38 Introduction. 



the column rises higher and higher until the strength of the 

 pull is balanced by the weight of the water column so 

 lifted. 



51. Evaporation. Very many substances, when they 

 have a free surface exposed, change in weight by evapora- 

 tion. The action of heat causes the molecules to vibrate 

 rapidly and by colliding with one another near the free sur- 

 face some are thrown out by the force and direction of the 

 blows. It is in this way that clothes dry rapidly on a 

 warm day, and that water evaporates from the surface "of 

 the soil, from the leaves of plants and from the bodies of 

 animals. Even snow and ice evaporate, as camphor does, 

 without first becoming liquid, but the process and the cause 

 are the same as the evaporation of water, namely, rapid 

 vibration and collision at the surface due to the absorption 

 of heat energy from .outside. 



It is often said that the air takes up the moisture and 

 the more rapidly the dryer it is or the stronger the wind 

 blows. The air itself is not the cause of the more rapid 

 evaporation observed, neither does evaporation stop when 

 the air becomes saturated with moisture. Evaporation 

 may be even more rapid in a vacuum and in rarefied air 

 than where the air is dense and the pressure heavy; and 

 when the air is saturated with water vapor and evaporation 

 appears to stop, it may be going on just as rapidly, only 

 condensation may be taking place at the same rate, that is, 

 just as many molecules of water return from the space 

 above the surface as leave it in a unit of time. 



52. Solution of Solids. The solution of solids, like sugar 

 or salt in water, is not fundamentally different, either in 

 cause or in manner, from the evaporation of water or of 

 camphor referred to in (51), and is due to the absorption 

 of heat from without which causes some of the surface 

 molecules to be thrown into such rapid motion that the 

 attractive force which draws them toward the center is no 

 longer able to retain them in place and they are thrown 

 out. 



