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ON PHYSICs oR NATURAL PHILosophy. - No. XIV. CAPILLARY ATTRACTION.
0&pillary Phenomena.-In the contact of solids and liquids, a series of phenomena are produced, to which the name of oqpillary phenomena is given, because that they are particularly observed in tubes, whose diameter is so small that it is com. parable to the thickness of a hair.
The effects of capillary attraction are very various; but in all cases, they are the result of the mutual attraction of the liquid particles to each other, and to that attraction which subsists between these particles and solid bodies. Take, for example, the following phenomena: when we immerse a solid rod in a liquid which will wet it, the liquid, in opposition to the laws of hydrostatics, rises around the solid rod, as in the case of glass and water; and its surface, instead of being horiZontal, takes a concave form, as shown in fig. 47; but, if a solid rod be immersed in a liquid which will not wet it, as in the case of glass and mercury, the liquid, instead of rising, sinks
round the solid rod, and its surface takes a convex form, as shown in fig, 48.
they produce an ascent or a depression. This ascent or depression increases as the diameter of the tube diminishes, see figs. 49 and 50. When the tubes are wetted by the liquid,
49, and when the tubes are not wetted by the liquid, the surface takes that of a &onvex: meniscus, as in fig. 50. The surface of the liquid assurfies the same concavity and convexity on the sides of the vessel which contains the liquid according as it wets or does not wet the
where the one represents the effect of a tube immersed in
water, and the other that of a tube immersed in mereury. + Fig. D.
Laws of Ascent and Depression in Capillary Tubes.—M, GayLussac has proved by experiment that the ascent and depression of liquids in capillary tubes, are regulated according to the three following laws: 1st. There is an ascent when the liquid wets the tubes, and a depression when it does not wet them : 2nd, this ascent and depression are in the inverse ratio of the diameters of the tubes, so long as they do not exceed the tenth part of an inch : 3rd, the ascent and depression vary with the nature of the liquid and with the temperature; but they are independent of the substance of the tubes and of the thickness of their sides, if the latter be previously wetted. | These laws hold good in a vacuum as well as in air.
The method employed by M. Gay-Lussac in the discovery of these laws was the following : 1st, he measured the interior
sides of that vessel, as shown in the following figures 9 and B,
quantity of mercury which completely filled them; the density
of the metal being known, it was then easy to deduce, from its
weight and the height of the column, the required diameter, as shown in a former lesson : 2nd, he then placed the liquid under consideration in a vessel AB c D, figure G, and vertically immersed in it, the capillary tubes which were successively submitted to experiment; close by each tube, he placed a rod RF, tapering to a point, which, by the motion of a screw, was made to reach the exact level of the liquid; then, by means of a cathetometer, he measured the vertical distance between the upper extremity of the column of liquid in the tube, and the lower extremity or point of the rod which came in contact with the liquid. The heights which different liquids reach are by no means the same, as may be seen in the following table; for, in a tube whose interior diameter was about one twenty-fifth part of an inch, the liquids rose to the different o here mentioned, above the level of the liquid in the Yeş861 : y
Laws of Ascent and Depression between two Plates Parallel or Inclined.—Phenomena analogous to those presented by capillary tubes, are produced between two bodies of any form immersed in a liquid, when they are sufficiently near to one another. For example, if we immerse in water two parallel plates of glass so near each other that the two curvatures formed at their contact with the liquid, are united, it is observed : 1st, that the water rises regularly between the two plates, in the inverse ratio of the interval which separates them; and, 2nd, that the height of ascent for a given interval, is the half of that which would have taken place in a tube whose diameter is equal to this interval. If parallel plates are immersed in mer
cury, depression takes place, but according to the same laws. Fig. 51.
it two plates of glass, A B and A C, fig, 51, be inclined to each
other so as to form an angle, and be immersed in a liquid which wets them, so that their line of contact be placed vertically, the liquid will rise towards the vertex of the angle between the two plates, and its surface, from the highest to the lowest point, will assume the form of the curve called an equilateral hyperbola. The asymptotés of this curve which is double, being traced on each plate, are the vertical straight line in which the edges of the plates meet, which is common to both, and the horizontal straight lines which determine the level of the liquid in which they are immersed, as shown by the dotted lines in the following figure H.
