PRESIDENTIAL ADDRESS. 4.95 
was 4:5; and, though Mr. Hill duly considered the eflect 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 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 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 meteorites and the 
interior of the earth is an additional and weighty argument in favour cf 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 con- 
sistent 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 temperature of 
only 1500° C. Whether the further conclusion, that this heat is due to the 
action of the radium in the crust, be established or not, it is gratifying to hear a 
physicist arguing in favour of a moderate and uniform internal temperature. 
All that the actual observations prove and that geological theories require is 
that the material within the earth be intensely hot, and that it lie under such 
overwhelming pressure that it would as readily change its form and as quickly 
fill up an accessible cavity as any liquid would do. Whether such a condition is 
to be described as solid, liquid, or gaseous is of little concern to geologists. 
