EADIO-ACTIVITY AND THE PEKIODIC SYSTEM* 



The periodic system of the elements has for nearly half a 

 century proved a most puzzling and absorbing problem to 

 chemists. It has been called a law, but while there is un- 

 doubtedly an underlying law or laws, I doubt whether we 

 have as yet any very clear conception of them. Certainly, 

 the usual statement that the properties of the elements are 

 periodic functions of their atomic weights was never strictly 

 true, even in days of partial knowledge, and is much less true 

 now. It was neither the periodicity "of the geometers," as 

 Mendeleeif himself said, nor the function of the mathema- 

 tician. Indeed, we have now come to a view where, appar- 

 ently, we must abandon the atomic weight as the only or even 

 the chief determining variable. 



The truth is that for many years after its announcement 

 it was more truly a working hypothesis, and a great deal of 

 work had to be and still has to be done before it can attain to 

 its completed form. It contains much that is true, has been 

 most useful as a guiding principle, and has shown a wonder- 

 ful power of adjustment to new facts and increasing knowl- 

 edge. 



It was in 1895 that the system had to adjust itself to the 

 first severe jolt which it received through the discovery of 

 argon and helium, and three years later, of other inactive, 

 monatomic elements. The necessity for readjustment here 

 had been in part foreseen. The abrupt change in the pro- 

 gression of the elements from strongly electro-negative fluo- 

 rine to strongly electro-positive sodium, and, in general, the 

 transition per saltum from period to period had been dis- 

 cussed by Reynolds and others. It needed explanation and 

 was impossible mathematically except by passing through 

 zero or infinity. Some, as Sedgwick and de Boisbaudran, 

 seem to have predicted such transition elements, and when 

 argon was discovered it was not difficult for Julius Thomsen 



* Reprinted from Science, N. S., Vol. XLI, p. 589, 1915. Read before the 

 Elisha Mitchell Scientific Society, March 9, 1915. 



