July 19, 1921 clarke: evolution of matter 295 



structure of atoms there is not as yet a complete agreement, and 

 much remains to be done before the problem can be definitely solved. 

 I cannot go into this subject in detail, but I may take it for granted 

 that the conception of an atom as consisting of an electropositive 

 nucleus attended by few or many electrons of opposite sign, is well 

 established. In such a structure symmetry would be conducive 

 to stability, and any deficiency in that respect would be unfavorable. 

 For a simple element, with few electrons, symmetry would be most 

 easily attained, for a complex element with many electrons it would 

 be more difficult. Furthermore, an atom to be stable must show an 

 exact balance between the electropositive and electronegative charges. 

 With a single ring or shell of electrons the force of attraction holding 

 nucleus and electrons together should be strongest; with concentric 

 rings of electrons the outer ones would be more loosely held, and the 

 atomic structure should be weaker. This seems to be the case with 

 uranium and its neighbors. Here the structures are the most com- 

 plex, and the number of electrons greatest. In short, the conditions, 

 internal and external, which determine stability are by no means 

 simple, and some of them operate in opposite directions. They all 

 agree, however, in favoring the evolution of the simpler elements 

 and so render the fact of their greater abundance more intelligible. 

 When an atom of uranium decays, an alpha particle, which is 

 an atom of helium, is first discharged, and with a very high velocity. 

 By a succession of such discharges a series of products is generated, 

 each one differing by four units in atomic weight from its predecessor. 

 The atomic weight of helium is 4. Three of these products, omitting 

 intermediate forms, are ionium, radium, and an "isotope" of lead. 

 Ionium is isotopic with thorium, but not identical with it except in 

 its purely chemical relations. The atomic weights are not the same. 

 A similar difference is found for the lead derived from radium, which 

 differs from normal lead in having an atomic weight about a unit 

 lower. The isotope of lead in the thorium series differs from the 

 normal by about the same amount in the opposite direction.-' In 

 short, the degradation path from uranium, and also that from thorium 

 is approximately parallel to the path of evolution, but not identical 

 with it. The short-lived products of radioactive decay might be 

 described as the debris of exploding atoms. They do not appear in 

 the ascending series of the elements. 



*For a discussion of the nature of isotopic lead see Clarke, Proc. Nat. Acad. Sci. 4 

 181. 1918. 



