36 INTRODUCTION TO GENERAL SCIENCE 



Milk may be kept from souring, and butter may be pre- 

 served in a more solid condition, by being placed in a saucer 

 of water and wrapped with a cloth which is kept damp by the 

 water in the saucer. Cooling closets are constructed on the 

 same principle; that is, a light framework is covered with 

 sacking which is kept moist by a tank on top. The heat 

 which the water requires in order to become a vapor is taken 

 from the food within the closet, and thus the food becomes 

 cooled. 



Those liquids which have the tendency to evaporate easily 

 necessarily lower the temperature of their surroundings more 

 than does water when it evaporates. Ether, alcohol, and 

 any of the volatile liquids do this to a very marked degree. 

 It is quite easy to freeze water by means of the evaporation of 

 ether. 



The liberation of heat by condensation is made use of in 

 the system of steam heating. Water is boiled in a boiler, and 

 a large amount of heat is absorbed in changing the water to 

 steam. The steam passes through pipes to the colder parts 

 of a building and condenses to water, thereby giving up to the 

 air of the rooms all of the heat which it had absorbed from 

 the fire. The water which leaves a radiator may be of the 

 same temperature as the steam which entered, although it 

 has given up a vast amount of heat in its condensation. Dew, 

 in forming, raises the temperature of the atmosphere, and a 

 foggy night is usually warmer than a clear night. Both these 

 facts are due to the principles of condensation already ex- 

 plained. 

 References: 



1. 1103 : 134-135. Formation of Ground Fog Prevents Fur- 



ther Cooling. 



2. 1803 : 102. Freezing by Evaporation. 



