1915] on Radiations from Exploding Atoms 41)5 



electric field. In order, however, to acquire a velocit}' equal to the 

 velocity of expulsion of an a particle, say, from radium C, even in 

 the most favourable case nearly four million volts would have to be 

 applied to the tube. 



In a similar way, in order to set an electron in motion with a 

 velocity of 98 per cent, the velocity of light, at least two million 

 volts would be necessary. As we have seen, it has not so far been 

 found possible to produce X-rays from a vacuum tube as penetrating 

 as the y rays. The study of the radiations from radio-active sub- 

 stances is thus of especial interest, not only for the information 

 obtained on the structure of the atoms themselves, but also in 

 providing for investigation special types of radiation of greater 

 individual intensity than can be obtained by ordinary experimental 

 methods. The enormous energy of motion of swift a and ^ particles 

 must exist in the atom before its disintegration, either in a potential 

 or a kinetic form, and may arise either from the passage of the 

 charged particles through the intense electric fields within the atom, 

 or from the very swift motion of these particles within the atom 

 before their release. In any case, there can be no doubt that 

 electric fields, and possibly magnetic fields, of enormous intensity 

 exist within the very small volume occupied by the essential structure 

 of the atom — fields many million times greater in intensity than we 

 can hope to produce in laboratory experiments. 



In order to explain certain experimental results, I have suggested 

 that the main mass of the atom is concentrated within a minute 

 volume or nucleus, which has a positive charge, and is of dimensions 

 exceedingly minute compared with the diameter of the atom. This 

 charged nucleus is surrounded by a distribution of electrons which 

 may extend to distances comparable with the diameter of the atom, 

 as ordinarily understood. The general evidence indicates that the 

 a and primary ^ particles are expelled from the nucleus, and not 

 from the outer structure of the atom. If this be the case, the 

 a particle which carries a positive charge would have its velocity 

 increased in passing through the strong repulsive field surrounding 

 the nucleus ; on the other hand, the /? particle which carries a 

 negative charge must be retarded in its escape from the nucleus, and 

 must possess great initial energy of motion to escape at all. There 

 appears to be no doubt that the penetrating y rays have their origin 

 in some sort of disturbance in the rings of electrons nearest to the 

 nucleus, but do not represent, as some have supposed, the vibrations 

 of the nucleus itself. 



a Kays. 



A brief account was given of the recent work of Rutherford and 

 Robinson in determining with accuracy the velocity of expulsion of 

 the a particles from certain radio-active substances. This was done 



2 K 2 



