

 or diminished, as when its direction is changed, and 

 accordingly the same holds good of the apparent total 

 force of inertia. 



The total apparent mass is therefore not the same 

 when the actual force applied to the corpuscle is 

 parallel with its velocity and tends to accelerate its 

 movement, as when it is perpendicular to the velocity 

 and tends to alter its direction. Accordingly we must 

 distinguish between the total longitudinal mass and the 

 total transversal mass, and, moreover, these two total 

 masses depend upon the velocity. Such are the 

 results of Abraham's theoretical work. 



In the measurements spoken of in the last section, 

 what was it that was determined by measuring 

 the two deviations ? The velocity on the one hand, 

 and on the other the proportion of the charge to the 

 total transversal mass. Under these conditions, how 

 are we to determine what are the proportions, in this 

 total mass, of the actual mass and of the fictitious 

 electro-magnetic mass? If we had only the cathode 

 rays properly so called, we could not dream of doing 

 so, but fortunately we have the rays of radium, whose 

 velocity, as we have seen, is considerably higher. 

 These rays are not all identical, and do not behave 

 in the same way under the action of an electric and a 

 magnetic field. We find that the electric deviation 

 is a function of the magnetic deviation, and by re- 

 ceiving upon a sensitive plate rays of radium that 

 have been subjected to the action of the two fields. 



