120 CONCEFllON OF THE CHEMICAL ELEMENT 



in minerals containing uranium and thorium assumed 

 a totally new interpretation, borne out by the 

 spectroscopic proof of the production of helium from 

 radium by Sir William Ramsay and myself, and 

 later from actinium, polonium, and even from 

 uranium and thorium, all at the rates to be expected 

 from radioactive data. The identification of the 

 a-particle with helium, after the weight of the 

 a-particle had been shown by new physical methods 

 to be four times that of the hydrogen atom, was 

 accomplished by enclosing the radium emanation 

 in a glass tube thin-walled enough to allow the 

 a-particle to go through, but perfectly impervious 

 to the passage of gas. In these circumstances, 

 helium in spectroscopically detectable quantity was 

 proved by Rutherford to make its appearance out- 

 side the tube. 



Such confirmations by the spectroscope, welcome 

 and gratifying as they are, are nevertheless in a 

 sense subsidiary to the main problem, namely, the 

 task of unravelling the complicated series of changes 

 into its individual steps, and the characterisation 

 by their radioactivity of the several intermediate 

 members of the series, such as by the determina- 

 tion of their periods and the physical constants of 

 the radiation a-, /3-, or y-, to which they give rise. 

 The determination of their chemical character, 

 although equally important, was only later fully 

 accomplished. 



THE RADIATIONS. 



In the successive radioactive changes, a- or 

 i8-particles are expelled, one a-particle per atom 

 disintegrating for each change, although for the 

 /3-particles our knowledge is less exact. In some 

 cases, certainly, although these are exceptional, 

 /3-particles seem to be expelled along with a-particles. 



