As already stated, the phenomena are not dependent on the intervention of tubes; any capillary cavity suffices to produce them. When two light bodies, such as two bits of cork, are left to float on water, near each other, they soon come together, moving at last with a rush. This is sometimes given as an example of the gravitation that draws the planets to the suu; but it is really owing to this capillary action that we are con sidering. When the liquid wets the floating bodies, it rises slightly all round them, and this sustained liquid hangs as a weight on them on all sides. So long as it rises equally there is no motion; but when the bodies come near each other, the space between them becomes like part of the inside of a capillary tube, the water rises higher there than on the other sides, and the bodies move towards the sides that are most strongly pulled. When the floating bodies are not wetted by the liquid, the surface between the two bodies is depressed, as that of mercury is inside a glass tube, and the bodies descend, as it were, down the opposing slopes, and meet iu the bottom of the hollow. If one of two bodies floating on water is smeared with oil so as to prevent the water from adhering, instead of coming together, the two will recede from each other, for reasons analogous to the above.
C. A. plays a most important part in nature in a great variety of ways. An instance of its employment by man is seen in the wicks of lamps and candles, which, being composed of fibrous materials, furnish hair-like channels by which the melted oil is elevated to the flame, and supplied as fast as it is consumed. C. A. influences the cir culation of fluids in the porous tissues of animals, and it is the principal mode in which water, with the various substances which it holds in solution, is supplied to the roots of growing plants. It is through it that in summer droughts moisture is raised to the sur face for the maintenance of vegetable life. C. A., too, affects many phenomena usually considered under the head DIFFUSION (q.v.) of fluids and gases. The reader, on referring to the article OSMOTIC ACTION, will also see that it enters into the explana tion of the phenomena known as exosmose and endosmose.
A familiar illustration of C. A. is furnished when one end of a towel happens to be left in a basin of water, while the other hangs over the side below the level of the water; the basin is soon emptied of its contents. It is important to observe that,
although the towel will become wet, not a drop will flow from it, unless the outside end reach below the level of the water in the basin. In this respect C. A. resembles that of the siphon. And this shows the error of supposing that water may rise through the earth by C. A., and flow out as springs at a higher level than the source whence it is drawn.
Some very interesting experiments have been made by M. Poisenille (Ann. de Chimie et de Phys. III. xxi. 76) concerning the flow of liquids through capillary tubes, the results of which must here be stated. It appears that when the tube exceeds a certain length—which is greater as the bore increases—the following laws regulate the rate of efflux of the liquid, the efflux taking place under pressure: 1. The flow increases directly as the pressure, so that under double the pressure, double the amount is discharged in equal times. 2. In tubes of equal diameter, the quantities discharged vary inversely as the length of the tube. If a tube 2 in. long discharge 100 grains in 5 minutes, a tube 4 in. long will only discharge 50 grains. 3. In tubes of equal lengths, but different diam eters, the flow is as the fourth power of the diameters. If one be of an inch in bore, and the other the efflux from the larger will be 16 times as great as from the smaller. It is further found that the efflux varies with the nature of the liquid, the material of the tube not appearing to affect the result in any great degree. No law of the rate of efflux has yet been discovered, depending on the density, capillarity, or fluidity of the fluids.
It may be mentioned, in conclusion, that the tubes to be used in the experiments on capillarity should be perfectly clean and dry. If wetted. the film of moisture on the tube forms a new tube, and the action will be the same as with a tube of the sub stance forming the film. The reader should consult Miller's Elements of Chemistry, and J. Clerk Maxwell's Theory of Heat, where the phenomena are treated from a differ ent point of view; the phrases "superficial energy " and " superficial tension" being substituted for "capillarity," and the hypothesis of molecular attraction being avoided.