The phenomena of surface tension are most obvious in soap films and in foams, where the mass of the liquid concerned is so small (rela tively to the surface) that the molecular forces which give rise to the so-called asurface tension* can easily preponderate over gravity, which is relatively powerful in liquid bodies of large mass and small surface. The French physicist Plateau devoted a vast amount of attention to the phenomena that are manifested by liquid films, and by masses of liquid that are freed from the influence of gravity by being sus pended in other liquids with which they will not mix, but which have the same density as the liquid to be studied. Olive oil can readily be freed from the action of gravity, by submerg ing it in a mixture of alcohol and water, whose composition is regulated by trial until the mix ture has precisely the same specific gravity as the oil. A mass of oil which is submerged in this manner, and is not constrained in any way, at once assumes a spherical form; for the sphere has a smaller surface than any other solid of the same volume.
The existence of surface tension can be shown readily and strikingly, even in a large mass of water, by several very simple experi ments. Of these, the camphor-movement experi ment is one of the best known. To perform it, a perfectly clean vessel is filled with clean water, some of the water being allowed to flow over the skies of the vessel, so that any superficial impurities may be washed away. Very fine scrapings of camphor are then allowed to fall upon the surface of the water; and if the water surface is sufficiently clean, these scrapings at once begin to execute the most violent move ments. The motion of the camphor is due to the fact that the surface tension of a solution of camphor in water is less than that of pure water. The camphor particles do not dissolve evenly on all sides; and the horizontal pull exerted upon them by the water is greatest in those directions in which the concentration of the solution in immediate contact with the par ticles is least. Hence the motions. The great importance of absolute cleanliness in this ex periment is well illustrated by touching with a slightly greasy finger a water surface upon which camphor particles are in rapid motion. The entire surface becomes contaminated al most instantly, so that the camphor movements become deadened, or cease altogether.
The effects of surface tension are observable in large masses of liquid, where those masses come in contact with the walls of their contain ing vessels. The slight elevation of the water in a drinking glass, where the water touches the glass, is due to this cause. This particular phenomenon is more marked in the case of a glass tube of small diameter, dipping in a verti cal position into a vessel of water (or any other liquid which actually wets the glass). Let the glass tube be inserted into the water, so that it is wetted up to a certain level, and let the tube he then raised slightly. The glass, in the region which has been submerged below the general level of the water and is now raised above it again, adheres to the water, and as the tube is raised, the column of water within it sinks at the centre, so that its surface becomes con cave, as is illustrated in Fig. 2. The weight of
that part of the water within the tube which stands above the general level of the water in the external vessel (that is, the weight of that portion which lies between the actual water surface in the tube, and the dotted horizontal line), is sustained by the tension of the curved surface (or ameniscusi) that bounds the col umn at the top; this tension acting everywhere in the direction of the surface of the water, and therefore having an obliquely-upward di rection around the edges, and hence a vertical component, which is capable of sustaining the water in the tube. In the case of a liquid which does not wet the tube (for example, in the case of mercury and glass), the curvature of the liquid surface is in the opposite direc tion from that observed with water and glass; that is, the meniscus is convex upwards, as shown in Fig. 3, and the liquid in the tube stands at a lower level than corresponds to the general level of the liquid surface in the con taining vessel. In a barometer, the meniscus of the column is convex upward, and the depres sion of the column due to the surface tension of the mercury is usually quite sensible; so that in order to be in a position to know the exact height at which the mercury in the column would stand if the tube were large enough in diameter for the effects of surface tension to be negligible, it is necessary to investigate, very carefully, the way in which the depression varies with the diameter of the tube, and with the height of the meniscus itself. Numerous observers have made extensive investigations of this sort, and have given their results in tables. A very good table of this kind, due to Men deleeff, is given in Guillaume's
The surface tension of a liquid is measured by the horizontal pull that the liquid can exert upon a straight line one unit in length, lying in its surface; the pull being perpendicular, of course, to the direction of the line. The ac companying table contains the surface tensions of various liquids as determined by Quincke, and quoted by Maxwell. Mercury, for example, is capable of exerting a pull of 21.58 grains upon a straight line one inch long, lying in its surface. The value of the surface tension of water given in this table is certainly too great. Brunner found it to be 75.2 dynes per centi meter, and Wolf found 76.5 and 77.3. Ray leigh's determination, based upon a study of the wave-length of ripples, gave 73.9 dynes at C.; and T. Proctor Hall found that at T° C. the surface tension of water, in the same units, is given by the expression.
75.48-0.140T.
Boys, Bubbles and How to Make Them' ; Plateau, (Statique ex perimentale et theorique des liquides soumis aux seules forces moleculaires' •, Risteen, 'Mole cules and the Molecular Theory of Matter.' Also, any extended treatise on physics.