Laplace, assuming molecular force to be insensible at distances greater than a small quantity a, finds the resultant molecular force on a unit particle at a distance x within the (plane) surface. This being called X, the internal pressure is

where p is the density of the liquid. But this is evidently the work required to take unit volume of the liquid (particle by particle) from the interior to the surface. And it is easily seen that to carry it from the surface beyond the range of the molecular forces requires just as much more work:—for the density of the surface-film is treated as equal to that of the rest of the liquid.