94 Prof. Challis on a Theory of Molecular Forces, 
of a cube in order that they may fill the space, then by the same 
reasoning as before, at a distance D! from the centre of the sphere 
! 
such that aoe 
waves of the ¢hird order. It is to waves of this order that the 
force of gravity may be attributed. Also the absorption of the 
second order of waves into the third, puts a limit to the sphere 
of activity of the second order of repulsion. 
As an illustration of the formation of the waves of the second 
order was drawn from what is observed to take place at the 
surface of water in consequence of its being disturbed through 
a limited extent, so the third order of waves are analogous to the 
ocean-swell, or series of long waves, which have been observed 
on shores at great distances from parts of the ocean which have 
been agitated by a violent storm. 
I have elsewhere made the remark, that even the attraction of 
gravitation may, according to these views, be changed by distance 
ito repulsion, so that neighbouring stars may be repulsive to 
each other, while at the same time this repulsion is counteracted 
by an attraction resulting in the manner above described from 
the composition of the waves propagated from all the other more 
distant stars. Thus the final waves may be said to be of the 
fourth order, and the masses of stars and planets may be regarded 
as molecules relatively to the material system of the universe. 
4, The above considerations respecting molecular forces apply 
equally to a mass in a fluid state, the number of atoms ina given 
space being not so different in the fluid and solid states of the 
same substance as to render any difference in the reasoning 
necessary. But experience shows that the molecular attraction 
of a fluid mass is much less powerful than that of the same mass 
when solid. This difference, which theoretically corresponds to 
a difference in the relative magnitudes of yw and gq, is chiefly ex- 
hibited in the different circumstances of the equilibrium of the 
atoms at the boundary of the mass, on which, in fact, the differ- 
ence between the solid and fluid states essentially depends. If 
we take an atom in the interior of a uniform mass, and regard 
only the action of forces having very small spheres of activity, 
it is evident that whether the mass be solid or fluid, the repulsions 
to which the atom is subject will counteract each other, as will 
also the attractions. But the case will be different if the atom 
be situated at the boundary of the mass; for there, to maintain 
its equilibrium, the resultant of the attractions must be just equal 
and opposite to the resultant of the repulsions. This point I 
have considered at length in an article “ On Capillary Attraction 
and the Molecular Forces of Fluids,” communicated to the 
Philosophical Magazine for February 1836, on the suppositions 
the waves of the second order will merge into 
