﻿Mass of the a. particles from Thorium. 371 



The observed heating effects o£ the emanation and radium A 

 are given together, as it is very difficult experimentally to 

 determine their separate effects. It will be seen that there 

 is a substantial agreement between the calculated and observed 

 values. 



Connexion between the Velocity and Amount of Ionization 

 'produced by the a particle. 



Bragg (Phil. Mag. Nov. 1905) has shown that the ioniza- 

 tion produced by a single a particle increases with the distance 

 from the source to nearly the end of its range, when the 

 ionization falls off very abruptly. He has shown that the 

 ionization produced by the a particle at a distance r cms. 

 from the end of the path is inversely proportional to y/r 4- c, 

 where c is a constant equal to 1'33. In a previous paper 

 (Phil. Mag. Aug. 1906) I have shown that the velocity of an 

 u particle at a distance r cms. from the end of its range is 

 proportional to \/r + d, where d is a constant equal to 1*25. 

 The close agreement between these two expressions shows that 

 the ionization produced per unit path by the a particle is 

 inversely proportional to its velocity. This is in agreement 

 with the theoretical views of Bragg, who supposed that the 

 rate of expenditure of energy of the a particle in ionization at 

 any point is inversely proportional to the energy of motion 

 which it possesses. 



A comparison of the velocities of the a. particles expelled 

 from the various products of the radio-elements, and a dis- 

 cussion of the connexion that exists between the velocity of 

 expulsion of the a particle and the character of the trans- 

 formation will be given in a later paper. 



I desire to express my thanks to Dr. Halm and Dr. Levin 

 for their assistance in the measurement of the numerous 

 photographs obtained in this investigation. 

 Berkeley, California, July 20, 1906. 



XLII. Mass of the a particles from Thorium. By E. 

 Rutherford, F.R.S., Macdonald Professor of Physics, 

 McGill University, Montreal, and 0. Hahn, Ph.D.* 

 [Plate V. tigs. 1-3.] 



THE present investigation, which has involved the 

 determination by the photographic method of the 

 deflexion of the oc rays from the active deposit of thorium in 

 a magnetic and electric field, has been rendered possible by 



* Communicated by the Authors. 

 2B2 



