CHAP. VII.

ON CAPILLARY PHENOMENA.

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111. In all that has hitherto been said respecting the general equilibrium of liquids, the only force which has been considered is gravity. But the attraction which the ticles of the solid exerts on those of the fluid, and which the fluid exerts on its own particles, all of which may be included under the term molecular forces, introduce modifications, which must now be briefly considered. It will be impossible to trace here the profound analysis by which Laplace has submitted the whole subject to rigorous calculation, but we shall endeavour to indicate the most important phenomena, and the principles of the theory which explains them.

If the lower end of a glass tube of small diameter be dipped into any liquid, the surface of the liquid which enters the tube will not coincide with the exterior level of the liquid. In some liquids, as in water, the column is elevated, or stands above the exterior fluid; in others, as is generally the case with mercury, the column is depressed, or sinks below the exterior surface. These phenomena were first observed in tubes whose internal diameters were so small, that they might be compared with the diameter of a hair; they occur, however, more or less, in every tube; and similar phenomena present themselves whenever water

is in contact with a solid; and the combined actions from which these phenomena of capillary spaces arise are expressed by the term capillarity.

112. Form of Surface.-Whenever there is an elevation or a depression of a liquid in a straight tube, the surface of the liquid column assumes and preserves a determinate form. When the column is elevated, the surface is nearly a hollow hemisphere, whose diameter is the same as the diameter of the tube. This which is represented by a b c is called a concave meniscus.

Again, when the column is depressed, its surface is always a full hemisphere, so that any section, as def, is a convex meniscus.

These particular forms of the surface are essential conditions of the elevation or depression. For if the interior surface of the tube

a

be smeared with any unctuous substance, the water will not only not be elevated, but it will be depressed, and the surface of the depressed column will be a convex meniscus, just as a column of mercury in an ordinary tube. It will follow from this, that the difference of height depends on the form of the meniscus, and consequently whatever prevents the latter from taking the form which it ought to have, will prevent the liquid from standing at its proper height.

113. Heights of the Columns.-A very simple law, made out by experiments with capillary tubes, is, that the heights of the columns, raised or depressed, are in the inverse ratio of the diameter of the tubes. This will be seen at once by immersing any number of tubes of different diameters in the same liquid. The column elevated will be longest, and the depression of the column will be greatest, in the tube of smallest diameter.

It appears also that the effects are entirely independent of the thickness of the tube; that the internal diameter

being unchanged, any increase or diminution in their external diameters will not produce any effect on the position of the top of the column. These important fundamental facts were clearly established by the experiments of Boyle and Hawksbee. The nature of the materials of the tube also appears to have no further influence, than so far as it determines whether the liquid can or cannot wet the tube; thus a tube of iron or the stalk of a vegetable, provided their inner surfaces are wetted with equal facility, will represent the same phenomena.

All experiments seem to shew that the height of the sustained column is inversely proportional to the diameter of the tube: and the curved form which water takes between two glass plates, placed together by their edges, appears, on varying the inclination of the plates, to follow the same law of the distance.

The preceding phenomena shew distinctly that solids and liquids cannot touch each other without the surface of the liquid experiencing near the point of contact some change of form; and the change of form, or the curvature which takes place, depends on the nature of the substances, as will be seen in the following articles.

114. Attractions and Repulsions.A single thin strip of metal suspended in a liquid does. not experience any motion of translation, whether it be wetted, as at a, or whether it be not wetted, as at b, for in these cases the forces are equal on both sides of the metal. Two similar pieces of metal immersed near each

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d

other do not experience any motion provided they are so far from each other that the curvatures of the elevated or depressed liquid are separated by a rectilinear space cd; but if the bodies are so near that the curvatures cross each other, there is a sensible attraction, and the bodies

approach each other. This attraction takes place equally, whether the bodies are wetted, or whether they are not, as when two pieces of glass are suspended in water and in mercury. But if one body is wetted and the other is not, the action is repulsive, and they recede from each other. Thus, if a strip of ivory which is readily wetted by water, and a strip of talc which cannot be wetted, be suspended sufficiently near each other in water, the curvatures of the surface caused by the two substances a and b will cross each other, so that the surface will take a form as represented in the figure; and in this case the two substances will recede from each other, or appear to be repelled.

Similarly, two balls of cork or wood, floating on water, or two balls of pewter floating in mercury, approach each other, when they are at a capillary distance, that is, at a distance so near that the curvatures can interfere. Thus,

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at the distances a b, or b c, they are without any action, but at the distance cd they attract each other. Two balls which do not wet with the liquid, as two balls of wax or smoked cork, floating on water, or two balls of iron in mercury, exert an attraction under the same circumstances. In general, all floating bodies experience from the same cause motions more or less rapid, when they approach each other, or when they approach the sides of the vessel at which the liquid is curved, whether by elevation or depression. In a vessel of water, for example, which is not full, all small bodies which are wetted ap

proach the sides, and those which are not wetted recede from them. The exact contrary takes place in a vessel which is quite full. The motion which takes place under these circumstances cannot be referred to the direct attraction of matter, but evidently depends on the curvatures of the surface; for the same substances which approach and recede from each other when floating on water, present no similar phenomena when wholly immersed in the fluid.

It is very well known that insects can walk on the surface of water. Their covering cannot be wetted; hence, a depression takes place in conformity with the preceding principles. Thus the quantity of water displaced becoming equal to the weight of their bodies, they are sustained in conformity with the well-known principles of Hydrostatics.

The experiment of making a needle, which is of iron or steel, float on the surface of a lighter medium, as water, is to be referred to the same principles. The surface of the metal, owing to its polish, or greasiness, cannot be wetted; hence, more fluid being displaced than is equal to the weight of the needle, it swims. Having once been wetted, however, it cannot rest on the surface.

115. Forces of Capillarity.-A solid plate placed in contact with a liquid cannot be raised up as if it were in free air, but will require a greater force than that required merely to sustain the plate. This may be readily measured by balancing a plate of metal on a scale beam, and observing the weights which must be added to raise it up from the surface of a liquid with which it is in contact. From these experiments it appears that every substance, provided the diameter of the plate be the same and it can be wetted by the liquid, requires exactly the same weight to raise it. The adhesion, then, as well as the capillarity, is independent of the nature of the solid, and depends solely on the nature of the liquid. It is easy to see the reason of this, for the plate always brings away with it a