324 BUTTER-MAKING. 



On another page a table of comparative cost of natural ice 

 and mechanical refrigeration is given. It was also stated in 

 that connection that the cost of mechanical refrigeration 

 would vary under different conditions. The chief factors 

 affecting the cost of mechanical refrigeration may be said to 

 be similar to those affecting the economic running of the re- 

 maining machinery, such as kind of fuel used, skill of fireman, 

 style and condition of boiler, proportion of boiler power to 

 work done, upon the correlative size of all machinery, upon 

 kind of insulation and care of cooling-rooms, and upon effi- 

 ciency of compressor and whole refrigerating system. 



Chemicals Used for Mechanical Refrigeration. The most 

 common substances used in mechanical refrigeration are am- 

 monia and carbonic acid. A number of others are in use, but 

 from a creamery standpoint, these only are of importance. 

 Ammonia is used chiefly. It is efficient, cheap, and not so 

 dangerous to life and property as are some of the others. 

 Anhydrous ammonia has a boiling-point of 27 below zero 

 at atmospheric pressure. The latent heat of ammonia is also 

 great. Ammonia has great chemical stability, and is not 

 explosive in nature. Ammonia attacks copper and brass, 

 but has no effect upon iron and steel pipes. If ammonia 

 should escape through a leak into a room, the operator can 

 protect himself from the effects of the gas by breathing through 

 a wet sponge held in the mouth. Ammonia leaks may be 

 detected by holding a glass rod dipped in hydrochloric acid 

 to the place where the leak may be. When ammonia comes 

 in contact with hydrochloric acid, white fumes are formed. 



Carbonic acid is used considerably in Europe, and is chiefly 

 favored because the gas is not highly poisonous; in case of 

 leak it does not spoil contents of refrigerator, and it liquefies 

 at a high temperature (90 to 100 F.), and is therefore favored 

 in tropical climes. 



Principles of Producing Cold Artificially. The chief principle 

 involved in producing artificial cold is that when a substance 

 passes from a liquid into a gaseous state, a definite amount of 



