Surface Tension
Collier's New Encyclopedia (1921)
in liquids, that property in virtue of which a liquid surface behaves as if it were a stretched elastic membrane-say a sheet of india- The idea is due to Segner rubber. (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. Pure water has the highest surface tension of any ordinary liquid except mercury. If a little alcohol be dropped on the wate 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 till the surface is reduced to uniformity, 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 produced 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 tension 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 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.
What Has Changed in the Last Century
The physics Collier's set out in 1921 is still the physics taught today: Thomas Young's 1805 insight and the Young-Laplace equation, filled out by Laplace and Gauss, remain the working tools, and water's tension of about 72 millinewtons per metre at 20°C against mercury's roughly 485 still bears out the claim that water is the highest of the ordinary liquids. What the entry could not foresee was the industry built on lowering that tension: Irving Langmuir, whose monolayer studies began in the years just before this was printed, won the 1932 Nobel Prize in Chemistry for surface science, and synthetic surfactants now underpin detergents, emulsions, inkjet printing and microfluidics. Medicine found its own case: the discovery that lung alveoli are lined with a natural surfactant led to replacement therapy that transformed survival for premature infants from the 1980s onward. The one detail a modern text would put differently is the ink-drop vortex ring, now explained mainly by the vorticity the falling drop's own momentum generates rather than by surface tension pulling its underside outward.
Written for this edition, 2026.