CAPILLARITY. The subject of capillarity takes its name from the circumstance that it was first studied in connection with the rise of liquids in tubes having a bore so fine as to be comparable in diameter with a hair (capillus). When one end of such a tube is immersed in water, the water rises in the tube above the general level of the surface outside it, in a way which is not in accord with the general law of hydrostatics, that a liquid will stand at the same level in two communicating vessels. Many other instances can be found in which liquid surfaces,' especially in the neighborhood of solid bodies, assume shapes and positions which are equally at variance with the laws of hydrostatics. All such cases are now treated as belonging to the general subject of capillarity. Examples of capillary actions are the soaking up of water by a sponge; the penetration of var nish into wood; the rising of oil in a lamp wick; the clinging of ink to a properly nibbed pen; the running off of the ink from pen to paper; the soaking up of the superfluous ink in blot ting paper; the falling of drops of uniform size from the lip of a bottle or from a medicine dropper; the rounding of drops of melted lead into pellets of shot as they fall in a shot-tower.
When we consider such examples as these, it becomes clear that they are to be explained as the result of forces acting between the parts of the liquid, or between the liquid and the solid with which it is in contact. \These forces are often called the forces of cohesion and ad hesion:...Very little is known about them. The one thing definitely known is that they are very great when the parts of the body or bodies be tween which they are exerted are so near to gether that they are said to be in contact, and fall off rapidly in magnitude as the parts are separated, so as to become inappreciable when the distance between the parts becomes discern ible. It is customary to think of these forces as exerted between the molecules of matter, and so to call them molecular forces. The very small distance within which the action of a molecule on its neighbors is appreciable is called the range of molecular action.
By reflection upon the effects of such molec ular forces acting in a liquid, Young (1804) was led to assume that a tension exists in a thin layer of molecules at the surface of a liquid, comparable in general to the tension in a stretched membrane. The magnitude of this surface tension depends upon the nature of the liquid, or, more exactly, upon the nature of the two media, of whatever sort they may be, sep arated by the surface. It is independent of the shape of the surface, provided that its radii of curvature are always great in comparison with the range of molecular action or the thickness of the surface layer. Young added to this hy pothesis the observation that the angle of con tact between a liquid surface and a solid is always the same for the same pair of substances. The angle of contact is generally measured, at the line of contact, between the external normals to the solid and the liquid surfaces. In the case of mercury and glass, which Young particu larly observed, this angle is obtuse, and seemed to him to be about 135°. In the case of water and glass it is acute, and so small as to seem evanescent, or equal to zero. Young assumed that the like is true for all contacts of liquids with solids which are wetted by them.
Young's two principles are clearly not proved to be consequences of the more fundamental hypothesis of molecular force; but, accepted as generalizations from observation, they may be used to explain all the forms of liquid surfaces.
example, let us consider the rise of water in a glass tube. The water wets the inner wall of the tube, and so meets it everywhere in the cir cle of contact at an angle equal to zero. Owing to this, the surface of the water in the tube will be concave upward. In a tube of very small bore, it will be approximately hemispherical. The tension strives to straighten out the sur face, and since the contact condition prevents its doing this, it lifts a column of water up the tube, to a point such that the weight of the uplifted column is sustained by the upward force due to the tension, while at the same time the curvature of the surface is consistent with the contact condition.