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Page:Popular Science Monthly Volume 35.djvu/623

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THE SURFACE TENSION OF LIQUIDS.

appear swimming over the surface of the mercury (Fig. 7). If, now, we breathe continuously from one side upon the mercury, the "tadpoles" will become more lively, and direct themselves against the breath, coming up to the very edge of the mercury. The breath, driving the vapors bank, clears a space in front of the "tadpole," leaving the tension of the mercury free to act upon it and draw it forward, while it clouds the rear, weakening the tension.

M. Devaux has exemplified the strength and persistence of the tensional force by connecting his camphor-boat with a float in the shape of a watch-glass. The movement of the boat continues, carrying

PSM V35 D623 Experiment of surface tension and weight on water.jpg
Fig. 8.—Tin Boat causing a Loaded Float to go bound with it.

the float around while it is loaded with weights rising to fifty or a hundred grammes, and even to a kilogramme (Fig. 8); and if forcibly stopped, it will begin again when the obstacle is removed.

The phenomena of capillary attraction are explained under the theory of superficial tension. The liquid rises in the tubes by virtue of the adhesion of its superficial membrane to their walls, and to a less height in the larger than in the smaller tubes because the mass of the liquid to be raised increases more rapidly than the power of the membrane to sustain it. Just as the tension of a liquid is diminished by adding a foreign substance, the capillary force of a tube is diminished by the presence of a foreign vapor. This is illustrated by M. Devaux as in Fig. 9, where water rises to the greatest height in the .tube A, which was filled simply with air, to a less height in E, which has been charged with the vapor of ether, and to a still less height in C, which was occupied with the vapor of camphor.

Other energies than this mechanical energy have been shown