MOSSOTTI ON CAPILLARY PHA NOMENA. 567 
each stratum, since each molecule will be situated in the midst 
of a number of horizontal forces all equal, arising from the 
molecules by which it is surrounded. But the existence of this 
individual equilibrium of the molecules, depending upon their 
uniform distribution, does not imply that the horizontal attrac- 
tion between the different parts of the fluid is zero. On the 
contrary, since we find the distances between the molecules in 
the upper strata greater than when the fluid is in a natural state, 
or greater than in the interior of the fluid where the pressure is 
zero, it follows, by the principles of hydrostatics (see note 1, 
p. 575), that if through any point in the surface we draw a vertical 
pplane, a thread of molecules perpendicular to this plane, situated 
‘in one of the above strata, and equal in length to the distance of 
molecular action, will be attracted toward the plane: hence at 
every point along the surface of the fluid a mutual attraction will 
take place between the parts, and this will produce a sort of force 
ef contraction at the surface, which Segner, Monge and Young 
had indeed foreseen, though they could not accurately explain 
the cause. 
_ 4, Let us now suppose the liquid surface to be limited, and to 
terminate at two opposite extremities in two planes perpendicular 
to it and formed of some solid substance. If the action of either 
of these planes on a horizontal prism of the fluid, equal in 
height to the range of the sensible molecular action, were equal 
to the action of the fluid, it is clear that no change would take 
place near that plane; but in general the action of the plane on 
the fluid is different from that of the fluid.. If it be less, the sur- 
face of the fluid, in consequence of the force of contraction, will 
detach itself from the plane ; and if it be greater, the fluid will be 
attracted and compressed to the plane and will rise along it. Let 
us consider these two cases severally. 
In the first case, the fluid detaching itself from the plane will, 
near the points where it detaches itself, increase its free surface, 
along which an equal force of attraction will successively be pro- 
duced ; and if the action of the solid plane on the liquid be zero, 
the separation of the fluid from the plane would cease when the 
cylindrical and free surface of the fluid had become convex to such 
a degree that the plane should be tangent to it; below that point 
the liquid would remain in juxtaposition with the plane, and would 
possess a force of attraction along it equal to that of its free sur- 
