214 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1931 



for the heavy atoms to settle gradually to the center is not certain, 

 but it appears to be fairly probable. 



The remarkable differences in the abundance of the elements are 

 as yet almost unexplained ; this is a problem for the next generation 

 of physicists, and a fascinating one. It is natural to imagine that 

 the many different elements have been }3roduced in some way or other, 

 out of a more primitive material— perhaps out of hydrogen, which is 

 the simplest, and in many ways seems fit to be the raw material for 

 the rest. Abundant elements would then be those that have been 

 formed in large amounts, wdiile the rare ones have either been less 

 likely to form, or more likely to change over into something else. 



One general relation, first pointed out by Harkins, of Chicago, is 

 conspicuous. Elements of odd atomic number are much less abundant 

 than those of the adjacent even numbers. There is hardly an excep- 

 tion among the 56 elements included in the study of the sun's atmos- 

 phere, and on the average the even elements are eight times as 

 abundant as the odd ones. 



This must have something to do with the stability of the atomic 

 nuclei, but no one yet knows how. 



Another very striking case has been accounted for. Lithium and 

 beryllium, the lightest elements next to hydrogen and helium, occur 

 in surprisingly small amounts in the sun, less than a hundred thou- 

 sandth part as much as either these lighter elements or heavier ones 

 like oxygen or iron. Atkinson, of Rutgers, has accounted for this 

 by a theory of the building up of atoms out of hydrogen in the 

 intensely heated interior of the sun or of a star. In such a region 

 the lighter atoms are stripped of all their electrons and reduced to 

 bare nuclei, -which collide violently with one another. Once in a 

 very long time (as atomic events go) a hydrogen nucleus, or proton, 

 will hit some other nucleus so hard that it penetrates it and stays 

 there, forming a new nucleus, of greater atomic weight. The wave 

 mechanics indicate that the probability of such penetration is much 

 greater when the charge on the nucleus is small, and hence the lith- 

 ium or beryllium atoms, if originally present in large numbers inside 

 a star, would get built up into heavier ones, till in the final " steady 

 state " very few were left. This process of atom building liberates 

 a great amount of energy, sufficient to keep the sun shining for 

 thousands of millions of years. 



This theory, though beautiful and most promising, is still provi- 

 sional. It is probable, however, that with increased knowledge 

 regarding the composition of the stars and the sun, as well as of 

 their masses, densities, and other characteristics, a great deal more 

 can be found out about the past history and the evolution both of 

 stars and of atoms than anj^one knows now. 



