SURFACE TENSION. The title under which many surface phenomena—including Capillarity—are usually considered.
When the surface of water (or other liquid) in a tank is care fully examined it is found not to be perfectly level. The state ment that water finds its own level is only approximate. The surface in fact becomes considerably curved near its edges where the liquid comes in contact with the wall of the tank.
If a vertical tube with a very narrow bore is placed with one end in the water, the liquid rises some distance above the level of the outer surface. The rise is greater the smaller the diameter of the bore. The rise of oil through a wick is a phenomenon of the same kind.
If a small quantity of mercury is poured on a horizontal plate it forms a drop. If the quantity is only a few cubic millimetres the drop is nearly spherical—a much larger quantity spreads out into a cake-shaped mass with a nearly flat top and with rounded edges.
A drop of oil placed on a clean water surface spreads almost instantaneously so as to form a very thin film. The thinness of the film can be judged from the colours that flash out (see INTERFERENCE) or from the size of the area covered by even a small drop.
The phenomena of soap bubbles and the formation of froth in solutions of soap are also familiar ones. In these cases, also, thin films are formed.
The propagation of ripples of very short wave-length follows a different law from the propagation of deep sea waves.
When camphor is scraped so that the fragments fall upon a clean surface of water, they rush about on the surface with very rapid and rapidly changing motion. A minute quantity of oil placed on the surface brings them almost instantaneously to rest.
These and many allied phenomena, can all be studied under one heading. They arise from the existence of surfaces separating one medium from another. The special forces which come into play are thence known as Surface forces. The rise in tubes of narrow bore—which are known as capillary tubes because the bore is as "fine as a hair"—is due to these forces. From the special
way in which the effects arise in this case the action is called capillary action. This name, though often applied in other cases, is hardly applicable except to the case of narrow tubes ; while the term surface action is applicable in all. The forces which are con cerned in these phenomena are those which act between neigh bouring parts of substances. These also produce the effects of cohesion. Newton in the third edition of his Opticks refers to them in the following passage :—"The parts of all homogeneal hard Bodies, which fully touch one another, stick together very strongly . . . I . . . infer from their Cohesion that their Par ticles attract one another by some force, which in immediate Con tact is exceeding strong, at small distances performs the chymical Operations above mention'd, and reaches not far from the par ticles with any sensible Effect. . . . There are therefore Agents in Nature able to make the Particles of Bodies stick together by very strong Attractions. And it is the Business of experimental Philosophy to find them out." These forces must be distinguished from those of gravity which also act between the particles of matter because these latter can act over very great distances. (The earth is controlled in its motion round the sun by gravitation.) Cohesion acts to any measurable extent only over minute distances. Sticks of chalk for drawing are made by highly compressing finely ground chalk and other materials. If a stick is broken and the two parts are brought into their original position as nearly as possible by hand they do not stick together because the neighbouring particles are still so far apart as to exert no sensible attraction. High pressure is needed for them to begin sensibly to attract each other. Before considering in detail how these attractions arise we give a his torical summary of the development of the subject.