RAIN-MAKING. 48 1 



duce saturation of the atmosphere under the conditions above 

 mentioned is more than twice as great at 80 F. as at 50 F. 



On the other hand, lowering the temperature of the liquid and 

 vapor by a like amount lessens the number of molecules given off 

 from the liquid surface more rapidly than it lessens the number 

 striking upon the surface. Accordingly, we say that raising the 

 temperature increases evaporation ; lowering the temperature 

 increases condensation. 



Now, it happens that this same force of cohesion may hold 

 water molecules upon the surface of most solid bodies as strongly 

 as upon the surface of water itself, and in many cases even more 

 strongly. Accordingly, if a solid body of this kind be placed in 

 the atmosphere, the same exchange of water molecules will take 

 place between its surface and the air as between a water surface 

 and the air. In fact, as soon as a layer of water molecules is 

 formed over its surface, it becomes a water surface. According- 

 ly, if a solid particle be placed in an atmosphere saturated with 

 water vapor and the temperature be lowered, the water molecules 

 will accumulate upon its surface faster than they are driven off, 

 and we say that a precipitation of dew is taking place upon it. 

 The air is accordingly said to reach its dew point when it reaches 

 its point of saturation. 



There are other substances which hold fast in a different way 

 the water molecules which strike upon their surface. These sub- 

 stances form either chemical compounds or solutions with water, 

 and in this way remove the water molecules from the places where 

 they strike to the interior of the compound or the solution. Sul- 

 phuric acid is a good example of this class of substances. If a 

 vessel of sulphuric acid be placed in a receiver filled with water 

 vapor, the acid holds fast all the water molecules which strike its 

 surface, and sends off no other water molecules to replace them. 

 Since all the water molecules in the receiver will in time come in 

 contact with the acid surface, they will ultimately all be held in 

 a liquid form by the acid. Accordingly, a receiver of moist air 

 can be changed to dry air by allowing it to stand for a sufficient 

 length of time over sulphuric acid. 



There are very many other substances which, like sulphuric 

 acid, have the property of condensing the water molecules from 

 a space which is not saturated with them. Such substances are 

 said to be deliquescent, or to gather moisture from the air. Com- 

 mon salt and caustic potash are good examples of deliquescent 

 substances. 



There is still another method of producing condensation. If 

 an inclosed space contain water vapor enough to bring it to the 

 point of saturation, and if the volume of the space be decreased 

 without changing the temperature, more molecules will strike 



VOL. XLV. 38 



