SOME PROPERTIES OF GASES 167 



on the earth's surface is compressed by the weight of 

 the miles of air above us. When the air is heated it 

 expands and becomes less dense, but its pressure remains 

 practically the same, because the molecules move with 

 greater speed and strike with greater force. 



111. Compression and Heat. When gases are heated 

 either their pressure or their volume will increase. When 

 air is heated it expands, becomes less dense, and occupies 

 more space. If this expanded air is compressed, it will 

 give out as much heat as it took up when it expanded. 

 (Recall the law of conservation of energy.) The fact 

 that air does give out heat when it is compressed has 

 been experienced by everyone who has used a bicycle 

 pump. After a few strokes are made the body of the 

 pump becomes warm. This heat is partially due to the 

 friction of the piston inside, but most of the heat is due 

 to the fact that the air is being compressed; the molecules 

 are being forced into less space, and they cannot move 

 so freely, so their heat is given out. If compressed air is 

 cooled and then allowed to expand, its temperature will 

 fall and will take heat from surrounding objects. Let the 

 escaping air of a bicycle tire touch your hand and see how 

 cool it feels. When ammonia gas whose temperature is 

 about 15 F. is compressed by a powerful engine, its tem- 

 perature at once rises very nearly to 200 F. This increase 

 in temperature is not due to the friction of the com- 

 pression pump, but it is due to the fact that the molecules 

 of ammonia are crowded closer together and the loss 

 of energy in molecular activity appears in the form of 

 heat. If this compressed ammonia gas is permitted to 

 expand again, its temperature falls to what it was at first. 

 In general, when gases are compressed, they give out heat, 

 and when they are allowed to expand they take up heat. 



