ORIGIN OF THE EARTH—PAGE 165 
Such a stratification within the earth might have a bearing on the 
original conditions. For instance, if the earth were once molten, we 
might expect heavier materials to sink to the center and lighter ones 
to come to the surface. A variety of measurements do prove that 
the earth is much more dense at the core than at the surface, and this 
central condensation was long used to support the concept of an 
originally molten globe. In fact, the central core itself was generally 
believed still to be molten. But a few years ago, observations of 
faint earthquake waves which could only have passed through the 
core if it were solid, disputed the point. 
It is now generally accepted that the earth’s interior is stratified in 
three distinct layers on a central core which is four times as dense as 
the surface rocks, and although probably as solid throughout as 
surface rocks, it yields to plastic flow over long intervals of time. 
(The molten lava of volcanoes is only in local pools liquefied by a 
temporary release of pressure.) Recent work by geochemists shows 
that at least some of the stratification is due to chemical compaction, 
the tremendous pressures favoring the formation of heavier chemical 
compounds in the interior. There is no satisfactory explanation of 
the dense core, which must be a material radically different from 
surface rock. But its existence can no longer be used with certainty 
to argue that the earth was once molten. 
CHEMICAL CLUES 
Geochemical studies give a somewhat better clue to the earth’s 
temperature at birth. Harrison Brown at Chicago has recently shown 
that all the elements which exist mainly in gaseous form—hydrogen, 
helium, neon, argon, krypton, xenon—occur in the earth, its seas and 
atmosphere, to a very much smaller extent than expected from studies 
of the abundances of elements, both from theory and from observations 
of the sun and stars. 
The low abundance of hydrogen and helium is easy to understand: 
at temperatures of 500 to 600 degrees Centigrade they would escape 
from the gravitational attraction of the earth in a few hundred million 
years because of the high velocities and small masses of their molecules. 
But the heavier atoms, krypton and xenon, could have escaped in 
quantity only if the material of the earth were at one time in much 
smaller pieces, with correspondingly smaller gravitational attraction, 
or if the earth had for some time a temperature of 10,000 to 30,000° C. 
Now this is hotter than most stars, and quite impossible for the earth 
to maintain, so we deduce that early in its history the material of the 
earth was in separate, small pieces. Since oxygen, nitrogen, and 
water-vapor molecules are all lighter than krypton (and would there- 
fore escape if krypton did), it appears that the earth’s atmosphere and 
