416 Notices of Memoirs—Professor J. W. Gregory— 
waves travel more slowly in the central core of the earth than in the 
intermediate zone, or are even suppressed altogether there; hence 
the centre of the earth may be occupied by matter less compact than 
that of the shell around it; and, according to Oldham’s calculations, 
the light central core occupies two-fifths of the diameter of the earth. 
The evidence of density alone, therefore, gives no convincing 
evidence of the nature of the earth’s interior; and geologists have 
been left with no conclusive reason for choosing between the con- 
densation 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 meteorites 
and the interior of the earth is an additional and weighty argument 
in favour 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 
geologists now justified in their belief that the deeper layers of the 
earth’s crust are in a state of fluxion, but, according to Arrhenius 
(1900), the earth is solid only to the depth of 25 miles, below which 
is a liquid zone extending to the depth of 190 miles; and below that 
level, he tells us, ‘‘ the temperature must, without doubt, exceed the 
critical temperature of all known substances, and at this depth the 
liquid magma passes gradually to a gaseous magma.”’ This distinguished 
physicist gives a description of the earth’s interior which reminds 
us of the views of the early geologists. Arrhenius’s theory rests, 
however, on the existence within the earth of exalted temperatures ; 
and this assumption a geologist may now hesitate to accept with less 
risk of getting into disgrace than he would have run a few years ago. 
It is improbable that the rapid increase of heat with depth which is 
observed near the surface should continue below the lithosphere ; for, 
if the earth consists in the main of iron, even although it be arranged 
as a mesh containing silicates in the interspaces, the heat conductivity 
might be sufficient to keep the whole metallic sphere at a nearly 
equal temperature. Here, again, Mr. Strutt’s work on radio-activity 
is in full agreement with the requirements of geologists, for he 
estimates that below a crust 45 miles thick the earth has a uniform 
