310 Lord Rayleigh on Laplace's 
the effect of the second term may be represented by the fric- 
tion of a constant tension in the superficial layer. According 
to Laplace's theory, however, the first term K is enormously 
the greater; only, being the same at all points in the interior 
of the fluid, whatever may be the form of the boundary, it 
necessarily escapes direct observation. 
When two liquids are in contact the difference of pressures 
within them will still be of the form (1), but the values of K 
and H will depend upon the properties of both kinds of 
matter. 
The existence of an intense molecular pressure K is a ne- 
cessary part of Laplace's, and probably of any similar, theory 
of these phenomena; but it has not met with universal accept- 
ance*. The difficulty which has been felt appears to depend 
upon an omission in the theory as hitherto presented. Before 
we can speak of K as a molecular pressure proper to the liquid, 
it is necessary to show that the change, which we may denote 
by K 13 , experienced in passing the surface dividing liquid I. 
from liquid III. is identical with the sum of the changes de- 
noted by K 12 and K 23 ; so that it makes no difference whether 
we pass from I. to III. directly or by way of II. That this 
should be the case upon Laplace's principles will be shown 
further on. The point, however, is so important that I pro- 
pose to give in addition a proof of much wider generality, by 
which the relation is placed upon a sound basis. The exist- 
ence of an intense internal pressure is probable for many 
reasons; and it is hoped that no further difficulty need be felt 
in admitting it as a legitimate hypothesis. 
Let us imagine different kinds 
of liquids, varying continuously a 
or discontinuously,to be arranged 
in plane strata, and let us examine 
the difference of pressure, due to p 
the attracting forces, at two points 
A and B, round each of which the q 
fluid is uniform to a distance ex- 
ceeding the range of the forces. 
The difference of pressure in 
crossing any infinitely thin stra- 
turn at P is due to the forces 
operative between P and all the other strata. The force be- 
tween one of the interior strata Q and P will depend upon the 
thicknesses of the strata, upon the nature and condition of the 
fluids composing them, and upon the distance PQ. But what- 
* Quincke, Pogg. Ann. 1870. Also Riley, Phil. Mag. March 1883. 
