CHEMICAL ELEMENTS AND ATOMS — URBAIN 217 



Such important laws could not pass unnoticed even by the eyes 

 of those who did not especially work in radioactivity. All which 

 relates to the periodic classification thus again assumes a general 

 character and interests both chemist and physicist. 



In what relates to radioactivity, experiment and theory are so 

 intimately bound that it is difficult to tell where one ends and the 

 other begins, and one willingly attributes to theory the preponderat- 

 ing part. We seek for the crucial experiment which dispels all 

 doubts. Radioactivity is found in two series in the Mendeleeff 

 classification, and theory assigns to the changing element one of two 

 ultimate forms — helium or lead, 



I spoke earlier of variable atomic weights. Th. Richards who 

 obtained for lead from galena the atomic weight of 207.18, found 

 for lead from carnotite the value 20G.41. The precision of these 

 measures was over 1 part in 10,000. There was no chance for doubt. 

 We must applaud him. 



There therefore exist different kinds of a body as common as lead. 

 This name then does not designate one but a family of substances. 

 Our atomic weights which served as bases for our highest theoretical 

 speculations can not be considered as true constants since they char- 

 acterise not pure bodies but mixtures. Once more we must make an 

 alteration in our definition of an element. 



Will the classification of Mendeleeff totter now that its foundations 

 seem wrong? How shall we connect the properties of an element 

 with its atomic weight? It is necessary to give up the rigor of the 

 atomic weights and resign ourselves to their being only average 

 values. The idea of Fr. Soddy comes as a great relief in the eyes of 

 chemists. Mendeleeff 's classification no longer should be considered 

 as dealing with elements but rather with families of elements. 

 Each family becomes a pleiad of elements, in the limiting case, an 

 element. The idea grows on us. It is in the scientific air which we 

 breathe. We are anxious as to how in actual experience it will turn 

 out with all the elements, for until now the trials have all been 

 with radioactive elements. The abnormal leads, studied by Th. 

 Richards and his associates, by Maurice Curie, by Baxter and 

 Grover, by Honigschmidt and Horowitz, by Soddy and Hyman, and 

 several others, were all radioactive. In the special domain of radio- 

 active elements, the existence of isotopes has passed the boundary 

 which separates probable hypothesis from certain reality. But what 

 next? 



Whatever value we may be disposed to attribute to this distinc- 

 tion we are only half satisfied. Some of the compartments of Men- 

 deleef are to contain families — e. g., in the case of the radioactive ele- 

 ments — and others single elements. Is it necessary to save the classi- 



