Surface-tension

Surface-tension, in liquids, is that property in virtue of which a liquid surface behaves as if it were a stretched elastic membrane—say a sheet of india-rubber. We owe the idea to Segner (1751); but it was Young who, in 1805, first applied it successfully to the explanation of various physical phenomena, such as those of capillarity. The whole subject was subsequently developed in its complete mathematical form by Laplace and Gauss. The general description of the meaning of surface-tension has been given under CAPILLARITY. Here we shall refer to a few other phenomena, which require for their elucidation the assumption of a tension existing in liquid surfaces. Pure water has the highest surface-tension of any ordinary liquid except mercury. If a little alcohol be dropped on the water, the surface-tension will be diminished there. The more powerful surface-tension over the pure water will show its superiority by pulling the alcohol over the whole surface until the surface is reduced to uniformity, and equilibrium produced. Again, a piece of camphor placed on water will dart from place to place in the most capricious manner. This is due to the irregular way in which the camphor dissolves in the water, so that the surface-tension is more weakened on one side than on the other. It is the action of surface-tension that draws out all over the surface any impurities that may settle on it; and hence arises the great difficulty of getting a clean water or mercury surface. Drops of liquid, free from all but their own molecular forces, assume spherical forms, this being the only shape consistent with equilibrium under the influence of equal surface-tension at all parts of the curved surface. Ripples on the surface of any liquid progress because of the action of surface-tension, which gives rise to an inward pressure on any convex surface. The laws governing the propagation of ripples on the surface of mercury have been recently studied by Professor C. Michie Smith and Lord Rayleigh. Some very instructive experiments may be made with soap-films (see SOAP-BUBBLES); while in cohesion figures we have some very exquisite phenomena, which have been closely studied by Tomlinson. These are produced by dropping a dark-coloured liquid into a transparent liquid of slightly smaller density. Ordinary ink dropped into water will serve the purpose very well, although better effects are obtained with a solution of permanganate of potash. As the drop meets the water-surface, the action of the surface-tension pulls the under surface of the drop outwards, and transforms the drop into a vortex ring, which slowly sinks through the clear fluid. As it so sinks it breaks up into smaller rings and shoots out fantastic ramifications of rare beauty. Ultimately, of course, under the influence of diffusion, the vortex motion decays and the dark liquid mixes with the clear liquid.