A ah 4 PRESIDENTIAL ADDRESS. 345 
When our ideas of the state of affairs below the surface thus begin to yield 
economic results, there is hope that they are at last steadying down, becoming 
more settled, and indeed more ‘ scientific.’ It may not be unprofitable, therefore, 
to review some of the advances recently made in developing theoretical concep. 
tions regarding the interior of the Earth that are of direct importance to 
geologists. In undertaking this review I am conscious of the fact that I shall 
be traversing ground that is generally familiar to all, and much of it the special 
property of ‘specialists whose views I hesitate to summarise and should not dare 
to criticise. As the author of the ‘Ingoldsby Legends’ said of the only story 
that Mrs, Peters would allow her husband to finish, ‘The subject, I fear me, 
is not over new, but will remind my friends— 
“Of something better they have seen before.” ’ 
The intensity and quantity of polemical literature on scientific problems 
frequently varies inversely as the number of direct observations on which the 
discussions are based: the number and variety of theories concerning a subject 
thus often form a coefficient of our ignorance. Beyond the superficial observa- 
tions, direct and indirect, made by geologists, not extending below about one 
two-hundredth of the Earth’s radius, we have to trust to the deductions of 
mathematicians for our ideas regarding the interior of the Earth; and they 
have provided us successively with every permutation and combination possible 
of the three physical states of matter—solid, liquid, and gaseous. 
Starting, say, two centuries back with the astronomer Halley, geologists were 
presented with’ a globe whose shell rotated at a rate different from that of its 
core. In more recent times this idea has been revived by Sir F. J. Evans (1878) 
to account for the secular variations in the declination of the magnetic needle, 
Clairault’s celebrated theorem (1743), on which Laplace based the most 
long-lived among many cosmogonies, gave us a globe of molten matter sur- 
rounded by a solid crust. Hopkins demanded a globe solid to the core, and, 
though his arguments were considered to be unsound, his conclusions have been 
revived on other grounds; while the high rigidity of the Earth as a body has 
been maintained by Lord Kelvin, Sir George Darwin, Professor Newcombe, 
Dr. Rudski, and especially by the recent observaticns of Dr. O. Hecker, supple- 
mented by the mathematical reasoning of Professor A. E. H. Love. Hennessy 
(1886), however, concluded that the astronomical demands could be satisfied by 
the old-fashioned molten Earth in which the heavier substances conformed to 
the equatorial belt. 
As long ago as 1858 Herbert Spencer suggested that, on account of its tem- 
perature being probably above the critical temperature of known elements, the 
centre of the Earth is possibly gaseous. Late in the ’seventies Dr. Ritter revived 
the idea of a gaseous core surrounded by a solid crust, and this was modified in 
1900 by the Swedish philosopher, Svante Arrhenius, whose globe with a solid 
crust, liquid substratum, and gaseous core is now a favourite among some 
geologists. 
Wiechert (1897) supposed that the core of the Earth, some 5,000 kilométres 
in radius, is composed mostly of iron with a density of 7:8, while this is sur- 
rounded by a shell of lithoidal material having a density of about 3:0 to 3:4; 
and this great contrast in density is about that which distinguishes the iron 
meteorites generally from those of the stony class. Arrhenius also assumes that 
iron forms the main part of the central three-quarters, and he shows that this 
distribution of substance may still be consistent with his theory of a gaseous 
core ; indeed, he not only imagines that the whole of the iron nucleus is gaseous, 
but also most of the siliceous shell, for he leaves only 5 per cent. of the 
radius as the depth of the solid and liquid shells combined. 
But the variety of ideas does not end with theories on the present constitution 
of the globe. Poisson required the process of solidification to begin from the 
centre and to progress outwards, while other mathematicians had been happy 
with the Leibnitzian consistentior status as the first external slaggy crust. 
Since the days of Laplace all naturalists have been forced to accept the idea of 
a solar system formed by the cooling and condensation of a spheroidal gaseous 
nebula; and all except those geologists who have vainly searched for traces 
of the primeval crust have been happy in this belief. 
Recently, however, Dr. F. R. Moulton and Professor T. C, Chamberlin 
. 
