Fig. 19.
70. Farther Illustrations.—When you see stems of plants rising above the surface of stagnant water you will observe that the water is considerably raised about them. This is from the attraction between them and the water. For the same reason water is not as high in the middle of a tumbler as it is at the sides. If you immerse a piece of glass in water, the water will rise at its sides as represented in Fig. 16. If you immerse two together, as in Fig. 17, the water will rise higher between than outside of them, because the particles between are attracted by two surfaces, while those outside are attracted only by one. It is for the same reason that two men can raise a weight higher than one of them can alone. And if the pieces of glass be brought quite near together, as in Fig. 18, the water will be raised higher still, because there is less to be raised by the two surfaces. It is just as two men can raise a small weight higher than they can a large one. The same thing may be beautifully illustrated in this way: Let two pieces of glass, as represented in Fig. 19, be immersed in colored water, with two of their edges joined together at A B, the opposite edges at E D C being separated. The height to which the fluid rises will make a curved line, A F C, it being lowest at the edges which are separated, and highest at the edges which are joined together.
Fig. 20.
Fig. 21.
71. Rise of Liquids in Tubes.—For the same reason that water rises higher between plates of glass than outside, so it will rise higher in a tube than it will outside of it. The diagram in Fig. 20 will make this clear. I represent in this a transverse section of a tube, enlarged so that the demonstration may be plain. We will take a particle on the inside and the outside at equal distances from the glass. It is clear that the particle a is not as near to as many particles of the glass as the particle b is. The lines drawn show this. The longest lines extending from the particles a and b to the glass are equal in length; that is, a e and a f are equal to b g and b h. It is clear, therefore, that all the glass between the lines at c and d is as near to the particle b as the glass between the lines at e and f is to the particle a. But this is not all. The particle b is near enough to all the inside of the tube to be attracted by it, while very little attraction is exerted upon a by any part of the glass beyond that which is included between e and f. The same difference can be shown with regard to all the particles on the inside of the tube compared with those outside. The former are nearer to more particles of the glass than the latter, and therefore are more strongly attracted. Again, as the nearer the plates of glass are (§ 70) the higher the water rises between them, so the smaller the tube is the higher will the water rise in it. You can try the experiment as represented in Fig. 21. It is obvious that the particle b (Fig. 20) would not be very strongly attracted by the part of the tube opposite if the tube were a large one; but it would be if the tube were very small, for then it would be quite near to that part.
72. Capillary Attraction.—The term capillary (derived from the Latin word capilla, hair) has been commonly applied to the attraction exhibited under the circumstances just noticed, because it is most obvious and was first observed in tubes of very fine bore. The same term is used when the attraction is seen in the rising or spreading of a liquid in interstices as well as in tubes. Thus capillary attraction causes the rising of oil or burning fluid in the wicks of lamps. The liquid goes up in the interstices or spaces between the fibres, as it does in the spaces of tubes. I will give some other examples. If you let one end of a towel be in a bowl of water, the other end lying over upon the table, the whole towel will become wet from the spreading of the water among the fibres in obedience to capillary attraction. If you suspend a piece of sponge so that it merely touch the surface of some water, or if you lay it in a plate with water in it, the whole sponge will become wet. So, too, if you dip the end of a lump of sugar in your tea, and hold it there a little time, the whole will be moistened. In very damp weather the wood-work in our houses swells from the spreading of water in the pores of the wood in obedience to capillary attraction. Especially will this be so in basement rooms, where the water can go up from the ground in the pores of the walls, as well as from the damp air. In watering plants in pots, if the water be poured into the saucers, it will pass up through the earth by capillary attraction. For the same reason plants and trees near streams grow luxuriantly, being abundantly supplied with water, which rises to their roots through the pores of the soil. The disposition of wood to imbibe moisture in its pores has sometimes been made use of very effectually in getting out millstones. First a large block of stone is hewn into a cylindrical shape. Then grooves are cut into it all around where a separation is desired, and wooden wedges are driven tightly into them. These absorb moisture from the dews and rain, and therefore swell so much as to split the stone in the direction of the grooves. The blotter which you use furnishes an illustration of capillary attraction, the ink being taken up among the fibres of the paper. Ordinary writing paper will not answer as a blotter, because the sizing fills up the interstices between the fibres.