PROPERTIES OF GASES- 



101 



1 



part that can show the pressure. When air is allowed to go 

 back into the receiver, the rubber becomes flat again. If the 

 bottle is placed in a horizontal position and the experiment 

 is repeated, the rubber bulges out in a horizontal direction. 



The laws of pressure in 

 gases are similar to those in 

 regard to liquids. 



110. Relation between 

 the Pressure and the Volume 

 of a Gas. Gases are very 

 compressible and highly 

 elastic. If pressure is ap- 

 plied to compress a gas into 

 a smaller volume and is then 

 removed, the gas returns to 

 its original volume. As it 

 expands, the gas may be 

 made to do work, by push- 

 ing back the body that 

 compressed it. Air guns, 

 air springs for closing doors, 

 air brakes, and other devices 

 make use of the energy of 

 compressed air. 



When a gas has been 

 compressed into half the 

 space it originally occupied, 



it exerts twice as great pressure as before compression. This 

 relation between the volume and the pressure of a confined 

 gas continues as long as the substance remains a gas. Under 

 a very great compressing force at low temperature, gases 

 even air become liquids. 



Steam at high temperature in a confined space exerts hun- 

 dreds of pounds of pressure to the square inch. This pressure 

 does an immense amount of useful work if properly controlled 



FIG. 48. RELATION BETWEEN 

 PRESSURE AND VOLUME OF GASES 



1. What is the pressure upon the 

 mercury in the long arm of the left 

 tube? 2. What shows that the 

 downward pressure of the gas in the 

 space a is equal to the downward 

 pressure in the long arm of the tube? 



3. How great is the downward pressure 

 in the long arm of the second tube? 



4. Why has the volume of the gas in 

 a' decreased? 5. How much pressure 

 does the gas in of now exert? 



