hough it appears from the mathematical theory that
there is a slight increase of tension where the mean curvature of the
surface is concave, and a slight diminution where it is convex. The
amount of this increase and diminution is too small to be directly
measured, though it has a certain theoretical importance in the
explanation of the equilibrium of the superficial layer of the liquid
where it is inclined to the horizon.

2. The surface-tension diminishes as the temperature rises, and when the
temperature reaches that of the critical point at which the distinction
between the liquid and its vapour ceases, it has been observed by
Andrews that the capillary action also vanishes. The early writers on
capillary action supposed that the diminution of capillary action was
due simply to the change of density corresponding to the rise of
temperature, and, therefore, assuming the surface-tension to vary as the
square of the density, they deduced its variations from the observed
dilatation of the liquid by heat. This assumption, however, does not
appear to be verified by the experiments of Brunner and Wolff on the
rise of water in tubes at different temperatures.

3. The tension of the surface separating two liquids which do not mix
cannot be deduced by any known method from the tensions of the surfaces
of the liquids when separately in contact with air.

When the surface is curved, the effect of the surface-tension is to make
the pressure on the concave side exceed the pressure on the convex side
by T (1/R1 + 1/R2), where T is the intensity of the surface-tension and
R1, R2 are the radii of curvature of any two sections normal to the
surface and to each other.

[Illustration: FIG. 3.]

If three fluids which do not mix are in contact with each other, the
three surfaces of separation meet in a line, straight or curved. Let O
(fig. 3) be a point in this line, and let the plane of the paper be
supposed to be normal to the line at the point O. The three angles
between the tangent planes to the three surfaces of separation at the
point O are completely determined by the tensions of the three surfaces.
For if in the triangle abc the side ab is taken so as to represent on a
given scale the tension of the surface of contact of the fluids a and b,
and if the other sides bc and ca are taken so as to represent on the
same scale the tensions of the surfaces between b and c and between c
and a respectively, then the condition of equili