MANUFACTURED ICE AND FREEZING MIXTURES 165 



temperature below the freezing point of water, we can then 

 allow this liquid to evaporate and absorb heat, taking it 

 from water till the water freezes. This is exactly the principle 

 employed in the manufacture of ice, and AMMONIA is the liquid 

 used. 



Nearly everyone is somewhat familiar with ammonia. It 

 is the gas that escapes from common AQUA AMMONIA, or 

 SPIRITS OF HARTSHORN, which may be purchased at any drug 

 store. This common aqua ammonia is simply water which 

 has absorbed large quantities of ammonia. When aqua 

 ammonia is exposed to the air, the ammonia escapes. Its 

 stinging, biting odor is familiar to all and is easily recognized. 

 The ammonia with which we commonly come in contact, then, 

 is always in the gaseous form. This same ammonia, however, 

 can be changed into liquid form by compressing it. The 

 pressure required to change ammonia gas into liquid form 

 depends upon its temperature; the higher the temperature, 

 the greater is the pressure required. For any given tempera- 

 ture, there is a corresponding pressure which is just sufficient 

 to liquefy the gas. This relation of temperature to pressure 

 is often stated in another way: We often speak of the boiling 

 point corresponding to a given pressure (see Art. 155 and 

 Table VI). It must be clearly understood, however, that the 

 boiling point corresponding to a given pressure is exactly the 

 same temperature as the liquefying point corresponding to 

 that pressure. Recall that water boils and steam condenses 

 at the same temperature. 



TABLE VIII. PRESSURE AND CORRESPONDING BOILING POINT 

 OF AMMONIA 



lb P p r e e r S s U q r ln. Atmospheres or g^g* 



15 lb 1 atmosphere -29F. 



30 Ib 2 atmospheres 0F. 



34 lb 2.3 atmospheres 5F. 



63 lb 4.2 atmospheres 32F. 



107 lb 7.1 atmospheres 60F. 



130 lb 8.6 atmospheres 70F. 



155 lb 10.3 atmospheres 80F. 



