ADIABATIC HEATING 123 



occur at a height of from 6 to 10 miles. Conditions affect- 

 ing weather, however, seem to extend to a height of not 

 much over 3 miles. 



59. Adiabatic Heating and Cooling of Air. Experiment 70. 

 Have a five-pint glass bottle fitted with a two-hole rubber stopper. 

 Pass through the holes in the stopper a chemical or air 

 thermometer and a short glass tube the lower end of 

 which extends into the bottle not near the bulb of the ther- 

 mometer, so that when the air is exhausted or allowed to 

 enter the bottle there will be no movement of the air 

 near the bulb of the thermometer. The end of the column 

 of the thermometer must be visible above the stopper. 



Attach the glass tube to an air pump by means of a 

 thick-walled rubber tube. Note the temperature of the 

 thermometer within the bottle and also of the air outside. ' ' 

 Quickly exhaust the air 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 tem- 

 perature 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 expands its temperature 

 falls and when it is compressed its temperature rises. This 

 heating and cooling of the air without the application of 

 external heat or cold, but simply by a change in the den- 

 sity of the gas itself, 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 in develop- 

 ing our wind circulation and storms. 



The heating effect of compressing air can be well seen 

 when a pneumatic 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 



