ft and 7 Rays from Radioactive Substances. 461 



into 7 rays in its escape from the atom. In the case of radium 

 E where the 7 rays are very weak in intensity and of slight 



penetrating power, it seems probable that the ft rays originate 

 near the surface of the atom, and consequently do not traverse 

 the regions where penetrating 7 rays can be set up. There 

 still remains the difficulty, however, of accounting for the 

 heterogeneity of the ft rays which are emitted, to which 

 attention has been drawn by Gray and by Gray and Wilson. 



There is one point of interest which has so far not been 

 considered. Bragg has given strong evidence for believing 

 that a ft and a 7 ray are mutually convertible forms of energy. 

 The energy of a 7 ray incident on matter is transformed into 

 the energy of a /3 ray, and vice versa. On this view, it has 

 generally been supposed that the whole of the energy of one 

 7 ray is converted into the energy of one ft ray, so that the 

 7 ray disappears and the ft particle takes its place. From 

 the point of view outlined in this paper, it is supposed that 

 the ft ray originating in the transformation of an atom loses 

 only part of its energy which is abstracted from it in definite 

 units, depending on the region of the atom through which 

 the ft particle passes. A swift ft ray may consequently give 

 rise to several 7 rays in escaping from the atom and yet retain 

 a part of its initial energy. 



In a previous paper* I have given reasons for believing 

 that the atom consists of a positively charged nucleus of very 

 small dimensions, surrounded by a distribution of electrons 

 in rapid motion, possibly of rings of electrons rotating in one 

 plane. The instability of the atom which leads to its disin- 

 tegration may be conveniently considered to be due to two 

 causes, although these are not mutually independent, viz., the 

 instability of the central nucleus and the instability of the 

 electronic distribution. The former type of instability leads 

 to the expulsion of an a particle, the latter to the appearance 

 of /3and 7 rays. The instability which leads to the expulsion 

 of ajS ray may be mainly confined to one of the rings of 

 concentric electrons, and leads to the escape of a ft particle 

 from this ring with great velocity. The ft particle in escaping 

 from the atom passes through the electronic distribution 

 external to it, and in traversing each ring may lose part of 

 its energy in exciting one or more 7 rays which have a definite 

 energy, which is characteristic for each ring. 



At present we have no definite information of the mode in 

 which the transformation of a ft into a 7 ray or a 7 ray into 

 a ft ray takes place, but it is no doubt connected with the 



* Rutherford, Phil. Mag. x.xi. p. 669 (10] 1). 



