this happens they become free gaseous molecules, and move off in straight lines under the impulse of the force which set them free until they come into collision with other molecules.
At the surface of separation between water and air the conditions are accordingly as follows: The surface layer of water molecules is held down by the force called cohesion, but the individual molecules of this layer are being continually bumped off by the vibrations of the molecules below them. Some of these free molecules are undoubtedly driven back by the bombardment of the air molecules above them, so that they escape much more slowly into the air than they do into a vacuum, but those which once escape into the air are knocked about by the air molecules and by each other until they are pretty evenly distributed throughout the air. After a time they become so numerous in the space above the water that, in their irregular excursions between their collisions with other molecules, they begin to strike the surface of the water, and then, under favorable conditions, they penetrate into the liquid and are held fast. This process continues until finally as many molecules enter the water as escape from its surface, and then, while a constant exchange is taking place between the liquid and gaseous molecules, the average number in the space above the liquid remains constant. This space is then said to be saturated with vapor molecules. The number of molecules required to saturate this space is the same whether the space already contains air molecules or not, but, on account of the number of water molecules which are beaten back by the air molecules, it takes much longer for the space to become saturated when it is already filled with air than it does when there are no other molecules in it. The air molecules, however, hinder the vapor molecules from striking the surface of the water as often as they prevent them from leaving the surface, so they do not influence the total number required to produce saturation.
When the point of saturation has been reached, an increase of temperature—i. e., an increase of the molecular vibration of the water—causes the molecules to be driven off faster than before. It also causes the gaseous molecules to strike the surface of the water oftener than before. But an increase of temperature means a corresponding increase of vibration of all the molecules; and, since there are very many more liquid than gaseous molecules in the same volume, the total increase of molecular vibration corresponding to a given rise of temperature will be much greater for the liquid than for the gas, and a correspondingly greater number of molecules will be thrown off at the surface of the liquid than will be returned to it. Accordingly, the higher the temperature, the more molecules are required to saturate the space above the water. In fact, the amount of water vapor required to pro-