116 ELEMENTS OF GENERAL SCIENCE 



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. 



129. Refrigerators. 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 surrounding 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 refrig- 

 erator 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 an ordinary wooden box, and the same 

 amount in a refrigerator, and records are kept showing the 

 length of time required for both pieces of ice to melt, an 

 interesting demonstration will be provided regarding the 

 efficiency of. the refrigerator. It will also prove interesting 

 to perform a similar experiment to determine the relative 

 efficiency of different kinds of refrigerators. 



130. Freezing mixtures. Some substances absorb moisture 

 very readily. Common salt is one of these. It absorbs it so 

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

 absorbs water even from the air and becomes wet, as all of 

 us have had occasion to notice when using a salt shaker in 

 damp weather. Salt not only absorbs gaseous water from the 

 air, but it can also absorb water from a piece of ice, thereby 

 changing the ice to the liquid form. This is the explanation 

 of the use of salt to melt ice on sidewalks and in other places. 



When ice is melted, heat is required to carry on the process. 

 When salt melts ice, the source of heat is the ice and the 



