Prof. Challis on a Theory of Molecular Forces. 97 
the Himalaya range a large solid mass is plunged into molten 
liquid of greater specific gravity, is liable to objection, unless it can 
be shown that the distinction between the solid and fluid states, 
and any difference of density, can exist under the pressure of 
the mountain mass, at the depth which the explanation requires. 
I have suggested a different explanation of the anomaly in the 
article on the Force of Gravity. 
If this theory of the internal molecular condition of solids be 
true, there must be limits to the heights and acclivities of moun- 
tains, and to the depths of ocean-basins, depending on the 
energy of the superficial molecular attraction. The separation 
of large masses into parts by faults and fissures, by increasing 
the quantity of containing surfaces, probably renders a greater 
amount of superficial irregularity possible. If these irregu- 
larities and the effect of centrifugal force be disregarded, large 
bodies, like the sun and planets, would, according to the theory, 
take the form of a sphere. The form of a very thin plate, like 
that of Saturn’s Rings, is also consistent with the theory: but 
it does not appear that any form very unlike these two would be 
possible. 
5. Passing now to the consideration of the gaseous state of a 
substance, the first remark to make is, that since the mean 
interval between the atoms is much larger than in solids and 
fluids, the atomic repulsion, which varies very rapidly with di- 
stance, may be supposed to be of insensible magnitude. Also 
the condensations and rarefactions of the waves propagated from 
a single atom, so far as they are dependent on the number of 
atoms in a given space, will be much diminished in the aériform 
state. At the same time the radius R of the spherical surface 
which includes the fixed number of atoms N, must be much 
greater, and the distance D at which the waves from the atoms 
merge into waves of the second order be proportionally in- 
ereased. Hence the condensation and velocity in the waves of 
this order originating in the molecule of radius R, may never 
rise to such a magnitude as to satisfy the condition of producing 
excursions of the ztherial particles large compared to the dia- 
meter of an atom. Consequently they will be throughout waves 
of repulsion, until they merge into those which act as gravity. 
The tendency of aériform substances to expand is in this manner 
accounted for by the theory. 
By the aid of this theory, it is also conceivable that a gas, by 
being greatly compressed, so that its atoms are brought into 
such proximity that molecular attraction begins to act, may be 
converted into a liquid. Another property of gases is also simply 
explained by the theory, viz. the facility with which the atoms 
of one gas permeate another. The comparatively large intervals 
Phil. Mag. 8. 4, Vol. 19. No, 125. Feb, 1869. 
