10 THE PRODUCTION AND USE OF LIGHT 



Exercise 6. Effect of Extent of Surface on Rate of Evaporation, 



Fill a small drinking cup about half full of water, measuring 

 exactly the amount used. Again measure the same amount of 

 water, and place it in a large, shallow pan or basin. Set the cup be- 

 side the pan in a safe place where it will not be disturbed. The two 

 vessels should have the same temperature and be exposed to the same 

 air currents. Examine them daily and note the rate of evaporation 

 in each, till one is dry. Measure the amount of water remaining 

 in the other. 



Which is dry first? What conclusion do you draw regarding 

 the effect of extent of surface on evaporation? 



Exercise 7. Effect of Temperature on Rate of Evaporation 



Place one drinking cup containing a measured quantity of water 

 on or near the stove or radiator. The water should not boil, but 

 be kept warm. Place another cup of the same size and shape con- 

 taining the same amount of water in a cooler place. Watch these 

 two cups till one is dry. Draw a conclusion regarding the effect of 

 temperature on the rate of evaporation. 



Exercise 8. Effect of Air Currents on Rate of Evaporation 



Place two drinking cups of the same size and shape, each about 

 one-half full of water (measuring amounts accurately), side by side 

 in an open window or in some other position where the wind can sweep 

 past them. Turn a large, 2-qt., cup or a small pail over one of the 

 cups of water. Examine the cups of water daily and note which suf- 

 fers the greater evaporation. What is your conclusion? 



Exercise 9. Rate of Evaporation Varies with Different Liquids 



Place two drinking cups of the same size and shape side by side 

 in some safe place. Fill one about half full of water, and place in the 

 other the same amount of alcohol or gasoline. (Caution. It is 

 dangerous to leave gasoline in a closed room. It should be in the open 

 air and no flames should be brought near it.} Note which evaporates 

 more rapidly. Draw your conclusion. 



Exercise 10. Effect of Evaporation on Temperature 



Let a few drops of alcohol or gasoline fall upon the back of your 

 hand. Does it produce the sensation of heat or cold? Wrap a 

 small piece of cotton cloth around the bulb of the thermometer and 

 tie with a cord or thread. Take the reading of the temperature. 



