310 Prof. W. D. Harkins on the 



7 (a). In approximately 13 out of 14 atoms which exist in 



the meteorites and in 85 per cent, of those in the earth's 

 crust, the atomic weight is divisible by 4. 

 (b). The most important electron group concerned in the 

 building of complex nuclei is the alpha particle. 



8 («). The above relations indicate that most atomic species 



of even atomic number have even numbers for atomic 

 weights, while most atoms, of odd atomic number have 

 odd atomic weights, or when P — N is even, P is usually 

 even (and N even), and when P — N is odd, P is usually 

 odd (and N even). 



The Nucleus considered as a Proton-Electron 

 Aggregate. 



If complex nuclei have been built up from alpha particles 

 or other similar electron groups, it might seem that there 

 would be little chance of securing generalizations concerning 

 nuclear stability by considering each complex nucleus as an 

 aggregate of positive and negative electrons without reference 

 to their grouping. It will be shown, however, that when 

 this treatment is adopted a number of remarkable general- 

 izations, already outlined in their experimental form, become 

 apparent. Nevertheless, it will also be found that if they 

 are to be applied to specific cases, other relations must also 

 ba considered, and it will be seen that in the departures 

 from the general regularities are to be found the best evi- 

 dence for the existence and nature of the secondary electron 

 groupings. 



Relation between Nuclear Stability and 

 Abundance. 



If the abundance of the radioactive atomic species in the 

 earth's crust is compared with the average life period, it is 

 found that elements whose atoms are relatively stable, such 

 as uranium and thorium, are relatively abundant, those 

 which are less stable, such as radium, actinium, and meso- 

 thorium, are less abundant, while radium C and other 

 similar extremely unstable species, are also extremely rare. 

 It therefore seems a natural assumption that the relative 

 abundance of the atomic species of low atomic iveight may be 

 used as an index (not necessarily a measure) of their relative 

 stability. It seems evident that the abundance of nuclei of 

 a certain type may be affected by the stability and abundance 

 of the primary proton-electron groups, such as the alpha 



