﻿146 Retardation of the a. Particle of Radium. 



quantitatively whether the intensity of the luminosity pro- 

 duced in a zinc-sulphide screen, as well as of the photographic 

 effect, is directly proportional to the range of the a particles 

 in air. Such a connexion is to he expected i£ both the photo- 

 graphic and luminous effects are directly or indirectly due to 

 ionization by the a particles of the substances involved. 



In regard to the question whether the a particle has a 

 " critical " velocity, below which it is unable to produce its 

 characteristic effects, it is difficult to give a definite answer. 

 The a particle certainly appears to have such a " critical " 

 velocity, but there appears to be no obvious reason why the 

 a particle should suddenly lose its ionizing power when its 

 velocity is still very great. Bronson has made an investi- 

 gation to see whether the a particle produced any appreciable 

 ionization beyond its normal range of strong ionization in air, 

 but with negative results. We have to conclude either that 

 there is a definite critical velocity (about *4 of the maximum 

 velocity of the a particle from radium C), or that the velocity 

 of the a particle decreases very rapidly when it falls below 

 that amount. The question is complicated by the slight 

 scattering of the a particles in passing through matter, for 

 this makes definite deductions from the experiments very 

 difficult. I hope to make further experiments to throw light 

 on this important question. 



We have seen that it is possible to determine with accuracy 



the value of for the a particles emitted by radium, 



radium A, and radium F by measurements of the retardation 

 of the a particles of the single product radium C in passing 

 through matter. Such a result affords an almost certain proof 

 that the value of ejm is the same for the a. particles expelled 

 from each of these products ; for while the measurement of 

 the magnetic deflexion of the rays gives the momentum of the 

 a particle, the range of the particle plus a constant is propor- 

 tional to the kinetic energy of the a particle. Unless ejm is 

 the same for each product, we should not expect the observed 



mv 

 and calculated values of to agree. 



It will be shown in a future paper, that this conclusion has 

 been confirmed by direct measurement of the value of ejm 

 for the particles expelled from the various radium products. 



McGill University, Montreal, 



