PRODUCTION OF HELIUM AND LEAD 119 



task may be likened to that of searching for a meteor 

 which a moment before lit up the heavens and now 

 has vanished into the night. 



THE ULTIMATE PRODUCTS. 



It is a matter for surprise that in all radioactive 

 changes so far studied there appear to be only 

 two ultimate products, helium and lead, the former 

 constituting the a-particles and the latter being 

 produced both by uranium and thorium, withal, as 

 we now know, not the same lead in the two cases. 

 There are sufficient experimental reasons for doubting 

 whether the disintegration of an atom into more 

 nearly equal parts would be within range of detection 

 by any of the known methods. A heavy atom like 

 oxygen, for example, if expelled as a radiant particle, 

 might not attain sufficient velocity to ionise gases, 

 or, even if it did, the range over which the ionisation 

 would extend, as we know from the ionisation 

 produced by the recoil atoms, would be extremely 

 small. It must be a matter for comment, however, 

 that hydrogen never appears in these changes, as, if 

 it were produced, it would almost certainly be as easy 

 to ascertain as helium. It has always seemed to me 

 a possibility that some genetic connection may exist, 

 after all, between thorium and uranium, although 

 I have never been able to frame even a possible mode 

 of so connecting these two elements. With a differ- 

 ence of atomic weight of six units, it is impossible to 

 pass from one to the other by addition or expulsion 

 of helium atoms alone. 



Both with regard to helium and lead, the com- 

 position of radioactive minerals gave the first clue 

 to the identity of the ultimate products. After 

 the discovery of radioactivity and the elucidation 

 of its nature, the fact that helium was found only 



