Capillarity (referred to on page 25) is the term used to define capillary force, and is derived from the word _capillus_, a hair; and so very small bore tubes are called capillary tubes. We know that when we plunge a glass tube into water the liquid will rise up in it, and the narrower the tube the higher the water will go; moreover, the water inside will be higher than at the outside. This is in accordance with a well-known law of adhesion, which induces concave or convex surfaces[9] in the liquids in the tubes, according as the tube is wetted with the liquid or not. For instance, water, as we have said, will be higher in the tube, and concave in form; but mercury will be depressed below the outside level, and convex, because mercury will not adhere to glass. When the force of cohesion to the sides of the tube is more than twice as great as the adhesion of the particles of the liquid, it will rise up the sides, and if the forces be reversed, the rounded appearance will follow. This accounts for the convex appearance, or “meniscus,” in the column of mercury in a barometer. Amongst the complicated experiments to demonstrate molecular attraction, the following is very simple and very pretty:—Take two small balls of cork, and having placed them in a basin half-filled with water, let them come close to each other. When they have approached within a certain distance they will rush together. If you fix one of them on the blade of your penknife, it will attract the other as a magnet, so that you can lead it round the basin (fig. 68). But if the balls of cork are covered with grease they will _repel_ each other, which fact is accounted for by the form of the _menisques_, which are convex or concave, according as they are moistened, or preserved from action of the water by the grease. [Illustration: Fig. 67.—An improvised syphon.] This attribute is of great use in the animal and vegetable kingdoms. The rising of the sap is one instance of the latter. Experience in hydrostatics can be easily applied to amusing little experiments. For instance, as regards the syphon, we may make an image of _Tantalus_ as per illustration (fig. 69). A wooden figure may be cut in a stooping posture, and placed in the centre of a wide vase, as if about to drink. If water be poured slowly into the vase it will never rise to the mouth of the figure, and the unhappy _Tantalus_ will remain in expectancy. This result is obtained by the aid of a syphon hidden in the figure, the shorter limb of which is in the chest. The longer limb descends through a hole in the table, and carries off the water. These vases are called _vases of Tantalus_.