298 
NATCURE 
[ Feb. 9, 1871 

So stood the matter until the summer of 1868, when, fortu- 
nitely for the advancement of this inquiry, M. Delaunay, now 
D rector of the Observatory at Paris, an authority equally emi- 
nent asa mathematican and an astronomer, was induced to un ler- 
take the reconsideration of this problem ; a labour which has 
not only resulted in altogether reversing the above decision, and 
demonstrating the complete fullacy of the premisses upon which 
so much elaborate reasoning ha'l been expended, but which proved 
conclusively by experiment that a sphere filled with liquid matter 
would, under circumstances such as are present in the case of the 
earth, behave in precisely the same manner as an entirely solid 
one, and, consequently, that the fact of the earth being either 
solid or liquid in its interior could neither have any influence 
whatever on the rate of precession or nutation, nor be of any 
use as a means of deciding as to the real or approximative 
thickness of the earth’s crust. 
It may also be added that the conclusions arrived at by Mr. 
Hopkins, even when supported by Sir William Thomson anil 
Archdeacon Pratt, were not universally acquiesced in ; the cele- 
brated German physicist, Helmholtz, amongst others, was not 
satisfied as to their correctness, and in opp sition to the deduc- 
tions of Sir William Thomson that the earth’s crust must be some 
1,009 miles in thickness, we have the entirely opposite con- 
clusions of Mr. Henessey, whose calculations tend to show 
that the earth’s cru-t cannot be less than eighteen miles or more 
than 600 miles in thickness. We may now, however, fairly 
conclude that all the objections as yet advanced from an astro- 
nomical point of view against the theory of the fluid condition 
of the interior of our planet, have been invalidated or explained 
away. 
The only other argument in favour of internal solidity is one 
which bases itself upon the law, announced upon theoretical con- 
siderations by Professor Thomson in 1849, that the fusing points 
of bodies must become more elevated when subjected to pressure, 
or, in other words, that under the influence of pressure, bodies 
will require more heat to melt them. 
Starting from this, Bunsen argued that the earth could not be 
other than solid to the core, since the enormous pressure accu- 
mulated at the centre would cause its internal substance to become 
so infusible that it could not remain ina molten state. To a 
certain extent this law was corroborated by the experimental 
researches of Bunsen and Hopkins, made upon some of the easily 
fusible substances like wax, spermaceti, paraffin, and sulphur; but 
as faras the later experiments went, it was not confirmed either 
in the case of metallic substances, nor did it appear to hold true 
with other than the more easily compressible bodies. 
In the case of the earth, therefore, the conclusions of 
Bunsen cannot be accepted, since we have to deal with materials 
to which, as yet, this law has not been proved to apply ; still, 
assuming, as seems most probable, that the materials composing 
the earth’s mass do become to some extent more and more ia- 
fusible according as they approach nearer to its centre, it must, 
on the other hand, be remembered that this would be more or 
less neutralised by the expansion which these substances would 
undergo from the action of the internal heat ; and as incontrover- 
tible evidence has been produced to prove that the temperature 
of the earth downwards from the surface increases in direct 
ratio with the depth, it seems most probable that the combined 
effects of expansion and elevated temperatures would more than 
counteract any tendency to solidification due to the effects of 
pressure. 0 
Having now taken into consideration the various objections 
which have been urged against the theory of the earth’s internal 
fluidity, as wellas devoted some consideration to the opposing 
view of its solidity, it will be noticed, if we pass in review some 
o* the distinctive features of the two hypotheses, that the former 
theory is a legitimate deduction from the data afforded by the 
direct study of the earth itself, whereas the latter, on the con- 
trary, instead of making the explanation of the earth’s phenomena 
its starting point, devotes itself almost exclusively to the task of 
proving that it could not be fluid. 
Thus, how is it possible, if the earth’s mass be solid through- 
out, to account for the great upheavals or sinkings down of large 
portions of the rock formations which compose its external crust? 
Do not these phenomena lead to the direct inference that the ex- 
ternal crust cannot, by any possibility, rest in depth upon an un- 
yielding mass o! matter in the solid state, but that it must 
necessarily be superposed upon some more or less fluid substance 
which by its mobility can, when some one portion of the crust 
above sinks down, become displaced, and so make room for it 

by elevating, or, as it were, floating up some other part of the 
same? 
