When waves are set up on the surface of water, they are transmitted across the surface at a rate which depends on the hydrostatic pres sure and on the surface tension. The surface tension is practically the only agent in transmit ting the waves when they are very short. Such waves may be set up by the use of a vibrating tuning fork, and the measurement of their lengths furnishes a means for the determina tion of the value of the surface tension. The ripples set up on the smooth surface of a pond by a breath of air, or which proceed in front of a slowly moving boat, are largely due to sur face tension.
When a glass tumbler is partly filled with waterfthe surface tension draws the water up the sides. As more water is carefully poured in, the line of contact rises until it reaches the edge of the glass. It often happens that the line of contact is checked at the edge, so that the water does not run out over the top of the glass. In this case the glass can be filled above the level of its edge, and the water will stand in it under a surface that is convex upward, the surface tension in which keeps the water from running out.
The surface tension in the convex surface of the mercury in the tube of a barometer pro duces a pressure downward, which has to be estimated and allowed for when accurate ob servations are to be made.
A fine needle that has been slightly oiled or greased if laid gently down on the surface of water will float there. It lies in a concave trough formed in the water surface. The water cannot wet the needle, because of its coating of oil, and so the needle is supported by the uplift due to the surface tension acting in the concave surface in which the needle rests. In a way generally similar, the insects which run over the surface of water are supported in little hollows in the water surface. Their feet are not wetted by the water.
When two light bodies, floating on the sur face of a liquid, are moved toward each other until the curved parts of the liquid surface near them intersect, they seem to exert forces on each other. If they are both wetted by the liquid, or are both not wetted by it, they move together and adhere to each other. If one of them is wetted by the liquid and the other not, they move apart. If water is run in between two parallel sheets of plate glass, they are drawn closely together and adhere very strongly to each other. These actions are ascribed to differences in the pressures on opposite sides of the bodies. In case the bodies are wetted
by the liquid, the pressure in the region between them, in the elevated portion of the liquid under its concave surface, is less than the pres sure on their outer sides and they are pushed together. This action takes place even in a vacuum, in which case the pressure under the concave surface is a negative pressure or ten sion. In case the bodies are not wetted by the liquid the liquid is depressed between them, and the pressure inward on their outer sides is greater than that acting outward, and they are pushed together. A curious effect, pre dicted by Laplace from the theory of capillarity, and verified by experiment, is exhibited by two bodies, one of which is wetted by the liquid and the other not. These bodies, as the distance between them is diminished, at first appear to repel each other, but as the distance is re duced the repulsion changes to an attraction and the bodies come together.
If a small lump of camphor is dropped on clean water, it begins to move about over the surface in an irregular way, and continues to do so, generally for some time. These motions are explained by noticing that one part of the lump of camphor dissolves more freely than the rest, and so, near it, the surface tension of the water surface is lowered below that near the other parts of the lump. The camphor is accordingly drawn toward that part of the sur face in which the tension is greatest.
If a thread of water is at rest in a horizon tal capillary tube, and one of its two end sur faces is touched by a wire that has been dipped in turpentine or benzine, the tension at that end will be diminished, and the greater tension of the other end will draw the water along the tube. This effect is taken advantage of in cleaning off grease spots from cloth. The sur face tension of benzine is very low, and when benzine is applied in a gradually narrowing ring around the spot of grease, the grease is drawn in toward the centre of the ring, and if' the cloth is laid on a piece of blotting paper, the grease will be taken up by it. This action is promoted if a hot iron is applied to the other side of the cloth, for the heat lessens the tension in the ends of the pores nearest the iron, and the greater tension at the other ends draws the grease into the blotting paper. Con sult Boys 'Soap Bubbles, and How to Blow Them' (Jew York 1900; new ed., London 1912) ; Lori Rayleigh, 'Collected Scientific Papers' (1901).