94 THE EVOLUTION OF MATTER 



these new changes cannot yet be artificially produced 

 or imitated, the doctrine of the unchangeability of the 

 elements remains in this limited sense still true. 



Sir Ernest Rutherford and the writer were forced 

 to the conclusion that the element thorium, and 

 ultimately all the radio-elements, are in the process 

 of slow spontaneous change. Their radioactivity 

 is due in large measure to minute quantities of 

 impurities, of totally different chemical character 

 from themselves, that can be readily and completely 

 removed by simple purification processes. But, once 

 removed, the substances so purified do not remain 

 pure. At a perfectly definite rate they regrow or 

 produce the radioactive impurities, and these can be 

 again separated as often as desired. Once separated, 

 the radioactivity of the products dies away or decays, 

 and the apparently steady continuous emission of 

 rays from the parent substance is due to an 

 equilibrium, in which new radioactive products are 

 formed as fast as the radioactivity of those already 

 produced disappears. Very rapidly a complete and 

 satisfactory theory of the whole phenomena was 

 developed, and fourteen years of further development 

 of the science has not necessitated any modification. 

 The atoms of the radio-elements are not permanently 

 stable. After a term of existence which may be long 

 or short, according to the nature of the atom in 

 question, and which for the individual atoms of the 

 same radio-element may have any actual value, but 

 is for the average of all the atoms of any one kind 

 a perfectly definite period, known as the period of 

 average life, the atom explodes. 'Fragments are 

 expelled from it at hitherto unknown velocities 

 constituting the rays, of which more anon. What is 

 left is the new atom of a new element, totally different 

 from the parent. The radio-elements are in course 

 of spontaneous transmutation into other elements, 



