and the emission of slowly moving cathode rays, etc. 53 



electroscope is negative, may be due to this cause. A series of 

 measurements were made varying the distance between the po- 

 lonium and the electroscope, and it was found that as the 

 distance increased the leak increased, when the charge in the 

 electroscope was positive, and diminished when it was negative. 



Experiments with Radium. 



Similar experiments were made using radium instead of polo- 

 nium, the radium used was one of the earlier samples procured 

 from de Haen and was not very strong. The radium was spread 

 over a horizontal disc placed about 2 millimetres below a similar 

 disc fastened to the electroscope, the results obtained with this 

 were the same as with the polonium. Thus when the electroscope 

 was charged positively the leak was 39 in 10 minutes with the 

 magnet off, 7 with the magnet on, while when the electroscope 

 was charged negatively the leak was 11 with the magnet off, 10 

 with the magnet on. Thus we have again the positive leak larger 

 than the negative when the magnet is off, and smaller when it 

 is on. The negative leak diminishes as the distance of the radium 

 from the electroscope increases, and when this distance is a few 

 centimetres the negative leak is very small compared with the 

 positive. 



The preceding experiments show that polonium and radium 

 emit copious streams of slowly moving negative corpuscles, these 

 streams only differ in the velocity of the particles from the /3 rays 

 hitherto investigated, and it is to be observed that they would 

 escape detection by the methods hitherto used to investigate /3 rays, 

 for such methods only detect ft rays able to traverse a considerable 

 thickness of aluminium foil, i.e. /3 rays of great velocity, in fact the 

 ability to penetrate this foil has been taken as the definition of 

 a /3 ray. We see too that if we define an a ray as one absorbed 

 by a thin layer of aluminium foil, that a rays may be of two kinds, 

 one carrying positive, the other negative charges. The existence of 

 these slow negative rays may have a very important bearing on the 

 transformations which radio-active matter undergoes, it may also 

 be the explanation why the radium emanation which does not 

 emit the fast /3 rays, yet seems to acquire a positive charge and 

 be attracted by negatively electrified bodies ; this would be the 

 case if the emanation like polonium gave out the slow /3 rays. 

 Investigations are in progress at the Cavendish Laboratory to see 

 whether the emission of these slow cathode rays is a general 

 property of radio-active substances. 



Another point brought out by these experiments is the way in 

 which these slow /3 rays tend to neutralize the positive charge on 

 the a rays. This tendency for the a. rays to lose their charge has 



