Page:Popular Science Monthly Volume 35.djvu/615

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HAT is it that keeps a drop of water in shape; that enables it to resist a considerable pressure or blow before it will collapse into a spatter; that holds it in its integrity to a leaf or the eaves till it is mature to fall, while it still maintains its round, independent individuality? Whatever the power is, it appears yet more distinctly in a globule of mercury, which will not be hammered out of shape or compelled to spread. Dr. Thomas Young conceived, for the explanation of this and some other phenomena exhibited by small, isolated liquid masses, the idea of their being surrounded by a thin, elastic membrane, less dense than the deeper parts of the drop, and capable of adhering perfectly to them, and more or less strongly to solid bodies. It seemed capable of opposing a certain resistance to being rent, and this was called its superficial tension. Some curious movements take place when certain solid substances are cast upon water, to account for which Dutrochet supposed a new force, which he called epipolic force. These phenomena of the drops, the "epipolic force," the calming effects of oil on storm-disturbed water, and a variety of other curious actions hitherto unaccounted for, have lately been referred to this property of superficial tension. Taking a drop of water as typically embodying the property, M. E. Gossart asserts that all the energies of nature may be found in its tenuous envelope. Besides M. Gossart, studies of the curious and protean properties of this superficial tension, or the envelope of the water-drop, have been published by M. H. Devaux and M. Van der Mensbrugghe. The present article is a summary of some of the results of their studies. Regarding water in a vessel, M. Van der Mensbrugghe finds that whatever may once have been thought on the subject, it is not equally constituted throughout. Its particles are solicited by attractive forces which are exhibited when, upon drawing out a pencil which has been dipped into the mass, a drop is found adhering to the point. If this drop be conceived to be cut by a horizontal plane, all the parts below the plane may be supposed to be sustained by those which are above it. It is also acted upon by repulsive forces tending to scatter the particles, the effects of which are seen in evaporation. When the