SOME PROPERTIES OF GASES 169 



leave your hand the more heat is required to supply the 

 energy necessary for them to fly away. 



The compound that is used for refrigeration by evapo- 

 ration is ammonia, (NH 3 ). Ammonia at ordinary pres- 

 sure and temperature is a gas. At 40 C. it condenses 

 and forms a liquid, or it can be made to liquefy at a 

 higher temperature if the pressure is increased. The 

 ammonia is purchased in liquid form in long cylindrical 

 tanks. The cold-storage buildings in which ammonia 

 is used have a system of closed iron pipes, compression 

 pumps, and a place to liquefy the ammonia by cooling 

 it after it is compressed by the pumps. The ammonia 

 is put into this system of pipes, from which it cannot 

 escape. It is made to circulate by the pumps. The 

 pumps compress the ammonia gas and it is then cooled 

 and liquefied while passing through pipes over which cool 

 water is kept flowing. The cooling pipes are usually in 

 the open air, often on top of the building. After the 

 ammonia is liquefied, it flows to the cold-storage rooms, 

 where it is permitted to pass slowly through valves, and 

 then it evaporates rapidly because the pumps draw off 

 the ammonia gas as fast as it is formed by evaporation. 

 The evaporating ammonia takes the heat from the pipes 

 in which it is enclosed and the pipes in turn take heat 

 from the air in the room, which is thus kept at a very 

 low temperature, even below freezing if desired. Large 

 meat-packing firms and sometimes small meat-markets use 

 ammonia for refrigeration. Ice is also used for refrigera- 

 tion, especially in refrigerator cars and in private houses. 



113. Artificial Ice. Ice in the summer used to be 

 regarded as a luxury, but now it is a necessity; without 

 the modern methods of making ice, it would be very 

 expensive. Boards of health require artificial ice to be 



