MR. HENNESSY’S RESEARCHES IN TERRESTRIAL PHYSICS. 
525 
Equation (28.) may be thus written, 
m!^( 
c'— — =a\nl( 
1--) 
\ 
?3/ 
(29.) 
, . m 
making — ; 
from which it is evident that decreases with «i, and consequently 
that the less the volume of the nucleus, the more does it approach the state of homo- 
geneity. 
10. If any grounds existed for believing that experiment could never even ap- 
proximately disclose the laws of density and contraction of the matter composing the 
earth’s interior, we should not be justified in attaching much importance to any hy- 
pothesis, however plausible, which may be formed relative to the forms of these 
laws. Such grounds would partially exist as long as doubts could be formed relative 
to the great or small specific density of the matter of the earth’s interior. The 
variation in density of the spheroidal strata composing the earth, could be due to an 
accumulation near its centre of substances possessing a high specific density. From 
what we know of the matter composing the earth, I believe that almost all substances 
having a density much greater than that of the outer portion of its crust are metals, 
bodies which from their chemical nature do not probably exist to any great extent 
within the globe. Recent experiments tend to show that many metals are but com- 
pound radicals which exist only under certain conditions, and it is probable that the 
constituents of any one of. these radicals have densities less than that of the metal 
itself. Kane, Laurent and Gerhardt are of opinion, for instance, that the known 
metals do not exist in their oxides, but are eliminated when the oxides are decom- 
posed. The metals alluded to are also simple bodies in the sense generally used, 
namely, that of being the lowest terms as yet known of compound bodies. Recent 
chemical researches seem to show that such simple bodies existed in the earth’s pri- 
mitive state, even in less quantity than we can at present discover them*. The 
chemical laws in virtue of which this would be true, would under certain conditions 
act as well in the interior as at the earth’s surface, if we are entitled at all to admit 
their generality, and hence we must conclude that the specific density of the sub- 
stances in the earth’s interior must be subordinate to the effects of mechanical and 
physical causes in producing the observed variation in the density of its strata. 
IV. THE FORMS OF THE STRATA OF THE SHELL. 
11. As every stratum of the shell was originally the surface stratum of the fluid 
nucleus, its form must depend on that of the shell’s inner surface, and of the iso- 
thermal surfaces passing through its poles and equator. At first I shall abstract the 
influence of the isothermal surfaces in order to simplify the problem of the deter- 
mination of the form of the stratum. 
As in art# (L), the shell is here supposed to be rigid, and to be perfectly filled 
* See Bischof, Lehrbuch der Chem. und Phys. Geologie, Bd. I. s. 584, and Bd. II. s. 6. 
MDCCCLI. 3 V 
