THE MANUFACTURE OF ICE 127 



it is " done." To hasten the process of cooking by boiling 

 in high altitudes, pressure cookers are often used. The 

 high pressure developed by keeping the steam imprisoned 

 raises the boiling point of the water within. The contents 

 of the cooker may thus be brought to a temperature of 

 170 C. or even more. This intense heat reduces the time 

 of cooking and thus saves fuel. 



The Manufacture of Ice ; Cold Storage. We saw hi 

 Experiment 53 that when air was compressed it gave up 

 heat and warmed its surroundings. When pressure was 

 removed, the air absorbed heat and cooled its surround- 

 ings. Other gases act in the same way. Water vapor, for 

 example, may be compressed until it gives up so much heat 

 that it returns to the liquid state. 



Ammonia is a gas that at ordinary temperatures is easily 

 condensed by pressure into a liquid. (This liquid must 

 not be confused with the aqua ammonia of our kitchens, 

 which is simply water that has absorbed ammonia gas.) 

 When the pressure is removed, the liquid ammonia quickly 

 returns to the gaseous state, and in so doing it absorbs 

 much heat. 



Figure 70 shows the essential construction of an ice 

 plant. The pump (A) compresses the ammonia gas into 

 the pipes at (B). The pressure condenses the gas into 

 liquid, and the cold running water absorbs the heat given 

 out in the process. The liquid thus cooled is allowed to 

 run very slowly through the valve (C), into the pipes at 

 (D). The valves in the pump (A) are so arranged that 

 while the pump increases the pressure in the pipes at (B) 

 it decreases the pressure in the pipes at (D). Because of 

 the low pressure in the pipes (D), the liquid ammonia evapo- 



