128 
rorULAR SCIENCE REVIEW. 
This hypothesis is purely theoretical, and in no way supported 
hy direct observation. It originated in the assumption that the 
central parts of the globe cannot consist of such substances as 
we find in its crust, as otherwise the condensation of such bodies 
under the accumulating pressure acting towards the centre 
would cause the globe to possess a far greater mean density 
than 5-^ times that of water, which in actuality we know is the 
case. 
This assumption is based entirely upon the supposition that 
bodies become more dense in direct ratio to the pressure to 
which they are subjected ; according to which idea, air at a 
depth of about 80 miles below the surface should be as dense as 
water, which in its turn at some 360 miles’ depth should be as 
heavy as mercury; and a solid like clay, which at the surface 
weighs about 125 pounds per cubic foot, ought to become so 
much condensed that a cubic foot would there weigh above 
G tons. It is for this reason contended that the central mass 
must consist of matter of extreme lightness (at the surface), 
such as gases which, upon being subject to such enormous 
pressure in the centre of the earth, could not assume a greater 
density than would fulfil the required conditions, as above 
explained. 
The experimental investigations which have been made into 
the compressibility of substances do not, however, prove any 
such unlimited rate of condensation, and demonstrate that very 
soon a point of what may be termed approximative maximum 
density is attained, beyond which the effects of pressure become 
so much smaller and smaller in relation to the force applied, as 
at last to become almost inappreciable. As a proof of how 
little the effects of great pressure have been understood, it need 
only be remembered that until lately it was a commonly 
received opinion that, owing to the pressure exerted by the 
column of supernatant water, no animals, even of the lowest 
type, could possibly exist in the great depths of the ocean ; and 
it was even advanced that any soft muddy deposits would, from 
the same cause, be consolidated into beds of compact shales, or 
even rocks. It required, however, only a few deep-sea sound- 
ings and casts of the dredge in the depths of the North Atlantic 
to dispel such illations, by bringing up abundance of soft and 
slimy deposits replete with animal life. 
The study of geological phenomena does not in any way 
countenance the idea of sucli a great body of compressed gases 
being imprisoned in the interior of our sphere, and, whilst the 
evidence of great internal heat is totally at variance with such 
a conclusion, experimental researches upon the compressibility 
of gases, as far as they have gone, are in direct opposition, 
since they tend to support the view that the gaseous form of 
