312 Sir E. Rutherford on the 



ft rays differs somewhat from that advanced in the earlier 

 papers (loc. cit.) . I there supposed that homogeneous groups 

 of ft rajs were due to the decrease of energy in definite 

 units of the primary ft particle in exciting the vibrations in 

 the atom. The present theory supposes that the homogeneous 

 groups of ft rays arise from the conversion of the energy of 

 the 7 rays into the ft ray form. In other words, the primary 

 effect in the atom is the excitation of 7 rays by the escape of 

 the ft particle from the nucleus. The appearance of groups 

 of homogeneous ft rays is a secondary effect due to the 

 partial conversion of the 7 rays into ft rays in their passage 

 through the radioactive atom. On the other hand, the 

 continuous ft radiation is ascribed mainly to the effect of 

 the primary ft particles escaping from the nucleus which 

 have lost energy, though not in definite quanta, in setting 

 the electronic system of the atom into vibration. 



Consideration of the experimental evidence. 



We shall now consider briefly some recent experimental 

 •evidence which has thrown light on this question. Robinson, 

 Rawlinson, and the writer"* have shown that the ft rays 

 excited by the 7 rays in their passage through matter 

 consist of definite groups which, no doubt, would be 

 homogeneous if the layer of matter in which the ft rays 

 were excited was exceedingly thin. As far as experiment has 

 gone, the velocities of these groups of " excited " ft rays are 

 in close if not complete agreement with the velocities of the 

 stronger groups of primary ft particles from the source of 

 radiation. The velocities of the corresponding groups 

 appear to vary slightly when the ft rays are excited in 

 different metals, but the differences, though no doubt real, 

 are not very marked. We may conclude from these results 

 that the primary ft particles, for example from radium B, 

 like the ft rays excited by the 7 rays in traversing absorbing 

 material, are due to the conversion of 7 rays into ft rays in 

 their escape from the radioactive atom. The observed 

 variations of velocity between the primary groups of ft rays 

 from the radioactive atom and the ft rays excited by the 

 7 rays in different substances, may be due to the variation of 

 the part — (qhv 2 + A) in the expression for the energy 

 'E=phv 1 — qhv 2 — A. The experiments, however, show that 

 the variation due to this cause is small for the swift groups 

 of ft rays excited by 7 rays in different kinds of matter, and 

 for simplicity it may be assumed as a first approximation 



* Phil. Mag. August 1914. 



