are partly depressed, and the space between them then becomes convex, as in A C, fig. 1. If we move the two balls nearer together, the mercury between them becomes more convex. C, fig. 2, represents the curve formed by the ball and the water; the former not being shewn in the figure, in order to give a general view of the disturbance of the liquid surface, and its elevation above the liquid level. In D, fig. 2, we have an interesting variation of A. One ball is completely wetted, and water rises up all over it, producing a general elevation above the level of the liquid, the other ball is kept perfectly dry, by giving it a slight coating of grease or of varnish, and in this condition it repels the water around it, and produces the hollow, or depression, as shewn in the figure. If the balls be moved along in the direction of the arrows, an elevation and depression will accompany them in the same manner as if they were stationary. We see, then, that if one of the balls be kept dry, and the other allowed to get wetted, the water will rise up round the latter, and be depressed round the former. Did any of our younger readers ever consider how it is that the oil in our lamps becomes consumed? We put oil into a hollow case, and attach a wick of cotton, which partly dips into, and partly rises above the surface of the oil, but we apply a light to the wick, at some distance above the cistern of oil; how, then, does the oil ascend? We may reply, that if there were no such thing as capillary attraction, our lamps would be of no use whatever. Oil, like other fluids, has in general no tendency to ascend, and it does so in this case only on account of the peculiar attraction of which we are now speaking. The wick consists of several filaments of cotton-thread, loosely twisted together, and in this form the intervals, or interstices, between them act like capillary tubes, up which the oil ascends. If we put a new wick and a new supply of oil into a lamp, we shortly see the upper extremity of the wick wet with pi; for it has ascended through the little channels between the filaments of cotton. As the oil burns away in form of flame, more oil ascends through the wick, and thus a supply is kept up. If we fill a glass tumbler with water, and put one end of a skein of thread, or of a wick of cotton, into it, and let the thread hang over the edge of the glass, so that the other end shall be outside, the glass will be entirely emptied. The little filaments carry up the water in minute streams and channels, and when it has arrived at the edge of the glass, it follows the course of the thread down the outside of the glass. If one end of a towel or handkerchief be placed in a wash-hand basin containing water, and if the towel hang over the the edge, and have the other end outside the basin, the water will be conveyed from the basin in a similar manner to that in the last experiment. The towel or the handkerchief is made of linen or silken threads, each of which is formed of a fibrous material, and the minute spaces between the fibres act as capillary tubes, and conduct the water through them. If a skein of cotton or of silk be placed partly into a glass tumbler full of water, and the other part be placed in an empty tumbler near the first, the water will flow from the first into the second glass, until it is at the same height in both. If we take a small square of glass, and dip it edgeways into a basin of water, we shall see that the water will slightly rise at the surface of the glass, but in so small a degree as to be hardly perceptible; but if we have two squares of glass, and place them face to face, but not absolutely touching, and then dip their edges into water, we shall find that the water will rise between them to a perceptible height: we have, in fact, a broad, flat, capillary tube, up which the water ascends. But the most pleasing way of producing this result is to place the pieces of glass in contact at one of their upright edges, and to let them gradually open towards the other upright edge. The water will then ascend to different heights between the plates, being highest at that side where the glasses are in contact. Fig. 3 may illustrate this. We have here two pieces Fig. 3. of glass connected by hinges at the left edges, while the opposite edges are clasped by an instrument which allows us to vary the distance of the two plates: when the plates are a little opened at the edges, they are then in the condition of a book very nearly closed. If now the lower edges be dipped in water, the water will ascend to a considerable height near the hinges, and to a gradually decreasing height as we go from those edges to the edges which are a little opened. The water forms a curved line, called hyperbola, which is represented by the darker lines of the figure. This experiment is more pleasing and striking if coloured water be employed. For this purpose we may drop some red or black ink into the water. The means by which we know whether or not a solid and a fluid exert this sort of attraction on one another is by observing whether the fluid wets the solid, or immersing the latter in the former, and drawing it out again. When we dip a piece of glass into water, and take it out again immediately, the glass is wetted, which is but another mode of saying that the glass has attracted the water. But if we dip the piece of glass into mercury, and take it out again, we find that the glass is not wetted; no mercury adheres to it, because mercury and glass do not attract each other. Also, if we grease the glass, water will not adhere to, or be attracted by it. The effect of attraction and repulsion exerted in this way may be further illustrated by fig. 4. Suppose we have water in a vessel, and have plates of different substances uspended by threads; and suppose some of the plates have the property of attracting water, and others the property of repelling it. The left hand figure, A, will represent the effect of dipping into the water a plate which attracts it; the water is raised a little on each side of the plate. The next adjoining one, B, is a plate having a repulsive tendency, so that, on dipping it into the water, a depression is seen on each side of the plate. Suppose, now, that we have two plates of the former, i.e., the attractive kind, and that we dip them into the water near each other, as at c and d; there is then an elevation of water on each side of each plate, and as the plates are drawn nearer together, in the direction of the arrows, the surface of the water between them gradually assumes a concave form, which becomes more decided as the plates approach each other. Lastly, if we have two plates with what we call the repulsive tendency; on dipping them in near each other, as at e and ƒ, the liquid is depressed at those two points; and on making the plates approach each other, the surface of the water between them will become more and more convex. If two dissimilar plates, such as D and E, be used, the water will rise round one and sink round the other. We may frequently see that, if a lump of white sugar be placed on a wet part of the table, the whole lump will become wetted. We may take a little water in a teaspoon, and place a lump of sugar in it, when we shall see the water gradually rising through the sugar, until the latter is all wetted. This is wholly caused by capillary attraction. The sugar is full of minute pores, through which the water ascends. If water coloured with red ink be employed, the experiment becomes more pleasing. The same may be said of a sponge. If we place the lower part of a sponge in water, the whole of the sponge becomes speedily wetted, by the ascension of the water through the little channels which pervade the sponge in every direction. If we observe the mercury in a barometer, its surface is sometimes flat, sometimes convex, and sometimes concave. It is convex when rising, concave when sinking, and flat when it has just begun to sink. These various appearances greatly influence the observer in predicting any changes in the weather. The various appearances of liquids in capillary tubes are collected in fig. 5. The first three tubes are curved at the bottom; the first tube is of unequal bore, being wider at the level h than in the straight part of the tube. mercury be poured into it, the fluid will rise considerably If above its level; it will rise to a, and form a convex surface. In the second tube, the mouth s is wider than the straight tube, and the latter is larger than an ordinary capillary tube. If water be poured in, it will be concave at s, and rise in the straight part of the tube to s', where it is also concave, but if the straight part of the tube be capillary, the water will rise up to a, as in the third tube, and its surface will be concave. The three straight tubes represent the elevation of different liquids above the level of the liquid into which they are plunged. |