PRESENT PROBLEMS IN RADIOACTIVITY. 9 



plate in a vacuum, but no appreciable charging was observed. The 

 /? rays were temporarily got rid of by heating the radium in order 

 to drive off its emanation. There was found to be a strong ioniza- 

 tion set up at the surface from which the rays emerged and the 

 surface on which they impinged. The presence of this ionization 

 causes the upper plate to rapidly lose a charge communicated to it. 

 Although this action would mask to some extent the effect to be 

 looked for, a measurable difference should have been obtained under the 

 experimental conditions, if the a rays were expelled with a positive 

 charge; but not 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 re- 

 markable 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 to notice, in this connection, that Villard 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 e/m 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 moment 

 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 examination. The 

 question is in a very unsatisfactory state and requires further in- 

 vestigation. 



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. 



* Recent experiments have confirmed this point of view. By the use of 

 a strong magnetic field, to remove the slow moving electrons, the charge carried 

 by the a rays has been detected both by J. J. Thomson and the writer. 



