MANUFACTURED ICE AND FREEZING MIXTURES 163 



attached to an ordinary refrigerator. On top of the refriger- 

 ator is a COMPRESSOR for compressing sulphur dioxide gas. 

 The compressor is run by a small electric motor. The com- 

 pressed gas passes through cooling coils where it is liquefied. 

 It then passes into the refrigerating coils which are in the ice 

 compartment of the refrigerator. Here the liquid sulphur 

 dioxide is released from pressure and changes into a gas, pro- 

 ducing a lower temperature and dryer air than can be secured 

 by the use of ice. 



II. MANUFACTURED ICE AND FREEZING MIXTURES 



192. Need of Manufactured or Artificial Ice. Without 

 manufactured ice it would be impossible to make use of the 

 refrigerator in many portions of the civilized world. Even 

 in climates where natural ice is produced in sufficient quan- 

 tities, artificial ice is regarded as a necessity for many purposes 

 on account of its greater purity. Most towns and cities of 

 any considerable size have their ice factories. Artificial ice is 

 today in such common use that we should learn something 

 about the principles involved in its manufacture. 



193. The Ice Plant. The essential parts of the ordinary 

 plant for the manufacture of ice are: (1) Steam boilers and a 

 steam engine; (2) an ammonia compressor (B, Fig. 124); (3) 

 cooling coils, through which the ammonia passes and over 

 which flows constantly a stream of cool water (C, Fig. 124); 

 (4) a tank of brine through which the ammonia pipes run 

 (Fig. 124); (5) cans containing purified water which is to be 

 frozen (G, Fig. 124). There are usually also mechanical de- 

 vices for handling the blocks of ice, and frequently additional 

 boilers for the distilling of the water which is to be frozen. 



Before we can understand the use of these parts of the plant 

 and the process followed, we need to know some of the special 

 properties of ammonia. 



194. Some of the Properties of Ammonia. When studying 

 evaporation (Art. 12, Ex. 10), we learned that when any liquid 

 evaporates much heat is absorbed, or, as we commonly say, 



