MOSSOTTI ON CAPILLARY PHASNOMENA. 575 
Nore 1. 
As the passage of the Lecture on Hydrostatics, which is here mentioned, 
contains the fundamental idea of the equilibrium of fluids, as conceived by 
Poisson, and is the key to the internal mechanism by which the molecular ac- 
tions at a distance act so as to resist external pressures or tensions, I will trans- 
fer it here for the convenience of the reader. 
“ Tf we consider bodies as a collection of molecules that keep one another in 
stable equilibrium at given distances in consequence of forces that are repulsive 
at a small and attractive at a greater distance, and all of which produce a sensi- 
ble action only within the limits of inappreciable distances, fluids differ from 
solids inasmuch as the forces which each molecule exerts on the others are, on 
account probably of the space between them being greater, independent of the 
position of the axes of its figure. These forces consequently act equally all 
round each molecule, and vary only with the distance; and in order that a fluid 
not acted on by external forces may be in equilibrium to a sensible depth 
within it by the action of the molecular forces alone, that is to say in order that 
any molecule whatever may be always in the midst of a number of forces act- 
ing symmetrically, and not be attracted or repulsed more in one direction than 
in another, the molecules must all be uniformly distributed about cach other, 
and consequently the density of the fluid must be uniform. 
“To conceive how in a mass of such a fluid a pressure or a tension can exist, 
let us suppose a plane (fig. 5) drawn through it, 
and upon an element of this plane and perpen- 
dicular to it a small prism of the fluid, of which the 
height be equal to the distance, to which extends 
‘the sensible action of the molecules on the other 
side of the plane. Equilibrium will not be de- 
stroyed if we suppose for an instant that this prism 
become solid. ‘The sum of the actions of the mo- 
lecules on the other side of the plane on the molecules of the small prism will 
vary according as the fluid is in a state of pressure or of tension. If the mo- 
lecules are at such a distance that the repulsive action of the fluid on the other 
side of the plane on the molecules of the prism which are respectively nearest 
be equal to the attractive action of the molecules which are respectively furthest 
from one another, the prism will not be either repulsed from or attracted to- 
wards the plane; and in this case the fluid is in a natural state, not subject to 
any pressure or tension. If the fluid be compressed, its molecules are, however, 
imperceptibly condensed, and since, in consequence of this condensation, the 
repulsive forces between the molecules respectively nearest increase in a greater 
ratio than the attractive forces between the molecules which are respectively 
furthest from one another, the prism will be repulsed; this repulsion resists the 
pressure that tends to push it through the plane, and thus this pressure is 
counteracted by the action of the fluid itself*. If the fluid be acted on bya 
* Those who are acquainted with the differential and integral calculus will here see 
the reason why, in estimating the resultants of the molecular forces, it is not allowable 
to substitute the integrals for the sums of the mutual actions of the molecules. For if we 
were to consider the fluid as a continuous mass, for every increase or diminution of its 
density the resultants of the attractive and repulsive forces on the small prism would all 
increase or diminish in the same proportion, viz. as the square of the density, and there 
would never be an excess of repulsion or attraction to counteract the pressure or ten- 
sion to which the small prisms were subject. This will not be the case if we consider 
the mass as discontinuous and formed of separate molecules. The repulsions and attrac- 
tions of the molecules being functions of their distances, any change in the distances be- 
tween the molecules will have amuch more perceptible effect on the sum of the actions 
of the molecules respectively nearest to each other, which are those that repel one an- 
‘other, than on the sum of the actions of the molecules respectively furthest from each 
other, which are those that attract one another. Consequently the repulsive force act- 
ing on the small prism will be either greater or less than the attractive force, according 
VOL. III. PART XII. a 
