376 COOIJNK; FACIUTIIoS KOR CKI';AAlERJIiS 



are received daily during the summer months, mechanical 

 refrigeration is considered practicable. 



On another page a table of comparative costs 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 remaining machinery, such 

 as kind of fuel used, skill of firemen, style and condition of boiler, 

 proportion of boiler power to work done, the correlative size of all 

 machinery, kind of insulation and care of cooling-rooms, and 

 efficiency of compressor and whole refrigerating system. 



Chemicals Used for Mechanical Refrigeration. — The most 

 common substances used in mechanical refrigeration are ammo- 

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

 from a creamery standpoint, these only are of importance. 

 Ammonia is the most used. It is efficient, cheap, and not so 

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

 drous ammonia has a boiling-point of 27° below zero at atmos- 

 pheric pressure. The latent heat of ammonia is also great. 

 Ammonia has great chemical stability, and is not explosive in 

 nature; it 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 hydro- 

 chloric 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 P^urope, and is chiefly 

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

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

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

 in tropical climates. 



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 