In like manner the hypothesis that the earth is essentially solid 
necessitated that the phenomena of volcanoes should be explained 
upon the supposition that they had cheir sources in numerous 
small isolated local basins of molten rock scattered over the sur- 
face o the globe: a view which is altogether inconsistent with 
the resu'ts of chemical and mineralogical investization, which 
proves that the ejected products are identical in constitution even 
if taken from volcanic ven’s the most distant from one another, 
nor does such a theory attempt to explain the tidal phenomena 
of volcanic outbursts and earthquakes previously alluded to. 
So far, therefore, as we have gone into this subject, we may re- 
gard the balance of evidence as proving that at a depth of about 
filty miles or less from the surface, there exists a continuous zone 
of molten rock or lava, such as is brought up to the surface by 
volcanic eruptions, Let us now consider how deep this zone or 
stratum of molten matter is likely to extend, and also what forms 
the more central mass of the earth below it. 
In order to answer these questions we must look to other than 
direct evidence, and first of all must inquire whether the con- 
sideration of the mean density, or in other words the actual 
weight, of the earth itself, can throw any light upon these ab- 
struse points. The consideration of the attraction which bodies 
exert upon one another in the ratio of their magnitude, has 
enabled the physicist to effect the at first sight apparently im- 
practicable task of determining the entire weight of the earth 
itself, but it is out of our province to describe the mode of doing 
so, and we must content ourselves by accepting as a fact the re- 
sults of such investigations, which prove that the total weight of 
our planet is as near as possible 54 times the weight of a similar 
globe of pure water. Knowing now that the mean density, or 
specific gravity, as it is also called, of the earth, is 53, and 
also from direct experiment that the mean density of the entire 
solid rock forming its external crust cannot be higher than 2}, or 
less than half that of the entire sphere, it naturally follows that 
the central parts must be very much more heavy in order to 
account for so high a mean figure as 53, and it has been calcu- 
lated that if we suppose that the earth was composed of three 
concentric portions of equal thickness, each in turn increasing 
in density towards the centre in arithmetical progression, we 
should then have an outer circle of specific gravity 2}, or as 
heavy as rock, an intermediate zone of 12, or as heavy as quick- 
silver, and a central nucleus of about twenty times tne density of 
water, or as heavy as gold. 
This increase of density has sometimes been erroneously re- 
presented as entirely due to the effects of the enormous pressure 
of the superincumbent mass ; but thissupposition is quite untenable, 
since thre tendency of all the numerous experiments made in this 
direction has been to prove that no substances can be compressed 
or condensed to an indefinite extent, since what may be termed 
their approximative maximum density is soon attained, beyond 
which the effects of pressure become so much smaller and smaller 
in proportion to the force applied, that at last the further con- 
densation effected by still greater pressure is all but inappreciable. 
Besides this, it must not be forgot that the crust of the earth is a 
species of dome like the shell of an egg, which supports itself 
without resting or floating upon its fluid centre; and further that 
the earth’s high internal heat, by causing the materials which 
compose it to expand, must also counteract the effects of super- 
incumbent pressure, so that when all these facts are taken into 
due consideration, it appears quite evident that the materials 
which actually form the mass of the interior must be infinitely 
denser than any of the rocks met with on the surface, and that 
they must be metallic in their nature, since no other bodies are 
known which could at all fulfil these conditions of density. 
If now we suppose that the earth’s interior is composed of a 
series of concentric zones or layers made up of substances which 
are of more and more dense nature as they are situated nearer 
the centre, and that the external one is rock of a density of 
2°5, a calculation will show that the centre or nucleus will be 
about 10, or as heavy as silver. If now we suppose that the 
zone of molten lava, which we have already concluded must 
exist at a depth of about 50 miles below the suriace, has a 
density of 3, o say even 4, to give the fullest allowance for the 
condensing effects of superincumbent pressure, then we should 
find by calculation that this zone could not extend deeper than 
about 400 miles, since below this depth the matter wonld be so 
heavy that its density can only be explained on the supposition 
that it is made up of metatlic compounds, and as the density of 
