482 THE POPULAR SCIENCE MONTHLY. 



upon a given surface of the containing walls than when the vol- 

 ume of the gas was greater. Since the temperature remains the 

 same, the same number of molecules will be driven off from a 

 given surface of these walls as before. There will, accordingly, 

 be a condensation upon the walls, which will continue until 

 enough gaseous molecules have been removed to make the ex- 

 change again even. 



These are the three known methods of changing water vapor 

 to the liquid form viz., by lowering the temperature of the vapor 

 and the other bodies in contact with it until the point of satura- 

 tion has been passed, by compressing the vapor until there are 

 enough molecules in unit volume to produce saturation, and by 

 allowing the vapor molecules to strike upon some surface which 

 will immediately take them into solution or into chemical combi- 

 nation. I know of no other method by which water vapor, or any 

 other vapor, can be changed into the liquid form. 



The conditions necessary for the precipitation of the aqueous 

 vapor from the atmosphere are, then, as follows : 



(1) The air must contain enough molecules of water vapor to 

 more than saturate it, and must contain at the same time either 

 solid or liquid bodies upon which these vapor molecules may be 

 held fast by cohesion ; or (2) the air which does not contain enough 

 water vapor to saturate it may come in contact with solid or liquid 

 substances, which combine with or dissolve the water molecules 

 which strike upon them. 



This latter condition can manifestly play no important part in 

 atmospheric precipitation. The only condition under which such 

 substances could cause condensation above the earth's surface 

 would necessitate their distribution throughout the atmosphere, 

 and if they were so distributed they would constantly absorb the 

 atmospheric vapor until, loaded down with it, they would sink to 

 the earth, and there would be a condition of perpetual rainfall. 



For the general precipitation of atmospheric vapor we must 

 accordingly depend upon the condensation due to cohesion. Of 

 this form of condensation, dew is the simplest illustration. Dur- 

 ing the day the earth and the solid bodies upon its surface are 

 raised by the sun's radiation to a temperature higher than that 

 of the surrounding air. So long as this is the case the atmos- 

 pheric vapor will not condense upon them, even if the air be 

 cooled to the point of saturation. In the night the same sub- 

 stances which absorbed the sun's heat fastest now radiate it fast- 

 est and soon become colder than the surrounding air. As soon 

 as they are cooled to the temperature of saturation of the sur- 

 rounding air the vapor molecules begin to condense upon their 

 surface. 



Now, the condensation of water vapor in the air above the sur- 



