306 Sir E. Rutherford on tie 



immediate and definite solution, but the experimental results 

 are sufficiently complete to afford some data for drawing 

 some tentative conclusions. 



In a paper published two years ago * I discussed the 

 possible connexion between ft and y rays emitted from radio- 

 active substances and outlined a general theory in explanation 

 of the magnetic " spectrum " observed when the ft rays are 

 analysed by their passage through a magnetic field. It was 

 pointed out that the emission of a large number of groups of 

 the ft rays of definite velocity from a single substance could 4 

 be most simply explained by supposing that it is a statistical 

 effect due to a large number of atoms each of which gave 

 rise to a few only of the groups of ft rays observed. 



In a transformation where primary ft and y rays appear, 

 it was supposed that each atom broke up with the emission 

 of a ft particle of definite speed. The latter in passing 

 through the external electronic system set it into vibration, 

 and energy was abstracted from the ft particle in definite 

 integral units depending on the vibrating system. If, for 

 example, the ft particle passed through two distinct vibrating 

 systems A 1 and A 2 , the final energy of the escaping ft particle 

 was given by E — (pE, + gE 2 ) |, where E was the initial 

 energy of the ft particle, p and q wmole numbers which 

 might have any values 0, 1, 2, 3, etc., and E^ E 2 the units 

 of energy abstracted in passing through A and B respectively. 

 It was supposed that the energy pE 1 + qE 2 which was 

 abstracted from the ft particle appeared in the form of 

 p gamma rays each of energy E 1? and of q gamma rays each 

 of energy E 2 . It was suggested that the 7 rays so excited 

 corresponded to one or more of the types of characteristic 

 radiations brought to light by the experiments of Prof. Barkla 

 on X rays. This theory has formed a starting point for a 

 number of subsequent researches. In the first place, in 

 order to test the theory, it was necessary to know the energy 

 of the ft particles comprising the different groups with the 

 greatest possible accuracy. The initial experiments made 

 by Baeyer, Hahn and Meitner, and by Danysz on the groups 

 of ft rays emitted from radium B and radium C were repeated 

 with great care by Mr. H. Robinson and myself J. By the 

 adoption of a modified method, the magnetic spectra due 

 to radium B and radium C were separately determined. 

 The spectrum of radium C was greatly extended and found 



* Phil. Mag. xxiv. p. 453 (1912). 



t The particular point of view of which this formula is an expression 

 has been modified subsequently. 



X Rutherford & Robinson, Phil. Mag 1 , xxvi. p. 717 (1913). 



