CAPILLARY ACTION. When a clean glass tube with a fine bore, open at both ends,
is plunged into a liquid capable of wetting it, such as water, the liquid is found (1) to
rise in the tube above the level of its surface in the vessel containing it; (2) to rise the
higher in the tube above that level the finer its bore is; (3) to stand above the general
level in the tube where it approaches the sides (as in fig. 1, which is drawn on a greatly
exaggerated scale), so that its upper surface in the tube is curved and concave. When
a similar tube is plunged into a liquid incapable of wetting it, such as mercury, phenomena
of a precisely opposite nature are presented. The liquid stands iu the tube below
the level of its surface in the vessel; and, where it
approaches the sides of the tube, it stands below its
general level in the tube, so that its upper surface is
curved and convex as in fig. 2, the convexity and
depression in the tube increasing with the fineness of
its bore. While such is the case with the two classes
of liquids described, there are others on which fine
tubes have no action, so that they stand in such tubes
at the same level as in the vessel, and with plane upper surfaces. These are the leading
phenomena to be explained by what is called C. A., the tubes with fine hair-like bores
being called capillary tubes, from Lat. capillus, a hair. The phenomena, however,
though connected by name with such tubes, are not dependent on them, but may be
produced without their intervention by any contrivance which gives room for the
so-called capillary action. For instance, if two plates of glass with parallel faces be
placed together with two of their edges in contact, and the two opposite be separated a
very little by a fine wedge; and then if they be put standing with their common edge 11,1 Wt../ ALA
placed in a trough containing mercury, the mercury will be depressed between the plates till its upper surface forms a hyperbola convex to the zenith.
To understand the peculiar action producing these phe . „ nomena, it must be Kept in view that the surface of a num at rest under gravity is a horizontal plane (see lIYDRosTATics), and that this plane is maintained by gravity and the mutual attractions of the particles of the fluid mass. Suppose now a fluid at rest in a vessel to have a foreign body, such as a capillary tube, suddenly plunged into it, and separating, as by walls, a portion of the fluid from the rest. By cohesion (q.v.), the fluid particles inside the tube will be held on—drawn downwards—to the mass of the fluid, while by adhesion (q.v.) they will be drawn upwards towards the sides of the tube. By the ordinary action of gravity, as in tubes of a large size, the fluid will at once tend to rise in the tube to its level in the vessel. Whether it will succeed in doing so, or whether it will rise still higher, must depend on the adjustmeut of the forces of collegian between the fluid particles and their adhesion to the solid of the tube. The relation of these forces may be generally ea ulained as fnlInvra• Let m>rn. (fin be the surface of the column, mn, of a liquid contained in a space, abb a, above or below the surface, nit, of the external liquid. There being equilibrium between the liquid in the tube and in the vessel, any line of liquid particles may be taken and supposed to he detached from the rest and inclosed in a tube, without altering the forces exerted. Let the line included between the dotted lines be conceived so detached. The actions which the particles of the liquid in the tube exert on each other, or sustain from the sides of the tube, have no tendency to make the liquid move either up or down. But the column, nab, in the tube has some action exerted on it by the sides of the tube above the surface, wine . Let A, depending on the force of adhesion, represent