126 ELEMENTS OF GENERAL SCIENCE 



leave the bottle excepting through the leaves. The water 

 piven off from the leaves soon saturates the space in the bot- 

 tle, and the excess is deposited on the inside of the bottle. 

 If the leaves are removed from the plant they should be 

 placed with their lower ends in water. 



A more satisfactory way to show the loss of water is to 

 weigh a potted plant from time to time, having first wrapped 

 the pot in sheet rubber (Why ?). If the area of the leaves is 

 ascertained, it is possible to know just how much water has 

 been evaporated per square inch in a day. It has been found 

 that a good-sized sunflower plant may evaporate as much as 

 a quart of water daily. A large tree will evaporate from 700 

 to 900 pounds of water on a favorable day, and the grass on 

 a vacant city lot may give off a ton of water in a day. 



144. The principle of the refrigerator. As water in freezing 

 does not change temperature until all of it is frozen, so when 

 ice melts, it does not get warmer than C. until all the ice 

 is melted. The heat necessary to melt it is taken from sur- 

 rounding objects, and so they are cooled as long as the ice 

 is melting. It is on this principle that refrigerators work. 

 In order to economize as much as possible in the use of ice, 

 the walls of the refrigerator are packed with substances 

 through which heat does not easily pass. If the refrigerator 

 is not well built in this particular, so much heat comes into it 

 from the outside that the ice does not lower the temperature 

 in the box very much, and a great deal of ice is consumed. 

 If a twenty-five-pound piece of ice is placed in one refriger- 

 ator or in an ordinary wooden box, and the same amount in 

 another refrigerator, and records are kept showing the length 

 of time required for both pieces of ice to melt, an interest- 

 ing demonstration will be provided regarding the efficiency 

 of refrigerators. ^ 



145. Freezing mixtures. Common salt and some other sub- 

 stances absorb moisture very readily. Salt absorbs it so readily 

 that when the humidity of the air is rather high, it absorbs 