When the line of contact of the two plates is horizontal instead of vertical, as shown in their sections represented in figs. 52 and 53, and the plates are placed so as to form a very small angle, a drop of water put between them is hollowed at both its extremities into a concave meniscus, as in fig. 52, and
is attracted towards the vertex of the angle of the two plates; but if the liquid does not, wet the plates as is the case with .mercury, the drop of the liquid is rounded at both its extremities, into a convex meniscus, as in fig. 53, and is repelled from the vertex of the angle. The directions of attraction and repulsion in these figures are indicated by the arrow heads.
The force of attraction of a liquid to the sides of a vessel lies between two extreme cases; it is equal to that of the liquid to itself, or it is zero; in the former case, the ascent of the liquid in tubes is the consequence; in the latter, depression is the result. Between these two extremes, there must be the case in which there is neither ascent nor depression; this occurs when the force of the attraction of the liquid to the Bolid is exactly equal to half of the force of the attraction of the liquid to itself. "Water brought in contact with well polished steel appears to realise this particular case; for the liquid seems, on the approach of the metal, to experience neither elevation nor depression.
sufficiently small diameter, these surfaces are hemispherical. Between two parallel plates they are semicylindric. Since the liquid columns in tubes rise in proportion to the smallness of their diameter, it follows that the meniscus which appears at the surface is proportionally increased in curvature, which furnishes us by its direction and force, or rather by the shortness of its radius, an expression for the force which acts at the extremity of the column; the concave meniscus indicating a force which acts from the interior to the exterior or a traction; and the convex meniscus, a force which acts from the exterior to the interior, or a compression. This view is verified by the following experiments. Take an inverted siphon, having two unequal branches both in length and in diameter, as shown in the following figures 1°, 2°, 3°, and such that the capillary action is very marked, in the narrow branch, and almost nothing in the other branch, on account of its great diameter. Pour water into it at three different times, so as to make it assume the levels indicated by these figures.
In fig. 1", the level being very low in the branch A, it is elevated in the branch B to a height corresponding to the capillary action at that point, and the meniscus is concave at B. , In,fig. 2”, on pouring an additional quantity of water into the branch A, up to the exact level of the extremity s, the two surfaces are then of the same height, and both become plane ; in fig. 3°, on pouring an additional quantity of water still into the branch A, up to the level which measures the capillary action in the branch B at that point, the water rises in the form of a convex meniscus, and exerts a force of compression sufficient to prevent any flow; but if the level at A be increased in height above this point, the water will then begin to issue by the narrow branch B.
Again, if in a conical tube, of which the following figures marked W and M, are sections through their axes, we introduce
a drop of liquid, it will take the former or the latter form, according as it wets or does not wet the sides of the tubes:
and if left to itself, it will be attracted in the direction of the arrow heads.
Attractions and Repulsions of Capillary Action.--The attractions and repulsions which we observe among bodies floating at the surface of liquids, and which arise from capillary action, are regulated by the following laws: Ist, When two floating bodies are wetted by a liquid, as, for example, two balls of cork in water, a powerful attraction takes place as soon as they are put so near each other that a plane surface of water no longer exists between them. 2nd, When two floating bodies are not wetted by a liquid, as, for example, two balls of wax in water, a strong attraction takes place as soon as they are put in the same circumstances as the former, 3rd, When two floating bodies are such that one is wetted by the liquid and the other not, as a ball of cork and a ball of wax in water, repulsion is observed to take place as soon as they are so near each other, that the two contrary curvatures of the liquid are found in contact. The phenomena just described, depending on the concave or convex curvature assumed by the surface of the liquid in which the bodies are placed, we shall inquire into the cause which determines the form of this curvature in our
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