NON-SEPARABLE ELEMENTS 129 



merely of knowledge, but of the means of obtaining 

 knowledge, be imagined? Two different elements, 

 thorium and radiothorium, which on account of their 

 chemical resemblance could not be individually 

 recognised, and in the original interpretation of the 

 thorium disintegration series were taken as one, 

 became individually knowable, because the latter is 

 the product of the former through the intermediary 

 of a third member, mesothorium, possessing chemical 

 properties totally unlike either. Radioactive change 

 thus became the means of a new analysis of matter, 

 for which there is no counterpart outside the radio- 

 elements. 



In turn, mesothorium suffered analysis into two 

 successive products, mesothorium- 1 and -2, the first 

 distinguished by long period of life and a rayless 

 disintegration into the second, which has a short life 

 and gives powerful (3- and y-radiation in" its change 

 into radiothorium. 



I then found that mesothorium- 1 was chemically 

 non-separable from radium, a discovery also made by 

 Marckwald at the same time, and in 1911 I pointed 

 out that in an a-ray change, such as ionium into 

 radium, radium into emanation, thorium into meso- 

 thorium- i, and other cases, the expulsion of the 

 a-particle causes the radio-element to shift its place 

 in the periodic table by two places in the direction of 

 diminishing mass and diminishing valency, whereas 

 in successive changes in which a-particles are not 

 expelled, it frequently reverts to its former position, 

 as, for example, radiothorium from mesothorium and 

 lead from radiolead. 



To those actually engaged in the task of trying 

 to separate the successive products of radioactive 

 change by chemical analysis, it soon became clear 

 that the chemical resemblances disclosed between 

 certain of the members was such as to amount to 



