170 



GENERAL SCIENCE 



made of distilled water, and for this reason it is purer 

 and more free from germs than the ice of rivers or 

 lakes. 



The water to be frozen is put into a metal tank that 

 holds about 300 pounds, as shown in the illustration. 



DRAWING 3oo-PouND CANS or ICE FROM THE SALT WATER 



These tanks are usually about four feet high and they 

 are lowered into a large tank of salt water, which 

 generally covers as much area as a large room, but not 

 deep enough to permit the salt water to flow into 

 the freshwater tanks. The temperature of the salt 

 water is kept at about 14 F. or -10 C. (A saturated 

 solution of salt water freezes at -22 C.) The tempera- 

 ture of the salt water is kept low by ammonia pipes which 

 are in the salt water between the rows of freshwater 

 tanks. The ammonia here goes through the same pro- 

 cess as that described in 112 on refrigeration with 



