SURFACE TENSION, in liquids, that property in virtue of which a liquid sur face behaves as if it were a stretched elastic membrane—say a sheet of india rubber. The idea is due to Segner (1751) ; but it was Young who in 1805 first applied it successfully to the ex planation of various physical phenom ena, such as those of capillarity. The whole subject was subsequently devel oped in its complete mathematical form by Laplace and Gauss. Pure water has the highest surface tension of any or dinary liquid except mercury. If a little alcohol be dropped on the water, the sur face 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 sur face till the surface is reduced to uni formity, and equilibrium produced. 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. In cohesion figures we have some very exquisite phenomena. These are pro duced by dropping a dark-colored liquid into a transparent liquid of slightly smaller density. Ordinary ink dropped into water will serve the purpose very well, though better effects are obtained with a solution of permanganate of potash. As the drop meets the water surface, the action of the surface ten sion pulls the under surface of the drop outward, 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 fan tastic ramifications of rare beauty. Ul timately, of course, under the influence of diffusion, the vortex motion decays and the dark liquid mixes with the clear liquid.