128 THE ATMOSPHERE AND ITS SERVICE TO MAN 



rates ; that is, returns to the gaseous state. In so doing it 

 absorbs heat very rapidly from its surroundings (page 105). 

 The gaseous ammonia returns to (A) from the pipes (D) 

 because of the exhaust action of the pump. It is again 

 compressed into the pipes at (B). Thus the action con- 

 tinues without loss of ammonia. 



The ammonia pipes pass through the brine into which 

 cans of water have been lowered. Brine is used to sur- 

 round the ice cans because it does not freeze unless its 



FIGURE 70. DIAGRAM SHOWING ESSENTIAL, CONSTRUCTION OF 

 AN ICE PLANT 



temperature is reduced many degrees below the tempera- 

 ture at which pure water freezes. The evaporation of the 

 ammonia in the pipes reduces the temperature of the brine 

 so low that the water in the cans is frozen, but the brine re- 

 mains liquid, so that the cans may be easily removed. 



In cold storage plants the pipes (D) are placed in the cold 

 storage rooms to reduce the temperature of the air in the 

 rooms, just as they reduce the temperature of the brine in 

 the ice plant. 



The Barometer. On account of the movements of the 

 air due to heating and cooling and to other causes, the 



