﻿the ol particles expelled from Radium and Actinium. 361 



It was found by experiment that the measured value of D 

 — the distance between the extreme edges ot the trace — was 

 always underestimated for a very feeble photographic trace. 

 An underestimate of the value of D gives too large a value 



Ft o 



for e/m. Taking this factor into consideration, the values of 

 ejm obtained for the « particles from radium A and from 

 radium C agree within the limit of experimental error. 

 This shows that the a. particles expelled from radium A and C 

 have the same mass and differ only in their initial velocities of 

 projection. 



Mass of the a particle from Radium F. 



A bismuth rod coated with radiotellurium was used as a 

 source of a rays. It is now definitely established that the 

 active constituent in both radiotellurium and polonium is 

 the same and consists of the transformation product of 

 radium, radium F. The active matter is deposited in the 

 form of a thin film on the bismuth rod. and the <x particles 

 all escape from the surface at practically the same velocity. 

 A piece of the rod was placed in position inside the electro- 

 static apparatus, and the photographic plate exposed for 

 four days to the action of the a rays from the bare rod. The 

 value / 3 = lO'OO cms, the voltage = -143, and the observed 

 value D = 2' 72 mm. 



This gives a value = 5'63x 10 u for the a. rays from 



the unscreened source. The value of — = 3*2xl0 5 was 



e 



deduced by me in a previous paper from measurement of 



the range of the « particles from radium F in air. The 



experimental value found directly by Mackenzie (loc. cit.) 



was 3'3xl0 5 . Taking the mean value — =-3*25 x 10 5 , we 



e 



find that e/m = 5'3x 10 3 , and ?z = l , 73 X 10 9 cms. per second. 

 The actual photograph obtained was very weak inintensitv. 

 and, for the reasons previously mentioned, there is no doubt 

 that the value e/m obtained is too large, We may con- 

 sequently conclude that the a particle from radium F has 

 the same mass as that expelled from radium C. Using a 

 more active rod or a longer time of exposure, the value of 

 e/m should be obtained with much greater precision ; but 

 there can be no doubt that it would be found identical with 

 that observed for the a. particle from radium C. 



