C'HAXOE* IX STATE OF MATTER 



countries water is cooled by being placed in porous 

 jars. A small quantity of water passes through the 

 pores of the jar, and, 

 on evaporating, with- 

 draws enough heat 

 from the water re- 

 maining in the jar to 

 cool it. 



Artificial ice. This 

 same principle is util- 

 ized in the manufac- 

 ture of artificial ice. 

 In countries where it 

 is never cold enough 

 to freeze the water in 

 ponds, all the ice 

 formerly used had 

 to be shipped from 

 colder countries. Of 

 late years, however, 

 men have devised 

 methods by which ice 

 may be made very 

 cheaply even in the warmest places. In fact, these 

 methods offer so many advantages, that even in our 

 northern cities great quantities of artificial ice are used. 



The method most commonly used in the artificial 

 production of ice is known as the ammonia method. 

 This depends upon the fact that pure ammonia, which 



Copyright by Henry G. Pedbody. 



Fig. 23. An Indian woman carrying 



an olla, a porous water jug, in which 



the water is cooled by the evaporation 



of water from the outside of the olla. 



