130 CONCEPTION OF THE CHEMICAL ELEMENT 



chemical identity. The most obstinate cases of 

 similarity previously known, among the rare earths, 

 for example, cannot be compared with them. In all 

 cases, radioactive methods afford the most delicate 

 means for detecting the least alteration in the 

 concentration of the constituents, and the most 

 prolonged and careful attempts fail to produce a 

 detectable separation. 



At my request, Fleck undertook in my laboratory 

 a systematic chemical examination of all the members 

 of the series still imperfectly characterised, from the 

 point of view of first finding which known element 

 they most resembled and then finding whether or not 

 they could be separated from that element. His 

 researches were the means of finally unmasking 

 the extreme simplicity and profound theoretical 

 significance of the process of radioactive change. 

 All the members of the series so far chemically un- 

 characterised he found to be chemically non-separable 

 from one or other of the known elements, meso- 

 thorium-2 from actinium, radium-^4 from polonium, 

 the three /^-members and radium-/? from lead, the 

 three C-members and radium-/? from bismuth, 

 actinium-/? and thorium-/? from thallium. 



RADIOACTIVE CHANGE AND THE PERIODIC LAW. 



In February, 1913, K. Fajans in Germany, from 

 electrochemical evidence, and in this country A. S. 

 Russell and I, independently, from Fleck's work, 

 pointed out the complete generalisation which 

 connects chemical character and radioactive change. 

 In addition to the shift of two places in the periodic 

 table caused by the expulsion of the a-particle, it was 

 now clear that the expulsion of the /3-particle caused 

 a shift of one place in the opposite direction. Since 

 the a-particle carries two atomic charges of positive 



