286 Dr. Norman Campbell on the 



and of about equal intensity. It appears certain that the- 

 groups of ft rays observed with the two latter substances are 

 due to the conversion into ft rays of some of the more pene- 

 trating 7 rays from radium B. It is well known that the 

 greater part of the 7 radiation from this substance is rapidly 

 absorbed in heavy elements like lead and gold, and no doubt 

 the greater intensity of the groups of ft rays from these 

 substances is a result of the rapid absorption of the 7 rays 

 and their conversion into ft rays, since the absorption of 

 7 rays of this type increases with atomic weight more 

 rapidly than the absorption of ft rays. In consequence, the 

 ft rays are more intense from heavy than from light 

 elements. 



The lines obtained with a silver screen were compared 

 with those obtained with a lead screen for the same fleld r 

 and no certain difference in the velocity of the three strong 

 groups of rays could be detected. 



On the other hand, the velocities of a number of the 

 ft rays excited in gold were distinctly higher than the corre- 

 sponding velocities of the /3rays excited in lead, the difference 

 being of the order of two per cent. In view of the method 

 of comparison it seems scarcely likely that the difference is 

 due to experimental error. 



Experiments on this subject will be continued by Robinson 

 and Rawlinson. It is intended to continue the investigation 

 into the region of the groups of swift ft rays of radium C 

 and to examine the velocities of the ft rays excited in a 

 number of different metals. Preliminary photographs have 

 shown that the ft rays excited by the very penetrating 7 rays- 

 from radium C are difficult to observe, as would be anticipated 

 from the considerations already advanced. 



University of Manchester, 

 June 29, 1914. 



XXXV. The Ionization of Platinum by Cathode Rays. 

 By Norman Campbell, Sc.D* 



1. rTIHE researches of many investigators f have shown 

 JL that if slow cathode rays fall on a metal plate 

 electrons leave the plate, the numbers and speed of which 

 depend on the speed of the incident rays. As the speed of 

 the rays is increased from zero, the number of electrons 

 leaving the plate increases to a maximum at about 5 volts- 



* Communicated by the Author. 



f For a summary of the literature see Phil. Mag. June 1913, p. 803. 



