PRESENT PROBLEMS OF RADIOACTIVITY 159 



mann was 95 per cent of the velocity of light. The power of electrons 

 of penetrating solid matter increases rapidly with the velocity, and 

 some of those expelled from radium are able to penetrate through 

 more than 3 mms. of lead. It is probable that a few of the electrons 

 from radium move with a velocity still greater than the highest value 

 observed by Kaufmann, and it is important to determine the value 

 of ^ and the velocity of such electrons. According to the mathemat- 

 ical theory, the mass of the electron increases rapidly as the speed of 

 light is approached, and should be infinitely great when the velocity 

 of light is reached. This leads to the conclusion that no charged body 

 can be made to move with a velocity greater than that of light. This 

 result is of great importance, and requires further experimental veri- 

 fication. A close study of the high speed electrons from radium may 

 throw further light on this question. 



Only a brief and imperfect statement of our knowledge of electrons 

 has been given in this paper. A more complete and detailed account 

 of both the theory and experiment will be given by my colleague, 

 Dr. Langevin. 



III. The a rays 



The /? rays are readily deflected by a magnetic field, but a very 

 intense magnetic field is required to deflect appreciably the a rays. 

 The writer showed by the electric method that the a rays of radium 

 were deflected both by a magnetic and electric field, and deduced 

 the velocity of projection of the particles and the ratio ^ of the 

 charge to the mass. The direction of deflection of the a rays is opposite 

 in sense to the /? rays. Since the /? rays carry a negative charge, the a 

 particles thus behave as if they carried a positive charge. The mag- 

 netic deflection of these rays was confirmed by Becquerel and Des 

 Coudres, using the photographic method, while the latter, in addition, 

 showed their deflection in an electric field and deduced the value of 

 the velocity and ^. The values obtained by Rutherford and Des 

 Coudres were in very good agreement, considering the difficulty of 

 obtaining a measurable deviation. 



/> 



Observer Value of Velocity Value of - 



m 



Rutherford 2.5 X 10 9 cms. per sec. 6 X 10 3 electromagnetic units 



Des Coudres 1.6 X 10 cms. per sec. 6 X 10 3 electromagnetic units 



Since the value of - for the hydrogen atom is 10 4 , on the assump- 



ffv 



tion that the a particle carries the same charge as the hydrogen atom, 

 this result shows that the apparent mass of the a particle is about 

 twice that of the hydrogen atom. If the a particle consists of any 

 known kind of matter, this result indicates that it is either the atom 



