1915] on Radiations from Exploding Atoms 497 



describes a circular path the radius of which is inversely propor- 

 tional to the momentum of the ^ particle. By the application of 

 special methods it has been found possible to obtain a veritable 

 spectrum of the ^ rays. The spectrum of the (^ rays from radium B 

 and radium C has been very carefully examined by the writer and 

 Mr. Robinson, and found to give a large number of well-marked 

 bands, each of which represents a group of (B rays, all of which are 

 expelled with identical speed. It was at first thought that most of 

 the energy of the jB rays was comprised in these groups, as some of 

 the bands on the photographic plate were very marked. Chad wick, 

 however, has recently shown that the fraction of the rays which 

 give a line spectrum is only a few per cent, of the total radiation. 

 The general evidence shows that the ^ radiation from these sub- 

 stances gives a continuous spectrum due to /? rays of all possible 

 velocities, on which is superimposed a line spectrum due to a small 

 number of jB particles of definite velocity comprising each group. 



Lines in the ^-ray spectrum have been observed for particles 

 which have a velocity not far from that of light, but the photo- 

 graphic effect of the particles becomes relatively feeble for such 

 high speeds. 



It is known from direct measurement that each atom of radium B 

 or of radium C in its disintegration emits on an average one (B particle. 

 In the )S-ray spectrum of radium C at least fifty definite bands are 

 observed, differing widely in intensity. It is thus clear that a single 

 atom in disintegrating cannot provide one ^ particle for each of 

 these numerous groups. It is thus necessary to conclude that each 

 atom does not emit an identical (B radiation. The results are best 

 explained by supposing that the /?-ray spectrum is the statistical 

 effect due to a large number of atoms, each of which may only give 

 one or two of the groups in its disintegration. In this respect a 

 ^-ray transformation is distinguished from an a-ray transformation, 

 for in the latter case each atom emits one a particle of characteristic 

 speed. It will be seen later that there is undoubtedly a very close 

 connection between the emission of (B and y rays from radio-active 

 atoms, and the probable explanation of the remarkably complex 

 ^-ray spectrum will be discussed later. 



With the exception of one element, radium E, and possibly 

 uranium X, all the radio-active substances which emit primary /? rays 

 give a line spectrum. For the majority of elements the strong lines 

 in the ^-ray spectrum have been determined by Baeyer, Hahn, and 

 Meitner, but more intense sources of radiation will be necessary to 

 map accurately the weaker lines. 



7 Rays. 



The earlier experiments on the y rays were mainly confined to 

 a determination of the absorption of the more penetrating radiations 



