110 



GENERAL SCIENCE 



gas in the evaporating pipes. As the liquid ammonia 

 passes through the valve at D, it evaporates so rapidly 

 that enough heat is taken from the surrounding pipes 

 and brine to cool them below the freezing point of water. 

 This brine then circulates around the cans containing the 

 water to be frozen. It is possible to use the same am- 

 monia gas over and over again. 



Cold Storage. The same kind of apparatus is now 

 used in the artificial cooling of cold storage rooms, except 

 that the cooled brine is forced through pipes placed in 

 the rooms to be cooled. It is possible with such a system 

 to have the temperature ranging from 16 Fahrenheit up. 

 Occurrence of Water. Water is one of the most 

 widely distributed compounds. It occurs in all three 

 states of matter and is very useful in each of these states. 

 Besides covering three quarters of the earth's surface, 

 water composes a large part of the earth and the plants 

 and animals that live on it. Our own bodies are 70 per 



cent water, and the 

 foods which we eat 

 are largely composed 

 of it. 



Water Pressure. - 

 We are all aware of 

 the pressure that is 

 exerted by water. If 

 a cork is held under 

 water and then re- 

 leased, it will rise to 

 the surface. This is 

 proof of the upward 

 It is this- pressure which causes 



FIG. 100. The pressure in liquids varies 

 with the depth. 



pressure of the liquid, 

 boats to float. 



