CAPILLARY ATTRACTION.
Capillary attraction is the attraction which causes the ascent of fluids in small tubes.This word is derived from the Latincapillus—a hair.
The tubes must be less than one-tenth of an inch in diameter in order to produce the most satisfactory results, and tubes whose bores are no larger than a hair present the phenomenon the most strikingly. But though the rise of water above its natural level, is most manifest in small tubes, it appears, in a degree, in vessels of all sizes and shapes, by a ring of water formed around the sides with a concavity upward.
The following are the leadingfactsrespecting capillary attraction.
(1.)When small tubes, open at both ends, are immersed perpendicularly in any liquid, the liquid rises in them, to a height which is inversely as the diameter of the bore.Though tubes of glass are usually employed in experiments on this subject, yet tubes made of any other material exhibit the same property. Nor does the thickness of the solid part of the tube, or its quantity of matter, make the least difference, the effect depending solely on the attraction of the surface, and consequently extending only to a very small distance.
(2.)Different fluids are raised to unequal heights by the same tube.Thus, according to experiment, a tube which will raise water 23 inches will raise alcohol only 9 inches.
(3.) A tube1⁄100of an inch in diameter raises water 5.3 inches; and since the height is reciprocally as the diameter,the product of the diameter into the height is a constant quantity, namely, the .053th part of an inch square.
(4.)Fluids rise in a similar manner between plates of glass, metal, &c., placed perpendicularly in the fluids, and near to one another. If the plates are parallel, the height to which a fluid will rise, isinversely as the distance between the plates; and the whole ascent is justhalf that which takes place in a tubeof the same diameter. If the plates be placed edge to edge, so as to form an angle, and they be immersed in water, with theline of their intersection vertical, the water will ascend between them in acurvehaving its vertex at the angle of intersection. This curve is found to have the properties and form of thehyperbola.
In the adjustment of the stems of barometers and the thermometer an allowance is made to compensate for the influence of capillary attraction.
Figs. 94-96.
Figs. 94-96.
Such are the leading facts ascertained respecting capillary attraction. Various explanations of them have been attempted but that of La Place is most generally received. According to this high authority, the action of the sides of the tube draws up the film of fluid nearest to it, and that film draws along with it the film immediately below it, and so each film drags along with it the next below, until the weight of the volume of fluid raised exactly balances all the forces which act upon it. The fact that the elevation of the water between the parallel plates is exactlyhalfthat in a tube of the same diameter, clearly indicates that the force resides in the surrounding body; and the additional fact that the thickness or quantity of that body makes no difference, proves that the force resides in the surface, and that the action extends only to a very small distance.
Note.—Several familiar examples of capillary attraction may be added. A piece of sponge, or a lump of sugar, touching water at its lowest corner, soon becomes moistened throughout. The wick of a lamp lifts the oil to supply the flame, to the height of several inches. A capillary glass tube, bent in the form of a syphon, and having its shorter end inserted in a vessel of water, will fill itself and deliver over the water in drops. A lock of thread or of candle-wick, inserted in a vessel of water in a similar manner, with one end hanging over the vessel, will exhibit the same result. An immense weight or mass may be raised through a small space, by first stretching a dry rope between it and a support, and then wetting the rope.The several figures above need no explanation to the attentive reader.
Note.—Several familiar examples of capillary attraction may be added. A piece of sponge, or a lump of sugar, touching water at its lowest corner, soon becomes moistened throughout. The wick of a lamp lifts the oil to supply the flame, to the height of several inches. A capillary glass tube, bent in the form of a syphon, and having its shorter end inserted in a vessel of water, will fill itself and deliver over the water in drops. A lock of thread or of candle-wick, inserted in a vessel of water in a similar manner, with one end hanging over the vessel, will exhibit the same result. An immense weight or mass may be raised through a small space, by first stretching a dry rope between it and a support, and then wetting the rope.
The several figures above need no explanation to the attentive reader.