14 C. E. VAN ORSTRAND 
about half way down the scale of diminishing thermal energy. In the 
course of this process, which has required a few billions of years for 
its completion, the earth is supposed to have been separated from the 
sun and subsequently cooled to its present state. 
In support of the preceding hypothesis is the fact that 61 of the 91 
known chemical elements that enter into the constitution of the earth 
have been identified, with more or less certainty, in the sun (2). Of 
particular interest are the elements—helium, carbon, oxygen, nitro- 
gen, and sulphur in the atomic state, and the compounds—the hy- 
droxyl (OH); ammonia, or some other compound of nitrogen and 
hydrogen; a compound of carbon and nitrogen, probably cyanogen; 
and a compound of carbon and hydrogen (3). Further evidence in sup- 
port of the hypothesis that the primitive earth was molten to the 
surface are the facts that it now acts as a magnet, and that the density 
increases, probably discontinuously, from about 2.7 at the SEU REG to 
a mean value of about 5.5 for the entire earth. 
According to Jeffreys (4), the loss of heat from the liquid surface 
of the earth must have been so rapid that radioactivity was a minor 
source of heat prior to the solidification of the outer shell. As the 
solidified state was approached, feeble convection currents carried off 
the radioactive heat and completed the transfer of the radioactive 
substances toward the surface of the earth. Evidence in substantiation 
of the hypothesis of an increasing radioactive content of the rocks as 
the surface of the earth is approached is contained in the following 
record (4) which gives the contents by weight of Finland granites of 
decreasing age. 
Ra(X 10%) Th(X 10%) K(X107) 
A 2.36 0.87 2.51 
B and C 4.60 2.67 3.61 
D 6.21 5.85 5.06 
It has also been established that volcanic rocks contain more radium 
than plutonic rocks. Another reason for believing that radioactive 
substances are concentrated chiefly in the outer layers of the earth is 
the fact that if the deep layers were as radioactive as the surface 
layers, the rocky shell could never have solidified. 
COOLING AFTER SOLIDIFICATION 
By taking into account radioactivity, heat conduction, and an 
assumed initial temperature distribution based on the increase of the 
melting point per unit depth in the crust of the earth, Adams (6) and 
Jeffreys (7) have estimated the temperature at great depths in the 
crust of the earth (Table I). Their calculations show that cooling may 
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