168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



the same place in the periodic table, and gave them the name of "iso- 

 topes." This was the first time that proof had been obtained that an 

 element might be complex and consist of atoms differing in mass and 

 structure. Tlie complexity of the radio-elements is now well estab- 

 lished. The three well-known radioactive series contain, for example, 

 six isotopes of thorium, three of radium, seven of lead, and seven of 

 polonium, and each of these isotopes shows a different mass and dis- 

 tinctive radioactive behavior. 



The next notable advance was the proof that the position of a radio- 

 element in the periodic table had a very simple connection with the 

 type of radiation emitted by the parent product. Soddy had early 

 noted several cases in which the emission of an a-particle, which car- 

 ries two units of positive charge, gave rise to a product which had the 

 chemical properties of an element two places preceding its parent in 

 the periodic table. We now know that the effect of an emission of a 

 y3-particle, which carries only one unit of negative charge, is a corre- 

 sponding displacement of one group in the opposite direction. The 

 more complete generalization had to await a more definite knowledge 

 of the chemical nature of some of the products, much of which was 

 supplied by the careful work of Fleck. The essential features of this 

 process, known as the displacement law, were put forward about the 

 same time in 1913 by Fajans, by A. S. Russell, and by Soddy, and not 

 only included within their scope nearly all the radioactive bodies in the 

 three main families, but even predicted the properties and positions of 

 elements hitherto unobserved. The proof of this relation indicated 

 that the periodic grouping of the elements was closely connected with 

 the loss or gain of charge of the atom due to the exf)ulsion of an a- or 

 ^-particle. 



Wliile the displacement law, as put forward, is quite independent of 

 any special theory of the atom, yet we see that it is in complete accord 

 with the nuclear theory if we suppose that the a- and the /^-particle are 

 released from the nucleus. The atomic number and atomic weights of 

 uranium and thorium being known, the atomic number and weight of 

 each of the successive elements can be written down from a knowledge 

 of the radiations emitted by each element, illustrating the extraordi- 

 nary simplicity of the laws which hold for atomic nuclei. 



The new conception of the isotopic constitution of the radio ele- 

 ments naturally had a great influence in promoting experiments to 

 decide whether the ordinary inactive elements also were complex. 

 Very largely owing to the pioneer work of Aston, the broad features 

 of the isotopic constitution of the great majority of the elements 

 were soon established, indicating that the varieties of stable atoms 

 were much more numerous than had been supposed. 



