164 Prof. E. Rutherford on 



some 



intensity. The amount of the magnetic deviation has been 

 measured, but the electric deviation has not yet been deter- 

 mined with the accuracy required. 



During the course of these investigations some facts of 

 interest and importance have been observed, of which a brief 

 account will be given at this stage. 



In the experiments previously made by the writer * and 

 Des Coudresf for determining the velocity and ratio ejm 

 of the a. particle, a thick layer of radium in radioactive 

 equilibrium was used as a source of rays. It was recognized 

 that the a rays from radium were complex, and consisted of 

 a particles projected at different velocities. This fact, coupled 

 with the difficulty of obtaining a large electric deviation of 

 the pencil of rays, prevented the determination of the velocity 

 and ratio ejm of the particle with the accuracy required to 

 settle definitely whether or not it is an atom of helium. 



It has long been known that the a rays emitted from 

 radium and its products differed in their power of penetrating 

 matter. Two valuable papers have been recently published 

 by Bragg and KleemanJ which have considerably extended 

 our knowledge of the mechanism of absorption of the a rays 

 by matter. A brief account will be given of their experi- 

 mental results, as they are of importance in connexion with 

 the work described in this paper. By using a very thin film 

 of radium bromide, in which the absorption of the a rays was 

 negligible, and by using narrow cones of rays, they showed 

 that the rays from radium consisted of four distinct sets, 

 each of which passed through a different but definite distance 

 in air before the rays ceased to ionize the gas. The ioniza- 

 tion produced per cm. of path of the gas by each a particle 

 was approximately the same over the whole range, and was 

 found to end fairly abruptly. Each of these sets of rays of 

 definite range in air was found to correspond to the rays from 

 one of the four a ray products present in radium in radio- 

 active equilibrium. For example, the rays from radium C 

 passed through about 6'7 cms. of air before ionization ceased. 

 The corresponding distance of the rays from radium itself 

 was about 3 cms. These results indicated that each product 

 of radium emitted ol particles of one definite velocity, but that 

 this velocity varied considerably for the rays from the dif- 

 ferent products. As Bragg has pointed out, these results 

 confirm in a novel and interesting way the theory of 



* Phil. Ma^. Feb. 1903. 



t Phijsik. Zeit. iv. p. 483 (1903). 



% Phil. Mag. Dec. 1904. 



