1 38 THE POPULAR SCIENCE MONTHLY 



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show identical general physical and chemical behavior. For example, 

 the elements radium B, radium D and lead, of atomic weights 214, 210, 

 and 207, respectively, are so closely allied in chemical and physical 

 properties that all attempts to separate a mixture of any two of them 

 have failed completely. This would be explained if the nuclear charges 

 were identical for those elements, as the generalization, already re- 

 ferred to, indicates. If this be the case, they should give identical 

 spectra under similar conditions. Unfortunately the elements radium 

 B and radium D are in too small quantity to determine their ordinary 

 light spectra, but we can compare the X-ray spectrum of lead with 

 that given by radium B under the excitation of its own beta rays. Ex- 

 periments of this kind were recently made by Dr. Andrade and the 

 writer, and the two spectra were found to be identical within the limits 

 of experimental error. It is to be anticipated that their light spectra 

 would also prove to be identical, or nearly so, for, as previously pointed 

 out, the effect of the mass of the nucleus on the spectrum is probably 

 very small. 



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 ordinary 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 investi- 

 gators 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 weight 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. 3 



The question naturally arises whether some of the ordinary elements 

 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 chemical methods. 



3 Since the delivery of this lecture, similar conclusions have been reached by 

 the experiments of Eichards in Cambridge and Honigschmid 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 and fundamental problem. 



