Theory of Capillarity. 
341 
surface is in equilibrium with itself, and then considers the 
action between this mass and the surface layers. 
Now to every action there is an equal and opposite reaction, 
and consequently the attraction K downwards of any topmost 
element dz at the plane sui face by the surrounding liquid is 
accompanied by an equal attraction of the surrounding fila- 
ments upwards exercised on portions whose depth is greater 
than dz, i. e. on portions in other respects in equilibrium; and 
since the sum of the actions K downwards in all the filaments 
is equal to the sum of the reactions upwards, it is evident that 
no pressure will be transmitted to the liquid in the canal at a 
sensible depth below the surface, but that the only effect of 
these equal and opposite forces is to create an elastic reaction 
within the liquid. 
This is equivalent to saying that each layer of the liquid of 
less than sensible thickness throughout the liquid clings to 
the next with a cohesive force producing what may be called 
a molecular pressure of the nature of an elastic reaction 
within the liquid. This molecular pressure is not transmitted, 
like an hydrostatic pressure, through the liquid to any sensible 
distance, but at any point has its origin in the molecular 
actions in the immediate neighbourhood of that point. 
Although Laplace consistently adopts the fiction that this 
molecular pressure K is transmitted along any infinitely thin 
canal from the surface and balanced by an equal pressure 
transmitted from the other end, yet he seems to have had a 
perfectly correct view of the physical meaning of the quantity. 
Thus, in his introduction on p. 351, Mecanique Cdeste, 
Supplement au Livre X., he says: — " Je pense que de ce 
terme (K) dependent la suspension du mercure dans un tube 
de barometre a une hauteur deux ou trois fois plus grande que 
celle qui est due a la pression de F atmosphere, le pouvoir re- 
fringent de corps diaphanes, la cohesion, et generalement les 
affinites chimiques." 
Again, in No. 11, p. 391, in n 
considering the pressure trans- 
mitted along an elementary 
canal V S B, to the plane sur- 
face N E, of an immersed solid, 
after pointing out that the pres- 
sure transmitted along S R is 
equal to the external pressure ~ 
P, +K, +<7.VS, he goes on 
to say : — " L'action dont le 
fluide du canal R S est anim£ 
est egale: 1° a Faction du 
