July 19, 1921 . clarke : evolution of matter 293 



between helium (atomic weight 4) and iron, either came later or were 

 developed at first in smaller quantities. As cooling went on more and 

 more elements were generated, and in the Sun all the possible ones are 

 presumably present. It is conceivable that elements of different 

 stability may have been formed simultaneously, one in that part of 

 the cooling mass where temperatures and pressures were highest, 

 another further away from the center under less rigorous conditions. 

 This, however, is something which cannot be proved. If the three 

 nebular elements were the raw material from which the other elements 

 were built, their relative amounts must have been continually chang- 

 ing, and so as each new element appeared a new environment was 

 established for all that followed. How far these changes may have 

 affected the evolution of the elements it is impossible to say, but in the 

 evolution of compounds similar conditions would be significant. 



That some of the chemical elements are very abundant, and others 

 comparatively scarce, is a familiar fact which bears directly upon the 

 theory of evolution. Their relative abundance in known terrestrial 

 matter has been repeatedly computed, by several workers and by 

 different methods. The results agree remarkably well, at least in the 

 orders of magnitude as expressed in percentages. This order, when 

 we combine the figures for the lithosphere, the ocean, and the atmos- 

 phere, is as follows. First, oxygen, then silicon, aluminum, iron, calcium, 

 sodium, potassium, hydrogen, and titanium, and these ten elements 

 form at least 98 per cent of the whole. Only 2 per cent remains for 

 all the other elements, some 80 or more in number."* 



For present purposes this estimate is obviously defective, for it 

 covers hardly more than a thin film on the surface of the Earth and 

 says nothing about the Earth's interior. This objection is easily met 

 if we take three facts into consideration. The Earth behaves like a 

 huge magnet, it resembles a huge meteorite, and its mean density is 

 double that of the rocks forming its crust. From these facts, and other 

 quite minor considerations which need not be discussed here, we may 

 fairly assume that the interior of the Earth contains a large propor- 

 tion of metallic iron, and the quantity of it needed to give with the 

 crust the mean density of the globe can be calculated. This has been 

 done by others, and it now appears that in the Earth as a whole, 

 iron predominates, oxygen and hydrogen fall to subordinate positions 

 while the order of the other elements is little changed. Probably 



*The details of the most recent computation of the relative abundance of the elements 

 will appear in a joint paper by Dr. H. S. Washington and myself, which is in course of 

 preparation. 



