8 SODDY, Evolution of Matter by Radio-active Elements. 



great. It will be seen at once that the only explanation, 

 on the discrete theory of the rays, of the different degrees 

 of radio-activity exhibited by the various radio-elements 

 is that a different number of particles are expelled from 

 unit weight of the substance in unit time. The general 

 character of the a particles expelled is similar in the case 

 of each of the radio-elements. Rutherford was the first to 

 recognise the preponderating importance of the a radiation 

 in the phenomenon of radio-activity. He showed that in 

 all cases they probably represent over 99 per cent, of the 

 energy radiated, so that the energy radiated by the j3 and 

 •y rays together is a very small fraction of the whole. We 

 now know that the j3 and 7 rays are secondary phenomena 

 in point of time. The expulsion of the a particle starts 

 the process and the expulsion of the j3, and the emission 

 of the 7 ray which accompanies it, occur much later on 

 as the process nears completion. 



A theory accounting for the cause and nature of 

 radio-activity was put forward by Professor Rutherford 

 and myself, as the result of an investigation of the radio- 

 activity of thorium, before the discrete nature of the a radia- 

 tion was known. For the sake of convenience the complete 

 theory^ in its present state will be here considered, without 

 reference to the historical order in which the steps were 

 evolved. The elements exhibiting radio-activity are 

 regarded as undergoing a slow spontaneous transformation 

 into other elements. The change is slow only with regard 

 to the mass of the material, an extremely minute fraction 

 of the total undergoing change in the unit of time. But 

 for the individual atoms changing, the transformation is 

 sudden, and of the nature of a disintegration or disruption. 

 The a particles are small fragments of the disintegrating 



"^ Phil. Mag., 1902, S.6, v.4, pp., 370 and 569; 1903, s.6, v.5, 

 pp.. 441, 445. 561 and 576. 



