PRESENT PROBLEMS OF RADIOACTIVITY 161 



the slightest evidence of a charge was observed. I understand that 

 similar negative results have been obtained by other observers. 



This apparent absence of charge carried by the a rays is very 

 remarkable and difficult to account for. There is no doubt that the 

 a particles behave as if they carried a positive charge, for several ob- 

 servers have shown that the a rays are deflected by a magnetic field. 

 It is interesting, in this connection, that Wien was unable to detect 

 that the "canal rays" carried a charge. These rays, discovered by 

 Goldstein, are analogous in many respects to the a rays. They are 

 slightly deflected by a magnetic and electric field, and behave like 

 positively charged bodies atomic in size. The value of ^ is not a 

 constant, but depends upon the nature of the gas in the tube through 

 which the discharge is passed. The apparent absence of charge on 

 the a particles may possibly be explained on the supposition that 

 a negatively charged particle (an electron) is always projected at the 

 same time as the positively charged particle. Such electrons if they 

 are present should be readily bent back to the surface from which 

 they came by the action of a strong magnetic field. It will be of in- 

 terest to examine whether the charge carried by the a rays can be 

 detected under such conditions. Another hypothesis, which has some 

 points in its favor, is that the a particles are uncharged at the mo- 

 ment of their expulsion, but, in consequence of their collision with 

 the molecules of matter, lose a negative electron, and consequently 

 acquire a positive charge. This point is at present under examin- 

 ation. The question is in a very unsatisfactory state, and requires 

 further investigation. 



It is remarkable that positive electricity is always associated with 

 matter atomic in size, for no evidence has been obtained of the exist- 

 ence of a positive electron corresponding to the negative electron. 

 This difference between positive and negative electricity is apparently 

 fundamental, and no explanation of it has, as yet, been forthcoming. 



The evidence that the a particles are atomic in size mainly rests on 

 the deflection of the path of the rays in a strong magnetic and electric 

 field. It has, however, been suggested by H. A. Wilson that the a 

 particle may in reality be a "positive" electron, whose magnitude 

 is minute compared with that of the negative. The electric mass of 

 an electron for slow speeds is equal to ^ . Since there is every reason 

 to believe that the charge carried by the a particle and the electron 

 is the same, in order that the mass of the positive electron should 

 be about 2000 times that of the negative, it would be necessary to 

 suppose that the radius of the sphere over which the charge is dis- 

 tributed is only ^ of that of the electron, i. e., about 10~ 18 cms. 

 The magnetic and electric deflection would be equally well explained 

 on this view. This hypothesis, while interesting, is too far-reaching 



