314 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
It is possible to modify within the limits of the error of the ana- 
lytical data the values for the abundances of the elements in meteorites 
in such a way that the abundances of the individual nuclear species 
as a function of their mass number form regular smooth lines for 
the odd-mass-numbered isotopes. A similar smooth line is obtained 
with these modified abundance values, if one adds up at each mass 
number the abundance values of even-mass-numbered species with 
the same mass number (isobars). The isotopic composition of ad- 
jacent elements with even atomic number (graphically represented 
as in fig. 2) then fit together like pieces of a jigsaw puzzle (see fig. 3). 
Careful estimates, weighing the possible errors in the empirical 
abundance data of the elements, were made by H. C. Urey and the 
author. They led to an abundance distribution as shown in figure 3 
and in table 1. 
THE ORIGIN OF THE ELEMENTS 
When in 1889 Clarke was looking for regularities in the relative 
abundances of the elements, he expected to find some connection with 
the periodic table. Spectral analysis of the stars and chemical anal- 
ysis of the meteorites together with determinations of the isotopic 
composition of the elements made it possible more than 40 years later 
to discern certain types of regularities, but these regularities followed 
different laws from those of the atomic structure and had nothing to 
do with the periodic table. An entirely new aspect began to reveal 
itself, promising to lead far deeper into the fundamental fields of 
science than Clarke had expected. 
We have seen that solar abundances of the elements reflect proper- 
ties of the atomic nucleus. The matter surrounding us represents the 
ashes of cosmic nuclear reactions. These reactions took place some 
6 billion years ago at a time before our sun and the planets existed. 
They led to the formation of many radioactive nuclear species which 
subsequently decayed into the stable isotopes of the existing elements. 
Only a few rare radioactive species such as the isotopes of uranium, 
thorium, and potassium 40 are still present today like smoldering 
sparks that survived from the time of the original nuclear fire. 
For many years scientists have tried to explain the mechanism of 
the nuclear reactions that led to the formation of nuclear species 
and their abundance distribution. Our detailed knowledge of nuclear 
abundances serves now as a firm basis for such considerations. Many 
theories have been advanced, none of which could account for all 
the empirical facts, even in a crude way. All the past theories had in 
common the assumption that the matter surrounding us was created 
by one and the same nuclear process. The existence of radioactive 
nuclei and their abundance lead to the conclusion that this process 
must have taken place some 5 to 7 billion years ago. 
