480 THE POPULAR SCIENCE MONTHLY. 



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 condi- 

 tions are accordingly as follows : The surface layer of water mole- 

 cules 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 be- 

 tween 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 es- 

 cape from its surface, and then, while a constant exchange is tak- 

 ing 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 corre- 

 sponding to a given rise of temperature will be much greater for 

 the liquid than for the gas, and a correspondingly greater num- 

 ber of molecules will be thrown off at the surface of the liquid 

 than will be returned to it. Accordingly, the higher the tempera- 

 ture, the more molecules are required to saturate the space above 

 the water. In fact, the amount of water vapor required to pro- 



