56 CAPILLAKY ACTION A TUBE, the bore of which is so small that it will only admit a hair (capilla), is called a capillary tube. Whea such a tube of glass, open at both ends, is placed vertically with its lower end immersed in water, the water is observed to rise in the tube, and to stand within the tube at a higher level than the water outside. The action between the capillary tube and the water has been called Capillary Action, and the name has been extended to many other phenomena which have been found to depend on properties of liquids and solids similar to those which cause water to rise in capillary tubes. The forces which are concerned in these phenomena are those which act between neighbouring parts of the same substance, and which are called forces of cohesion, and those which act between portions of matter of different kinds, which are called forces of adhesion. These forces are quite insensible between two portions of matter separated by any distance which we can directly measure. It is only when the distance becomes exceedingly small that these forces become perceptible. Quincke 1 has made experiments to determine the greatest distance at which the effect of these forces is sensible, and he finds for various substances distances about the twenty- thousandth part of a milli metre. Poggendorff 2 tells us that Leonardo da Vinci 3 must be considered as the discoverer of capillary phenomena. The first accurate observations of the capillary action of tubes and glass plates were made by Hauksbee. 4 He ascribes the action to an attraction between the glass and the liquid. He observed that the effect was the same in thick tubes as in thin, and concluded that only those particles of the glass which are very near the surface have any influence on the phenomenon. Dr Jurin 5 showed that the height at which the liquid is suspended depends on the section of the tube at the surface of the liquid, and is independent of the form of the lower part of the tube. He considered that the suspen sion of the liquid is due to " the attraction of the periphery or section of the surface of the tube to which the upper surface of the water is contiguous and coheres." From this he shows that the rise of the liquid in tubes of the same substance is inversely proportional to their radii. Newton devotes the 31st query in the last edition of his Opticks to molecular forces, and instances several examples of the cohesion of liquids, such as the suspension of mercury iu a barometer tube at more than double the height at which it usually stands. This arises from its adhesion to the tube, and the upper part of the mercury sustains a considerable tension, or negative pressure, with out the separation of its parts. He considers the capillary phenomena to be of the same kind, but his explanation is not sufficiently explicit with respect to the nature and the limits of the action of the attractive force. It is to be observed that, while these early speculators ascribe the phenomena to attraction, they do uot distinctly assert that this attraction is sensible only at insensible distances, and that for all distances which we can erectly measure the force is altogether insensible. The idea of such forces, however, had been distinctly formed by Newton, who gave the first example of the calculation of the effect of such forces in his theorem on the alteration of 1 Porjg. Ann., cxxxvii. p. 402. " I oyg. Ann., ci. p. 551. 3 Died 1519. 4 Physico-Mechanical Experiments, London. 1709, pp. 139-169 and Phil. Trans., 1711 and 1712. 5 Phil. Tr. ns., 1718, No. 355, p. 739, and 1719. No. 363, p. 1083. the path of a light-corpuscule when it enters or leaves a dense body. Clairaut 6 appears to have been the first to show the necessity of taking account of the attraction between the parts of the fluid itself in order to explain the phenomena. He does not, however, recognize the fact that the dis tance at which the attraction is sensible is not only small but altogether insensible. Segner 7 introduced the very important idea of the surface-tension of liquids, which he ascribed to attractive forces, the sphere of whose action is so small " ut nullo adhuc sensu percipi potuerit." In attempting to calculate the effect of this surface-tension in determining the form of a drop of the liquid, Segner took account of the curva ture of a meridian section of the drop, but neglected the effect of the curvature in a plane at right angles to this section. But the idea of surface-tension introduced by Segner had a most important effect on the subsequent development of the theory. We may regard it as a physical fact estab lished by experiment in the same way as the laws of the elasticity of solid bodies. We may investigate the forces which act between finite portions of a liquid in the same way as we investigate the forces which act between finite portions of a solid. The experiments on solids lead to certain laws of elasticity expressed in terms of coefficients, the values of which can be determined only by experiments on each particular substance. Various attempts have also been made to deduce these laws from particular hypotheses as to the action between the molecules of the elastic substance. We may therefore regard the theory of elasticity as consisting of two parts. The first part estab lishes the laws of the elasticity of a finite portion of the solid subjected to a homogeneous strain, and deduces from these laws the equations of the equilibrium and motion of a body subjected to any forces and displacements. The second part endeavours to deduce the facts of the elasticity of a finite portion of the substance from hypotheses as to the motion of its constituent molecules and the forces acting between them. In like manner we may by experiment ascertain the general fact that the surface of a liquid is in a state of tension similar to that of a membrane stretched equally iu all directions, and prove that this tension depends only on the nature and temperature of the liquid and not on its form, and from this as a secondary physical principle we may deduce all the phenomena of capillary action. This is one step of the investigation. The next step is to deduce this surface-tension from an hypothesis as to the molecular constitution of the liquid and of the bodies that surround it. The scientific importance of this step is to be measured by the degree of insight which it affords or promises into the molecular constitution of real bodies by the suggestion of experiments by which we may discriminate between rival molecular theories. In 1756 Leidenfrost 8 showed that a soap-bubble tends to contract, so that if the tube with which it was blown is left open the bubble will diminish in size and will expel through the tube the air which it contains. He attributed this force, however, not to any general property of the surfaces of liquids, but to the fatty part of the soap whhh he supposed to separate itself from the other constituents c Clairaut, Theorie de la Figure de la Tern, Paris, 1808. pp. 105, 128. 7 Scgner, Comment. Soc. Reg. Gcitting., i. (1751), p. 301.
8 De aquae commuuix nonnvllis q-Mditatibus troctclt s, Duisburg.