136 CONCEPTION OF THE CHEMICAL ELEMENT 



to its relatively small quantity as a branch product 

 and to the fact that, itself, it gives no rays, the 

 characteristic radioactivity of its products only 

 making their appearance slowly after it has been 

 separated, actinium has always been a difficult 

 element to extract from the mineral and very easy 

 to lose in chemical operations. There is now, how- 

 ever, another reason which will assist in the study 

 of this element. 



THE ORIGIN OF ACTINIUM. EKATANTALUM. 



The generalisation has now led to the elucida- 

 tion of its origin and the discovery of its direct parent. 

 From its constant association with uranium minerals, 

 and the relative activity therein of its products in 

 comparison with the activity of those of radium, 

 it was considered to be a branch product of the 

 uranium series, only 8 per cent, of the atoms of 

 uranium disintegrating passing through the actinium 

 series and 92 per cent, through the radium series. 

 Its definite location in the periodic table, by virtue 

 of its isotopism with mesothorium-2, made it clear 

 that its parent must either be in the radium or the 

 ekatantalum place, the former if it is produced in a 

 /3-ray change and the latter if it is produced in an 

 a-ray change. 



The ekatantalum place was vacant when the 

 generalisation was first made, but it was necessary 

 to suppose that uramum-X, like mesothorium, com- 

 prised two successive products, uranium-A^ and 

 uranium-^, both giving /3-rays, and the latter occupy- 

 ing the vacant place in question. This prediction was 

 confirmed within a few weeks of its being made by 

 the discovery by Fajans and Gb'hring of uranium--Y 2 , 

 or brevium, a new member responsible for the 

 more penetrating /3-radiation given by uranium-^, 



