PKESSUKE AND CHANGE OF STATE 



69 



heat from it by means of a stream of running water flowing 

 over the pipes in which it is compressed. When the cooled 

 liquid is allowed to expand and become a gas again, it takes 

 the heat necessary for this process from the substances to be 

 cooled. The gas commonly used is ammonia or sulphur 

 dioxide. After the gas has been compressed and cooled, it 

 is allowed to expand into other pipes surrounded by strong 

 brine. The brine thus cooled is pumped away to the place 

 where refrigeration is desired. The gas is used over and over 

 again, the refrigerating system being so arranged that the 



FIG. 24.- 



-The air pump for removing air from the bell-jar .R. 

 and Turton.) 



(Tower, Smith 



pump which compresses it in one set of pipes also serves to 

 remove it from the set of pipes into which it expands. This 

 process also reduces the pressure in the pipes where expansion 

 occurs. 



66. Other Uses of Pressure. The fact that a compressed 

 gas expands instantly when pressure is removed is often taken 

 advantage of in engineering operations. By admitting air 

 under pressure to first one side and then the other of a piston 

 moving in a cylinder, motion is developed which will run drills 

 and riveting machines and do much other useful work. The 



