612 Velocity of Swiftly Moving Electrified Particles. 



Summary. 



According to the theory discussed in this paper, the 

 decrease of velocity of a and (3 rays in passing through 

 matter depends essentially on the characteristic frequencies 

 of the electron in the atoms, in a similar way as the pheno- 

 mena of refraction and dispersion. 



In a previous paper it was shown that the theory leads to 

 results which are in close agreement with experiments on 

 absorption of a. rays in hydrogen and helium, if we assume 

 that the atoms of these elements contain 1 and 2 electrons 

 respectively, and if the frequencies of these electrons are 

 put equal to the frequencies calculated from experiments on 

 dispersion. It was also shown that an approximate explana- 

 tion of the absorption of a rays in heavier substances can be 

 obtained, if we assume that the atoms of such elements, in 

 addition to a few electrons of optical frequencies, contain a 

 number of electrons more rigidly bound and of frequencies 

 of the same order of magnitude as characteristic Rontgen 

 rays. The number of electrons deduced was in approximate 

 agreement with those calculated in Sir E. Rutherford's theory 

 of scattering of a rays. These conclusions have been verified 

 by using the later more accurate measurements. 



In my former paper, very few data were available on the 

 decrease of velocity of ft rays in traversing matter and the 

 agreement between theory and experiment was not very close. 

 The agreement between theory and experiment is improved 

 materially, partly by using new measurements and partly 

 by taking the probability variations in the loss of energy 

 suffered by the individual ft particles into account. In this 

 connexion it is pointed out that it appears that measurements 

 on the decrease of velocity of ft rays afford an effective test 

 of the formulae for the energy and momentum of a high 

 speed electron deduced on the theory of relativity. 



In connexion with the calculations of the absorption of a. 

 and ft rays, the ionization produced by such rays is considered. 

 It is shown that the theory of Sir J. J. Thomson gives results 

 in approximate agreement with the measurements if the 

 secondary ionization produced by the electron expelled by 

 the direct impact of the a and ft rays is taken into account. 



I wish to express my best thanks to Sir Ernest Rutherford 

 for the kind interest he has taken in this w r ork. 



University of Manchester, 

 July 1915. 



