318 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
It has been objected that the weight of the earth is not great 
enough for much of it to be composed of metallic iron or of meteor- 
itic material. The specific gravity of iron under the pressure at the 
earth’s surface is about 7.7, and it would be even greater when com- 
pressed in the interior. But the barysphere is doubtless impregnated 
with much stony material that would lessen its weight. An estimate 
by Farrington (1897) of the average specific gravity of the meteor- 
ites of which the fall had been recorded is only 3.69. According to 
the Rev. E. Hill (1885), the mean specific gravity of all the meteor- 
ites in the British Museum was 4.5; and, though Mr. Hill duly con- 
sidered the effect of compression, he concluded that “the density of 
the earth is perfectly consistent with its being an aggregation of 
meteoric materials.” Moreover, within the metallic barysphere there 
may be a core of lighter material; for earthquake waves travel more 
slowly in the central core of the earth than in the intermediate zone, 
or are even suppressed altogether there; hence the center of the earth 
may be occupied by matter less compact than that of the shell around 
it; and, according to Oldham’s calculations, the hght central corps oc- 
cupies two-fifths of the diameter of the earth. 
The evidence of density alone, therefore, gives no convincing evi- 
dence of the nature of the earth’s interior; and geologists have been 
left with no conclusive reason for choosing between the condensation 
and segregation theories. Radio-activity has, however, unexpectedly 
come to our aid, and has disclosed a further striking resemblance 
between the internal mass of the earth and the iron meteorites. It 
has supplied direct evidence about the constituents of the earth at 
depths which have hitherto been far beyond the range of observation. 
Mr. Strutt has shown that radium is probably limited within the 
earth to the depth of 45 miles, that the deeper-lying material is free 
from radium, and that this substance is not found in iron meteorites. 
The agreement in radio-active properties between the iron meteor- 
ites and the interior of the earth is an additional and weighty argu- 
ment. in favor of the view that the earth is largely composed of nickel 
iron. 
3. Physical conditions and temperatures.—The physical condition 
in which the material exists is now of secondary interest. The old 
controversy as to whether the earth has a molten interior inclosed 
within a solid shell has lost its importance, because it has become 
a mere matter of definition of terms. The facts which led geologists 
to believe that the interior of the earth is fluid are consistent with 
those which prove that the earth is more rigid than a globe of steel. 
For under the immense pressure within the earth the materials can 
transmit vibrations and resist compression like a solid; but they can 
change their shape as easily as a fluid. They are fluid just as lead is 
when it is forced to flow from a hydraulic press. Not only are geolo- 
