106 GENERAL SCIENCE 



into the air, and so as the pressure is reduced the rising 

 molecules of vapor encounter less resistance and escape 

 more easily. Sometimes the pressure over evaporating 

 liquids is reduced by having vacuum pans, which are 

 pans with air-tight covers connected with a pump. 

 When air is partially exhausted from these pans, evapora- 

 tion is greatly hastened, 



4. There is a limit to the amount of water that the air 

 can hold ; and so, if the atmosphere is already saturated 

 with moisture in the form of water vapor, no more evapo- 

 ration can take place from the liquids under it. This 

 simply means that molecules of water are entering the 

 surface of the liquid from the air as fast as they are enter- 

 ing the air from the liquid. 



5. If there were no movement of air over an evaporating 

 liquid, the air would soon become saturated with water 

 vapor and evaporation would cease. The oftener the 

 air is renewed the more rapid will be the rate of evapora- 

 tion. 



Some liquids evaporate much more rapidly than others 

 under the same conditions. Alcohol, gasoline, and ether 

 are examples of such liquids. Liquids which evaporate 

 rapidly are said to be volatile, while others like heavy 

 oils are said to be non- volatile. 



Evaporation, a Cooling Process. Place a little alcohol 

 on the back of the hand and blow on it. Fasten some 

 cotton around the bulb of a thermometer, noting the 

 temperature. Now wet the cotton with ether and take 

 the reading of the thermometer again. What is the 

 effect in each of these cases? 



Evaporation is a cooling process, because heat is used 

 in the change. Evaporation involves a change of state, 

 and energy in the form of heat is necessary for such a 



