PRESSURE AND THE BOILING POINT 125 



from the bottle, carefully noting the action of the thermometer. 

 See that the temperature of the air in the room does not change 

 during the experiment. Allow the air quickly to enter the bottle 

 and note the action of the thermometer. The temperature inside 

 the bottle changes as the air is quickly exhausted, or as it is allowed 

 to enter the bottle again and thus to increase the density of the 

 air in the bottle. 



It has been found that when air or any other gas expands, 

 it absorbs heat and cools its surroundings ; and when it is 

 compressed, it yields heat and warms its surroundings. 

 This heating and cooling by changes in the density of gas 

 is called adiabatic heating or cooling. It is taken advantage 

 of in the manufacture of liquid air and is the same principle 

 which is utilized in cold-storage plants. This property of 

 air has. much to do with developing our wind circulation and 

 storms. 



The heating effect of compressing air can be well seen 

 when an automobile tire is filled. No matter how well the 

 piston of the pump may be oiled, as the density of the air 

 in the tire begins to increase, the pump will grow warm 

 rapidly. This rapid heating cannot be due to friction, as 

 the pump is not being worked any more swiftly than at 

 first. It is due to the greater compression of the air. As 

 this compression increases, the heating increases, the effect 

 of friction in a well-oiled pump being of small value. 



Pressure and the Boiling Point. Experiment 64. (Teach- 

 er's Experiment.) Fill a strong 500 cc. round-bottomed flask 

 about one third full of water. Boil the water. While the 

 water is briskly boiling, remove the flask from the heat, quickly 

 close its mouth with a rubber stopper, and invert it in a ringstand. 

 (Figure 69.) (Be sure not to insert the stopper until the flask is 

 fully removed from the heat.) Pour cold water upon the flask. 

 The water will again begin to boil. 



