[Illustration: Fig. 115.] 167. =Pressure of the Air on Liquids.=--If you plunge a tumbler into a vessel of water, and turning it over hold it so that its open part is just under the surface, it will remain full. The reason is that the weight of the air pressing upon the surface of the water in the vessel prevents the water in the tumbler from passing downward. Now if you introduce a bent tube under the tumbler, as in Fig. 113, and blow through it, the air that you force up into the tumbler presses the water down, taking its place. That is, the pressure of the air acts in opposition to the pressure of the air outside upon the surface of the water in the vessel. You take a jar, _a_, Fig. 114, and filling it with water, turn it over with its open end downward, the water will remain in the jar. You have here a representation of the pneumatic trough used by the chemist in collecting gases. To fill the jar _a_ with gas he puts the mouth of the retort from which the gas issues under the jar _a_, and the gas passing upward expels the water, as the water is expelled by the breath from the tumbler in Fig. 113. In Fig. 115 (p. 125) is represented an experiment which shows not only that the pressure of the air sustains the column of water in the cases cited above, but also that it makes no difference in what direction this pressure is exerted. Take a large tube, _a_, closed at one end and open at the other, and fill it even full with water. Place, now, a piece of writing-paper over its mouth, and carefully invert the tube, as seen in the figure. The paper will remain, and the water will not run out. It is the pressure of the air that sustains the water, and the paper only serves to maintain the surface of the water unbroken. If the paper were not there the particles of the air would insinuate themselves among those of the water, and pass upward in the tube. You can try this experiment with a wine-glass, and may even succeed with a tumbler. We see in these experiments the reason that a liquid will not run from a barrel when it is tapped, if there be no vent-hole above, unless there be so large an opening made as to let the air work its way in bubbles among portions of the liquid. It is this entrance of the air that causes the gurgling sound in pouring a liquid from a bottle.