NATURE OF THE RADIATIONS 121 



The a-particle is an atom of helium charged with 

 two atomic charges of positive electricity, or, as 

 we should now say, is the helium nucleus, deprived 

 of the two electrons which are combined with it in 

 the helium atom. The /3-particle is the negative 

 electron, and when expelled with sufficiently high 

 velocity is accompanied with y-rays. The latter are 

 X-rays of exceedingly short wave-length, varying 

 from 1-3 to o i Angstrom units. 1 A connection exists 

 between the speed of the change and the speed of 

 the particles expelled, and the more rapid the change 

 the faster in general and the more penetrating are 

 the attendant a- or /3-particles. In the case of the 

 a-particle, an empirical logarithmic relation, known 

 as the Geiger-Nuttall relation, enables us to calculate 

 approximately the period of the changing element 

 from the velocity or range of the a-particle, and 

 vice versa, and by this means periods too long or 

 too short to be directly measurable have been 

 estimated. In the case of the /3-rays, no definite 

 quantitative law has yet been made out, but it is 

 clear that a similar relationship must exist. One 

 of the important corollaries is that changes much 

 slower than the slowest known, namely, those of 

 uranium and thorium, would probably not be detect- 

 able, as, even were a- or /3-particles expelled, they 

 would be of too low velocity probably to ionise 

 gases or show fluorescent or photographic actions. 

 Indeed, for mesothorium-/and actinium this appears 

 to be the case. No detectable radiation is expelled, 

 although the products conform to what would occur 



1 The shortest wave-length so far resolved by the crystal 

 reflection method is 0072 A. in the spectrum of the 7-rays of 

 radium-C Ishino and Rutherford have recently concluded, how- 

 ever, that the main ^-radiation of radium-C" must have a wave- 

 length lying between 0-02 and 0-007 A. (Phil. Mag., 1917, [vi.] 33, 

 "9; 34. 



