Capillarity

Capillarity (from capillus, a hair) is the cause of various phenomena of surface tension, and exhibited by the behaviour of liquid in fine, hair-like tubes. To explain the nature of surface tension, it must be understood that the particles of a body exert considerable force of attraction on one another, but only at very minute distances. Thus a molecule of water in the middle of a glassful of that liquid is acted on by the mass of congregated molecules in its immediate vicinity, only those enclosed in a very small sphere round the specified particle having any appreciable effect on it. From the symmetry of the arrangement it is clear that there is no resultant pull on the particle in any one direction. But a molecule of water on the surface of the liquid is only acted on by a hemisphere of molecules of water in its neighbourhood, and these exert a resultant pull on the particle at right angles to the surface. It is true there is also a hemisphere of particles of air acting on the molecule of water, but their action is not so great. Consequently we see that all the particles on or near the surface are pulled downwards and therefore cause the surface to act as a sort of elastic membrane or skin, with the important difference that, however extended the surface may be, the force of attraction, or surface tension, is always the same per unit length. Thus the surface of a liquid will assume a definite form, the tendency being to minimise its area as far as circumstances permit. A raindrop falling through air, or a soap-bubble floating in it, will assume the spherical form, the surface of a sphere being less than that of any other solid of the same volume. At the edge of the glass of water, where we have glass, water, and air meeting, the three sets of forces draw the surface up the side to a slight extent. If a glass tube of very fine bore be put vertically in the water, the liquid is drawn up the tube to a definite height, and its surface is markedly concave upwards. If mercury be the liquid used instead of water, opposite effects will be seen, the level of the mercury inside the tube will be lower than that outside, and its surface will be convex upwards. Much may be explained concerning the behaviour of oil on "troubled waters," the motion of sap in plants, the formation of clouds, the shapes of the heavenly bodies, and concerning other physical subjects, by the study of capillarity.