96 Prof. Challis on a Theory of Molecular Forces. 
repulsion, extending, as above stated, to small distances from the 
surface, it is conceivable that mere mechanical compression of 
fragments together, by acting in aid of the molecular attraction, 
may suffice entirely to get rid of the gradation of density, and 
thus to effect a perfect union. The very important and instruct- 
ive experiments of Professors Tyndall and Huxley, detailed in 
the Transactions of the Royal Society (vol. cxlvii. pp. 829-331), 
are actual instances of the production of this effect by crushing 
together fragments of ice. 
As a theory of forces, of the nature of that which I am advo- 
cating, can be expected to be established only by the number 
and variety of the explanations of physical phenomena which it 
gives, I take this opportunity of remarking that the foregomg 
molecular theory, taken in conjunction with the experiments just 
referred to, seems to afford a simple explanation of some of the 
phenomena of glaciers. Both from the experiments and from 
the theory, it may be inferred that the mutual pressures of the 
parts of a glacier are continually tending to obliterate fragmen- 
tary composition and make it a continuous whole ; and as, ac- 
cording to the theory, the interior of a continuous solid mass is 
not different from that of a fluid mass, the glacier has a tendency 
to flow. The strength of its rigid casing, on account of the 
feeble molecular attraction of ice, not sufficing to keep the parts 
in the same relative positions, it flows as a stream, as was experi- 
mentally proved by Professor Forbes. The rigid envelopes can 
accommodate themselves to this motion only by perpetual cracks 
and fissures, longitudinal and transversal, alternating with per- 
petual reunions by pressure, or by filtration and congelation. 
The same theory of the internal condition of solids and fluids, 
accounts for a fact relating to the form of the earth, which other- 
wise seems difficult of explanation. What is the reason that 
being solid it takes the form which allows of a great portion of 
its surface to be covered to a comparatively small depth with a 
fluid? The answer which the theory gives to this question is, 
that the mass of the earth, taken as a whole, must be regarded 
as a fluid in the mathematical investigation of its form, and the 
rigidity of the superficial crust only accounts for local elevations 
and depressions, without having sensible influence on the general 
form. The effect of internal pressure would cause the distinction 
between solidity and fluidity to cease, probably at no great depth ; 
and consequently any theoretical investigation which admits a dif- 
ference between solid and fluid parts at considerable depths below 
the surface, would seem to be inconsistent with the laws of mole- 
cular forces. For this reason also the explanation which the 
Astronomer Royal has offered of the anomalous deviation of the 
plumb-line in India, by making the special hypothesis that under 
