﻿The a particles expelled from Radium and Actinium. 341) 



intense magnetic and electric fields*. I deduced that the 

 value of e/m — the ratio of the charge on the a, particle to its 

 mass — was about 6 x 10 3 , and that the swiftest a particles 

 emitted from radium had a velocity of about 2'5 x 10 9 cms. 

 per second. Shortly afterwards, these experiments were re- 

 peated by Des Ooudresf, using the photographic method and 

 with pure radium bromide as a source of rays. He found 

 the value of e/m to be 6*3 x 10 3 , and the average velocity to 

 be 1*65 x 10 9 cms. per second. 



On account of the difficulty of obtaining a sufficiently large 

 deflexion of the a rays in passing through an electric field, 

 the values of e/m and of the velocity of the a. particles obtained 

 by Rutherford and Des Coudres could only be considered as 

 a first approximation to the true values. 



Recently the question has again been attacked by Mac- 

 kenzie J, using the photographic method and pure radium 

 bromide as a source of rays. Fairly large deflexions of the 

 pencil of rays were obtained by using strong magnetic and 

 electric fields. He showed that the a. particles emitted by a 

 thick layer of radium bromide were unequally deflected in a 

 magnetic and electric field, and presumably consisted of a 

 particles moving with different velocities. By assuming that 

 the value of e/m was the same for all the a particles, he 

 deduced that the value of e/m for the average ray was 4*6 x 10 3 , 

 and that the average velocity was 1'37 x 10 9 cms. per second. 



It will be seen that all of these investigators have used a 

 thick layer of radium in radioactive equilibrium as a source 

 of rays. We know that the a particles from radium in equi- 

 librium come from four distinct a ray products. The a 

 particles from each of these products have different ranges 

 of ionization in air and different velocities of projection. In 

 addition, the a particles from each single product reach the 

 surface from different depths of radioactive matter, and con- 

 sequently have different velocities. It is thus seen that the 

 a radiation from radium is very complex, and consists of four 

 groups of « particles, each of which is made up of a particles 

 which escape at widely different velocities. 



On account of the dispersion of the pencil of rays in 

 passing through an electric and magnetic field, it is difficult 

 to interpret with certainty the deflexions observed. The 

 difficulties which arise are clearly pointed out by Mackenzie 

 in his paper (loc. cit.) 



In a previous paper (Phil. Mag. July 1905) I pointed out 



* Rutherford, Phys. Zeit. iv. p. 235 (1902) ; Phil. Mag. Feb. 1903. 

 t Des Coudres, Phys. Zeit iv. p. 483 (1903). 

 % Mackenzie, Phil. Mag. Nov. 1905. 



