12G Professor Frederick Soddy [May 18, 



Secondly, came the sweeping generalisations in the interpretation of 

 the Periodic Law. Lastly, there has been the first beginnings of 

 our experimental knowledge of atomic structure, which got beyond 

 the electronic constituents and at the material atom itself. 



Li pursuance of the first, Alexander Fleck, at my request, com- 

 menced a careful systematic study of the chemical character of all 

 the radio- elements known of which our knowledge was lacking or 

 imperfect, to see which were and which were not separable from 

 known chemical elements. Seldom can the results of so much long 

 and laborious chemical work be expressed in so few words. Every 

 one, that it was possible to examine, was found to be chemically 

 identical either with some common element or with another of the 

 new radio-elements. Of the more important characterisations, 

 mesothorium-II was found to be non-separable from actinium, 

 radium-A from polonium, the three B-members and radium-D from 

 lead, the three C-members and radium-E from bismuth, actinium-D 

 and thorium- D from thallium. These results naturally took some 

 time to complete, and became known fairly widely to others working in 

 the subject before they were published, through A. S. Russell, an old 

 student, who was then carrying on his investigations in radioactivity 

 in Manchester. Their interpretation constitutes the second line of 

 advance. 



Before that is considered, it may first be said that every case of 

 chemical non-separability put forward has stood the test of time, and 

 all the many skilled workers who have pitted their chemical skill 

 against Nature in this quest have merely confirmed it. The evidence 

 at the present day is too numerous and detailed to recount. It 

 comes from sources, such as in the technical extraction of meso- 

 thorium from monazite, where one process is repeated a nearly endless 

 number of times ; from trials of a very great variety of methods, as, 

 for example, in the investigations on radium-I) and lead by Paneth 

 and von Hevesy ; it is drawn from totally new methods, as in the 

 Ijeautiful proof by the same authors of the electro-chemical 

 identity of these two isotopes ; it is at the basis of the use of radio- 

 active elements as indicators for studying the properties of a common 

 element, isotopic with it, at concentrations too feeble to be otherwise 

 dealt with, imd from large numbers of isolated observations, as well 

 as prolonged systematic researches. One of the finest examples of 

 the latter kind of work, the Austrian researches on ionium, will be 

 dealt with later. The most recent, which appeared last month, is by 

 T. W. Richards and N. F. Hall, who subjected lead from Australian 

 carnotite, containing therefore radium-D, to over a thousand fractional 

 crystallisations in the form of chloride, without appreciably altering 

 the atomic weight or the y8-activity. So that it may be safely stated 

 that no one who has ever really tested this conclusion now doubts it, 

 and after all they alone have a right to an o]tinion. 



This statement of the non-separal)ility by chemical methods of 



