Heat 



95 



FIG. 46. The expansion of the compressed gas freezes the moisture on the tube. 



had some heat in it, as everything has. When you let 

 it loose, it used up much of its heat in pushing the air 

 in the test tube and all around it out of the way. In 

 this way it lost its heat, and then it became cold. Cold 

 means absence of heat, as dark means absence of light. 

 So when the compressed gas used up its heat in pushing 

 the air out of its way, it became so cold that it froze the 

 water in your test tube. 



One reason why it is always cold high up in the air. 

 Even on hot summer days aviators who fly high suffer 

 from the cold. You might think that they would get 

 warmer as they went up nearer the sun ; one reason that 

 they get colder instead is this : 



As you saw in the last experiment, a gas that expands 

 gets very cold. Air is a kind of gas. And whenever 

 air rises to where there is not so much air crowding 



