=22. Surface Tension.=--The cohesion of liquids is also indicated by the tendency of films to assume the smallest possible surface. Soap bubble films show this readily. Fig. 10 _a_ represents a circular wire form holding a film in which floats a loop of thread. The tension of the film is shown in Fig. 10 _b_ by the circular form of the loop after the film within it has been pierced by a hot wire, Fig. 11 shows a rectangular wire form with a "rider." The tension in the film draws the rider forward. [Illustration: FIG. 11.--The rider is drawn forward.] [Illustration: FIG. 12.--Surface tension causes the pointed shape.] A soap bubble takes its spherical shape because this form holds the confined air within the smallest possible surface. A drop of liquid is spherical for the same reason. Many illustrations of the tension in films may be given. Users of water colors notice that a dry camel's-hair brush is bushy. (Fig. 12 _A_). When in water it is still bushy. (Fig. 12 _B_.) But when it is taken from the water and the excess is shaken from it, it is pointed as in Fig. 12 _C_. It is held to the pointed shape by the tension of the liquid film about the brush. [Illustration: FIG. 13.--A needle depresses the surface when floating.] The surface of water acts as if covered by a film which coheres more strongly than the water beneath it. This is shown by the fact that a steel needle or a thin strip of metal may be floated upon the surface of water. It is supported by the surface film. (See Fig. 13.) If the film breaks the needle sinks. This film also supports the little water bugs seen running over the surface of a quiet pond in summer. The surface film is stronger in some liquids than in others. This may be shown by taking water, colored so that it can be seen, placing a thin layer of it on a white surface and dropping alcohol upon it. Wherever the alcohol drops, the water is seen to pull away from it, leaving a bare space over which the alcohol has been spread. This indicates that the alcohol has the weaker film. The _film of greasy benzine is stronger_ than the film of the pure material. If one wishes to remove a grease spot and places pure benzine at the center of the spot, the stronger film of the greasy liquid will pull away from the pure benzine, and spread out, making a larger spot than before, while if pure benzine is placed _around the grease spot_, the greasy liquid at the center pulls away from the pure benzine, drawing more and more to the center, where it may be wiped up and the grease entirely removed. [Illustration: FIG. 14.--The molecule at _A_ is held differently from one within the liquid.]