Let us study the attractions acting upon a molecule at different distances from the surface, as shown in Fig. 1. At A the molecule is attracted equally in all directions by the molecules that are within the distance of molecular attraction (cohesion); hence it can move readily in any direction. At B, very near the surface, the horizontal attractions are equal in all directions, but the inward (downward) attraction is greater than the outward (upward). At the surface the molecule C has no outward attraction, and hence it is held in place by the inward force.
As this is true of every molecule on the surface, the result is a tension upon the surface layer much greater than upon any other layer. The inward attraction tends to pull each molecule in from the surface layer, but if a molecule were drawn into the interior of the liquid, it would displace some other molecule and force it to the surface against a similar attraction; so there is no change unless the shape of the liquid body can be changed so as to decrease the area of the surface. The surface tension causes a tendency of the surface to contract to the smallest area possible. This is the reason why liquids take the spherical ,a sphere has a smaller surface than any other form of solid of the same volume.
Surface tension varies with the liquid and with the temperature of the liquid. The surface tension of pure water, which is very great, compared with that of most other liquids, is illustrated by the following:
Demonstrations. - Pour some hot water into a shallow dish, like a soup plate. Cover the surface with pepper. Hold a small piece of butter in the surface of the water at the middle, and observe how the pepper goes away from the melting butter to the sides of the plate Spread a thin layer of clean water upon a clean glass plate, and then let a drop of alcohol fall upon the middle of it. The water will at once retreat, leaving a space around the drop of alcohol. Why?
Viscous liquids are stronger than water though their surface tension is less, and for this reason oil is sometimes thrown upon the water around a ship during a storm. The effect of this is to smooth out the surface as though a strong elastic blanket were stretched over the water; and the waves are then kept from breaking over.
A drop of kerosene placed upon water has less surface tension than the water and hence is pulled out by the tension of the water into a thin circular film.
