198 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1915. 



The fact that the atoms of these three elements are not identical as 

 regards mass or radioactive properties shows that the structure of 

 the nucleus is different in each case. 



There is another important deduction that should be mentioned. 

 The end product of the uranium-radium series is an inactive element 

 which has long been considered to be lead, but it has been difficult 

 to verify this conclusion by direct experiment. We have seen that 

 the end product has the same atomic number as lead, but should have 

 an atomic weight about 206 instead of 207, as found for ordinarj'^ 

 lead. In a similar way it has been concluded by Soddy and Fajans 

 that the end product of thorium has the same atomic number as lead, 

 but should have an atomic weight about 208.5. In order to test these 

 remarkable conclusions experiments are now in progress by a number 

 of investigators in different countries to examine whether the lead 

 always found in radioactive minerals, and which presumably has 

 partly, if not wholly, a radioactive origin, shows the same atomic 

 w^eight as ordinary lead. Soddy has already found evidence that 

 there is a distinct difference in the atomic weights in the direction 

 predicted by the theory.^ 



The question naturally arises whether some of the ordinary ele- 

 ments may not prove to be a mixture of two or even more of these 

 isotopes, as they have been termed. Unless the component isotopes 

 are present in different proportion in different natural sources of the 

 element, it will be difficult to settle this problem by ordinary chemi- 

 cal methods. There is one element, however, besides lead, from which 

 some interesting evidence has been obtained on this point. Sir J. J. 

 Thomson found by examining the deflection of the positively charged 

 particles produced by an electric discharge through the rare gas 

 neon that two elements were present of atomic weights about 20 

 (neon) and 22. Aston was able by diffusion experiments to separate 

 partially the two components of neon and to show that they differed 

 in density, but failed in attempts to separate them "by fractional dis- 

 tillation in charcoal cooled by liquid air. Such results are to be 

 anticipated if neon is a mixture of two isotopes; i. e., elements of 

 identical nuclear charges but different atomic weights. 



It is obvious that this new" point of view will result in a systematic 

 examination of the elements to test for the ])ossible presence of 

 isotopes, and thus give fin additional reason for the accurate deter- 

 mination of atomic weights for elements obtained from widely dif- 

 ferent sources. 



^ Since the delivery of this lecture similar conclusions have been reached by the experi- 

 ments of Richards and Lembert in Cambi-idge and Hiinigschmid in Vienna. There still, 

 however, remains some doubt as to the actual difference in atomic weight of uranium 

 lead, thorium lead, and ordinary lead. A very promising beginning has thus been made 

 on the attack of this most important ang fundamental problem. 



