171. Relation of the Air's Pressure to the Boiling Point.—Water heated to 212 degrees of Fahrenheit boils, that is, it becomes vapor. Now if water be heated on the summit of a high mountain it boils before it arrives at this degree of temperature. On the top of Mont Blanc it boils at 180 degrees, that is, 32 degrees below the boiling point of water at the foot of the mountain. This is because the pressure of the air acts in opposition to the change of water in to vapor, and the less the pressure is the less heat will be required to vaporize the water. We may illustrate this influence of the pressure of air upon boiling by the following experiment. Let a cup of ether (which boils at 98 degrees) be placed under the receiver of an air-pump. On rarefying the air by the pump the ether will boil. The general effect of pressure upon boiling may be prettily illustrated by another experiment. Boil some water in a thin flask over a spirit-lamp. Blow out the lamp, and, corking the flask tightly, let the boiling cease. If, now, you pour some cold water over the flask the boiling will commence again with considerable force. Why? Because you condense the steam which is over the water by the application of cold, and thus take off the pressure. Then, again, if, while the water is boiling, you pour hot water over the flask, the boiling ceases, because the heat favors the accumulation of steam, and therefore renews the pressure on the surface of the water.
You can see from what has been stated that most liquids have the liquid form because of the pressure of the atmosphere upon them. If there were no atmosphere, ether, alcohol, the volatile oils, and even water, would fly off in vapor; and the earth would be enveloped in a vaporous robe, for the particles of the vapors would be held to the earth by attraction, just as the particles of the air are now, § 151.
Fig. 117.
Fig. 118.
172. Syphon.—The pressure of air upon fluids is beautifully exemplified in the operation of the syphon. This instrument is simply a bent tube having one branch longer than the other. Its operation is shown in Fig. 117. The tube having been first filled with the liquid, has its shorter branch in the liquid of the vessel A, which is to be emptied, and the other in the vessel B, which is to receive the liquid. As you see it here, the opening of the long branch is below the surface of the liquid in B. It is manifest, therefore, that the air presses equally upon the surfaces in both vessels, tending to support the fluid in the tube, just as the fluid is supported in the jar in Fig. 114. But, notwithstanding these equal pressures, the liquid runs up the tube from A, and down its longer branch into B. Why is this? As the pressure of a column of fluid is as its height, there is greater pressure or weight in the longer branch than in the other; and it is this difference in weight that causes the flow from A into B through the syphon. The difference in the columns in the two branches is not the difference in length of these branches, but the distance between the levels of the fluid in A and B, that is, the distance from a to b. The operation, then, of the instrument is this. There is a constant tendency to a vacuum at C, the bend of the tube, from the influence of gravitation on the excess of fluid in the long branch over that in the short one. This tendency is constantly counteracted by the rise of fluid in the short branch, it being forced up by the pressure of the air upon the surface of the fluid in A.
Fig. 119.
If the syphon were so placed that the surface of the liquid in A is precisely on a level with that in B, as represented in Fig. 118, the liquid would remain at rest, for as pressure is as the height, § 121, and the pressures on the two surfaces are equal, there would be an exact balance. But let the surface in B be in the least lower than in A and the flow will begin. And the greater the distance between the two levels the more rapid will be the flow, for the greater will be the influence of gravitation in the long branch.