﻿the a particles expelled from Radium and Actinium. 363 



was then directly compared with that due to the rays from 

 radium C under the same conditions. For this purpose, the 

 copper plate was removed and made active by exposure to 

 the radium emanation. It was then placed back in its 

 original position, and another photograph taken. The mica 

 plate, covering the opening in the base of the magnetic 

 apparatus, was of the same thickness as that used in the base 

 of the electrostatic apparatus. The following numbers illus- 

 trate the results obtained : — 



Distance between centres of deflected bands due to rays 



from actinium B = 1'85 mm. 



Distance between centres o£ deflected bands due to rays 



from radium C=1'53 mm. 



We have previously shown that the value of — for the 



a particles of radium C after passing through the standard 

 mica screen, absorbing-power equal to 3*5 cms. of air, is 



o"10x 10 5 . Consequently the value of — for the a particles 



from actinium B is given by 



25 = ^ x 3-10 x 10 5 = 2-50 x 10 5 . 

 e 185 



It is interesting to note that the comparative magnetic 

 deflexions observed for the rays of actinium B and of 

 radium C agree with those to be expected from their known 

 ranges in air, assuming the value of e/m for the a particle to 

 be the same in both cases. 



I have shown in a previous paper that the velocity T of 

 an a particle of range r cms. in air is given by 



~ = -348 A /r+l-25. 

 ' o 



where Y is the maximum velocity of the rays from radium C 

 which have a range of 7*06 cms. Xow, after passing through 

 the mica screen, the rays from actinium B have a range 

 5'5 — 3'5 = 2*0 cms., while those from radium C have a range 

 7*06 — 3*5 = 3-56 cms. 

 Consequently, 



velocity of rays from radium C after passage through mica_ 

 velocity of rays from actinium B after passage through mica 



/3-56 + 1-25 ., ™ 



= '\ / = 1'22 



V 2-0 + 1-25 



