Prof. Thomson, Some experiments on Ganal-Strahlen. 213 



velocity is very small, is shown by the fact that they are 

 completely stopped when the metal plate is charged to a small 

 positive potential. 



Secondary ionization is produced in the path of the canal- 

 strahlen. If the metal plate is charged positively, the boundary 

 of the canal-strahlen becomes fluffy and bends towards the 



Fig. 3. 



Fig. 1. 



Fig. 2. 



positive plate, as in Fig. 3, showing that negative corpuscles 

 are present ; the fluffy boundary is very susceptible to magnetic 

 force. These negative ions come out of the beam of the 

 canal-strahlen, which thus ionize the gas. The a particles 

 emitted by radium may be regarded as canal-strahlen of very 

 great velocity, enormously greater than the velocity of those 

 produced in a discharge tube. Now Rutherford has shown that 

 when the a particles, by passing through a gas or thin sheet of 

 metal, have lost about 18 °/ of their original velocity, they 

 suddenly lose their power of ionization, yet this reduced velocity is 



VOL. XIII. PT. IV. 



15 



