ON THE EVOLUTION OF THE ELEMENTS. 125 



A theory elucidating the mystery was worked out in the laboratories 

 of McGill University, Montreal, under Rutherford four years ago, and 

 having proved itself adequate to exphiin the known facts and to suggest 

 innumerable new discoveries, it has received general acceptance. I need 

 only recall the discussion which took place on the subject in this Section 

 three years ago, which assisted materially in the favourable reception ac- 

 corded the disintegration theory, and in this Section at least I need only 

 briefly refer to its main outlines. 



The atoms, in the case of the radioactive elements, are spontaneously 

 breaking up into atoms of lighter elements, and this change proceeds in 

 each case according to a very simple law. The same definite fraction of 

 the total number of atoms breaks up in the unit of time in the case of any 

 one radioactive element. The a particle expelled with enormous velocity 

 consists, as we have seen, of an atom a little heavier in mass than the 

 hydrogen atom, and this it seems probable, though it has not been com- 

 pletely proved, is an atom of helium. The residual atom which is left, in 

 many cases breaks up again and again, the same law as before being fol- 

 lowed, only in the succeeding products the fraction changing in unit time 

 is usually much larger than in the original elements, so that the conse- 

 quence is that these products have a very limited term of existence, and 

 can only accumulate in minute quantities. The energy that is evolved in 

 this change is so enormous that very slow changes can be observed, where 

 the actual amount changing is so infinitesimal that it is hopeless to attempt 

 to detect it by ordinary methods. This appears at once when it is con- 

 sidered that the smallest quantity of any element that can be detected by 

 the spectroscope contains between lO'-'and 10" individual atoms, whereas 

 the disintegration of a single atom accompanied with the expulsion of one 

 a particle is not greatly, if at all, below the limit of detection by present 

 methods. 



At first put forward on radioactive evidence alone, it was not long 

 before additional evidence of the correctness of the view was obtained by 

 the older methods. With the discovery in the laboratories of the Univer- 

 sity College, London, under Sir William Ramsay, of the production of 

 helium from radium, the fact of the gradual evolution of one element into 

 others passed beyond doubt, and the new methods of investigation received 

 the support of the old in a case where it was found possible to examine 

 the process from both points of view. 



The essential features distinguishing the type of evolution revealed by 

 radioactive processes from previous conceptions chiefly claim our atten- 

 tion. 



The first consists in the reconciliation of the idea of a gradual evolu- 

 tion of one element from another with the facts of chemistry and spectro- 

 scopy, which we have seen were at one time interpreted to prove the exact 

 opposite. On the new view the change, though of any degree of slowness 

 so far as the mass of the matter is concerned, is sudden and abrupt for 

 each individual atom. There is no gradual alteration of one element in 

 properties until it changes into another, but an abrupt, or more usually 

 a series of abrupt, step-by-step changes in property accompanying the 

 sudden expulsion of each a particle. As great a difference exists between 

 radium and its emanation, which is its first product, as between any pair 

 of elements known. Yet it is gradual in the sense that only a small frac- 

 tion of the total quantity of the radium changes in each unit of time. 



Slight as this addition is to make the requirements of chemistry and 



