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XL I. The Mass and Velocity of the a, particles expelled from 

 Radium and Actinium. By E. Rutherford, F.R.S., 

 Macdonald Professor of Physics, McGill University, 

 Montreal*. 



[Plate V.] 



THE present paper contains an account of investigations 

 that have been made to determine, as accurately as 

 possible, the mass and velocity of the a particles expelled 

 from some of the products of radium and actinium. At the 

 present stage of our knowledge of radioactivity, such measure- 

 ments have an important theoretical value in throwing light 

 on the following questions : — 



1. Has the a particle expelled from all radioactive products 



the same mass ? 



2. Does the value of e/m of the a particle vary in its 



passage through matter ? 



3. What is the connexion between the velocity of the a 



particle and its range of ionization in air ? 



4. What is the connexion, if any, between the a. particle 



and the helium atom ? 



5. Is the heating effect of radium or other radioactive 



substance due to the bombardment of the radioactive 

 matter by the a particles expelled throughout its own 

 mass ? 



In the course of these investigations, sufficient data have 

 been accumulated, if not to answer completely all of the 

 above questions, at least to indicate with some certainty the 

 relations that exist between the various quantities. 



The experiments outlined in this paper have been in pro- 

 gress for more than a year f , but publication has been delayed 

 in order to determine the mass of the a particle from thorium 

 and actinium as well as from radium. 



The investigations on the mass of the a particle from thorium 

 have been made in conjunction with Dr. Hahn, and are 

 described in a following paper. 



Determinations of the mass and velocity of the a. particles 

 from radium have been made by several observers. In 1902, 

 using the electroscopic method and radium of activity 19000, 

 I showed that the a particles from radium consisted of posi- 

 tively charged particles which were appreciably deflected in 



* Communicated by the Author. 



t A preliminary account of the measurements of the value of e/m for 

 the particle from radium was given before the American Physical 

 Society, December 1905. An abstract of the results appeared in the 

 Physical Keview, Feb. 1906. 



