molecules within, it is clear that molecules must continually move toward the center of the mass until the whole has reached the most compact form possible. Now the geometrical figure which has the smallest area for a given volume is a sphere. We conclude, therefore, that if we could relieve a body of liquid from the action of gravity and other outside forces, it would at once take the form of a perfect sphere. This conclusion may be easily verified by the following experiment: Let alcohol be poured upon the top of water in a wide flat-sided bottle. Then with a pipette insert a large globule of oil at the bottom of the layer of alcohol. The oil will be seen to float as a perfect sphere within the body of the liquid (Fig. 92). FIG. 92. A spherical globule of oil, freed from action of gravity ་ Liquids are not commonly observed to take the spherical form because ordinarily the force of gravity is so large as to be more influential in determining their shape than are the cohesive forces. As verification of this statement we have only to observe that as a body of liquid becomes smaller and smaller—that is, as the gravitational forces upon it become less and less it does indeed tend more and more to take the spherical form. Thus, very small globules of mercury on a table will be found to be almost perfect spheres, and small raindrops or minute floating particles of all liquids are very nearly spherical. 116. Contractility of liquid films; surface tension. The tendency of liquids to assume the smallest possible surface furnishes a simple explanation of the contractility of liquid films. Let a soap bubble 2 or 3 in. in diameter be blown on the bowl of a pipe and then allowed to stand. It will at once begin to shrink in size, and in a few minutes will disappear within the bowl of the pipe. The liquid of the bubble is simply obeying the tendency to reduce its surface to a minimum, a tendency arising entirely from the mutual attractions which its molecules exert upon one another. A candle flame held opposite the opening in the stem of the pipe will be deflected by the current of air which the contracting bubble is forcing out through the stem. Again, let a loop of fine thread be tied to a wire ring, as in Fig. 93. Let the ring be dipped into a soap solution so as to form a film across it, and then let a hot wire be thrust through the film FIG. 93 FIG. 94 FIG. 95 Illustrating the contractility of soap films inside the loop. The tendency of the film outside the loop to contract will instantly snap out the thread into a perfect circle (Fig. 94). The reason that the thread takes the circular form is that, since the film outside the loop is striving to assume the FIG. 96. Some of the stages through which a slowly forming drop passes smallest possible surface, the area inside the loop must of course become as large as possible. The circle is the figure which has the largest possible area for a given perimeter. Let a soap film be formed across the mouth of a clean 2-inch funnel, as in Fig. 95. The tendency of the film to contract will be sufficient to lift its weight against the force of gravity. The tendency of a liquid to reduce its exposed surface to a minimum, that is, the tendency of any liquid surface to act like a stretched elastic membrane, is called surface tension. The elastic nature of a film is illustrated in Fig. 96, which is from a motion-picture record of some of the stages through which a slowly forming drop passes. The external layers of molecules act like an elastic bag to hold the rest of the liquid within. 117. Ascension and depression of liquids in capillary tubes. It was shown in Chapter II that in general a liquid stands at the same level in any number of communicating vessels. The following experiments will show that this rule ceases to hold in the case of tubes of small diameter. Let a series of capillary tubes of diameter varying from 2 mm. to 1 mm. be arranged as in Fig. 97. FIG. 97. Rise of liquids in capillary tubes When water or ink is poured into the vessel it will be found to rise higher in the tubes than in the vessel, and it will be seen that the smaller the tube, the greater the height to which the liquid rises. If the water is replaced by mercury, however, the effects will be found to be just inverted. The mercury is depressed in all the tubes, the depression being greater in proportion as the tube is smaller (Fig. 98 (1)). This depression is most easily observed with a U-tube like that shown in Fig. 98 (2). (2) T FIG. 98. Depression of mercury in capillary tubes Experiments have established the following laws: 1. Liquids rise in capillary tubes when they are capable of wetting them, but are depressed in tubes which they do not wet. 2. The elevation in the one case and the depression in the other are inversely proportional to the diameters of the tubes. It will be noticed, too, that when a liquid rises, its surface within the tube is concave upward; when it is depressed, its surface is convex upward. 118. Cause of curvature of a liquid surface in a capillary tube. All the effects presented in the last paragraph can be explained by a consideration of cohesive and adhesive forces. However, throughout the explanation we must keep in mind two familiar facts: first, that the surface E E R of a body of water at rest (for example a pond) is at right angles to the resultant force Condition for elevation of a liquid near a wall (that is, gravity) which acts upon it; secondly, that the force of gravity acting on a minute amount of liquid is negligible in comparison with the cohesive forces acting within the liquid (see § 115). E F Consider, then, a very small body of liquid close to the point o (Fig. 99), where water is in contact with the glass wall of the tube. Let the quantity of liquid considered be so minute that the force of gravity acting upon it may be disregarded. The force of adhesion of the wall will pull the liquid particles at o in the direction oE. The force of cohesion of the liquid will pull these same particles in the direction oF. The resultant of these two pulls on the liquid at o will then be represented by oR (Fig. 99) in accordance with the parallelogram law of Chapter V. If, then, the resultant oR of the adhesive force oE and the cohesive force of lies to the left of the vertical om (Fig. 100), since the surface of a liquid always assumes a position at right angles to the resultant force, the liquid must rise up against the wall as water does against glass (Fig. 100). FIG. 101. Condition for the depression of a liquid near a wall If the cohesive force oF (Fig. 101) is strong in comparison with the adhesive force oE, the resultant OR will fall to the A SMALL FARM TRACTOR Farm tractors are used extensively in dry farming, which consists in specially preparing the soil for reception of water and Motor Company) |