THE NATURE OF THE INTERIOR OF THE EARTH. 
129 
matter is not compatible with such enormously high pressures, 
for even by the comparatively low pressures at the experimen- 
ter’s command, many of the gaseous bodies have already been 
condensed into the liquid or even solid form. 
Having now entered pretty fully into the consideration of 
the physical character of the interior of the earth, it may be 
enquired as to whether any light can be thrown upon the 
chemical nature of the materials which it consists of. It is to be 
feared, however, that this problem is even more difficult of solu- 
tion, for excepting the proof afforded by the matter emitted 
from volcanic orifices — which is in greatest part composed of 
silicates of the oxides along with some compounds of sulphur, 
boron, selenium, &c. — we have no means of direct examination 
whatsoever. 
The consideration of the specific gravity of the earth affords 
some opportunity, however, for speculative enquiry into this 
subject. As is known, the mean density of the earth’s mass is 
about times that of water, whilst the average of such parts 
of its exterior as we are acquainted with is reckoned at only 
about 2 J ; it follows, therefore, that the central parts must be 
infinitely more heavy, in order to account for its mean total 
density of 5J. 
It has been calculated that if the earth was composed of 3 
concentric portions of equal thickness and of densities respec- 
tively increasing towards the centre in arithmetical progression, 
we should have — an outer crust, as before stated, of specific 
gravity ; an intermediate zone of about 12 ; and a central 
nucleus of about 20 times the density of water ; whilst, if we 
were to imagine more than 3 zones, it would follow that the 
central nucleus would be found still denser in proportion as 
more zones are conceived. 
As before remarked, the old idea that such great increase in 
density can be due merely to the effects of superincumbent 
pressure is not borne out by the results of experiment, and 
further appears manifestly inadequate, when we also take into 
account the counteracting effects of the expansion produced by 
the earth’s internal heat; it would follow, therefore, that the 
substances forming the interior of the earth must in themselves 
be of a much denser nature than the generality of the bodies 
which we meet with at its surface. 
Of all the elementary bodies recognised by the chemists, it is 
only some few of the heavy metals which at all approach in 
density that of either the nucleus or intermediate zone, as already 
calculated, and consequently it has been inferred that it requires 
not only the assumption that bodies do become very consider- 
ably denser when subjected to pressure, but that there must 
also be a great accumulation of the heavy metals and their 
