84 



WATER 



which is now at 4 C., is cooled further, it becomes lighter 

 than the water at 4 C., and floats upon the warmer water. 

 When, finally, the surface of the water freezes, the ice 

 that is formed also floats. Hence the water below the 

 ice is rarely cooled below 4 C. Because of this fact water 

 animals and plants survive the winter, and live even in 

 Arctic waters. 



88. Artificial Ice. The supply of natural ice is so un- 

 certain, especially in warm climates, that men have been 



Hot Gas 130C 



17 C 



-15C 



Compression Pump Expansion Coils and l.ce Molds 



FIG. 71. 



Cooling Brine for Ice Making by the Expansion of 

 Compressed Liquid Ammonia. 



forced to make ice by artificial freezing. To freeze the 

 water some liquid is used that boils at a low temperature. 

 Usually the liquid is liquid ammonia. This is not "am- 

 monia water," but the gas, ammonia, which has been 

 liquefied by pressure. The liquid ammonia is made to 

 evaporate rapidly by the removal of the pressure. The 

 turning of the liquid into the gas requires heat, just as 



