■Section III., 1905. [ S ] Trans. R. S. C. 



I. — Some Properties of the oc Rays from Radium. 



By E. Rutherford, F.E.S., Macdonald Professor of Physics, McGill 

 University, Montreal. 



(Kead May 24, 1905.) 



The proriont paper contains the preliminary results of an investi- 

 gation made for the purpose of accurately determining the velocity and 



ratio - of the a particle expelled from radium. The usual method 

 has been adopted of measuring the deflection of a pencil of homogeneous 

 rays when passed through a magnetic and electric field of known 

 intensity. The amount of the magnetic deviation has been measured, 

 Imt the electric deviation has not yet been determined 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 



Coudras,^ for determining the velocity and ratio - of the a particle, 



a thick layer of radium in radioactive equilibrium was used as a source 

 OÎ rays. It was recognized that the oc rays from radium were complex, 

 and consisted of particles projected at different velocities. This fact, 

 xîoupled with the difficulty of obtaining a large electric deviation of the 



g 

 pencil of rays, prevented the determination of the velocity and ratio — 



of the o! particle with the accuracy required to settle definitely whether 

 or not it is an atom of helium. 



It has long been kno^\Ti that the a rays emitted from radium and 

 its products differed iu their power of penetrating matter. Two valu- 

 able papers have been recently published by Bragg and Kleeman,^ which 

 have considerably extended our knowledge of the mechanism of 

 absorption of the tv rays by matter. A brief account will be given 

 •of their exj^erimental results, as they are of importance in connection 

 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 



^ Phil. Mag-., Feb., 1903. 



^ Physik. Zeit, 4, p. 48.3. 1903. 



• Phil. Mag., Dec, 1904. 



