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Page:Popular Science Monthly Volume 35.djvu/616

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THE POPULAR SCIENCE MONTHLY.

attractive and repulsive forces are at equilibrium within the liquid, there is supposed to be in the immediate vicinity of the free surface a tendency to the dispersion of the particles which is constantly opposed by the attractive forces. The condition of the superficial layer may be compared with that of a thin, elastic membrane under stretch, the cohesion of which constantly opposes itself to a more considerable elongation. The superficial layer of a liquid is thus subject to a contractile force or tension, by virtue of which it tends to become as small as possible. M. Gossart, comparing the relative situation of two molecules, A within the drop, and B at its surface, against the air or another liquid or a solid body, shows that each molecule is attracted by the others only from a certain distance (less than ten thousandth of a millimetre), which is as formidably great to it as it seems little to us. Those molecules which are at a greater distance from A and B will have no more action upon them than the stars have upon our sun, earth, and planets. Regarding these spheres alone, A, equally solicited in all directions by an equal number of molecules, will be free in its movements, and obedient to Pascal's principle; while B has not the same surrounding in every direction. Hence a kind of rarefaction which extends to only a slight depth in the drop; and hence also, on the surface, the elastic membranous or resistant quality.

This property is illustrated in some experiments described by M. Van der Mensbrugghe. Take two pencils, one of which should be of light wood and thinner than the other (Fig. 1); place them alongside and in contact; drop a little clear water in the angle between them, so as to moisten the line of contact. There will be formed a slight liquid mass, adherent to both pencils, of concave outline, the section of which is represented by a b in the corner diagram of Fig. 1. The lighter pencil will hang from the other by virtue of the tension of the concave surfaces a b, that bound either side of the line of contact. With the pencils twelve centimetres long, a weight of eighteen hundred milligrammes may be sustained in this way. In a second experiment, a ring of copper wire a millimetre thick and three and one quarter inches in diameter, is laid carefully upon the surface of pure water, when—if everything be entirely clean—it will float, as in Fig. 2, section a, and this, notwithstanding copper is 8·8 times heavier than water. This takes place because all the tensions of the liquid that touches upon the ring produce an upward resultant. A ring weighing seventeen hundred and thirty milligrammes may be thus upheld, while the maximum effect of the tensions is three thousand seven hundred and seventy milligrammes, or more than double the weight of the ring. Needles, globules of mercury, a thin ring of platinum, etc., may be similarly made to float on water.