PRESSURE IN GASES 41 



7. What is the use of a lifting pump? Explain its action. 

 How does a force pump differ from a lifting pump in its 

 action ? Of what different use is it ? Explain the use of the 

 dome. 



8. Explain the action of the siphon. What forces are used by 

 it ? How is the flow stopped ? 



9. Would a lifting pump serve its purpose if the piston did 

 not fit tightly in the pipe ? Why? 



SECTION III 

 PRESSURE IN GASES 



50. Expansion of Gases. Gases differ from liquids 

 and solids in that their molecules are not kept near 

 together by cohesive force ( 14). Therefore, since 

 their molecules are always in rapid motion, there is 

 no force exerted by the gaseous particles to prevent 

 their becoming widely separated. Thus if a bottle of 

 some gas be left open in a room, its molecules soon mix 

 with the air and move to all parts of the room. Open 

 the bottle of gas in a large vacuum, and the same thing 

 happens ; the molecules do not increase in size, but the 

 spaces between them increase greatly. This increase in 

 the volume of a gaseous body by the wider separation 

 of its molecules is called expansion. 



Gases therefore may be said to tend always to ex- 

 pand; and as their molecules exert no cohesive force 

 to oppose this expansion, a gas can be kept in a certain 

 space only by inclosing it completely within walls that 

 may supply the necessary force. If a vessel is filled 

 with a gas under ordinary pressure of the air, the gas 

 is said to be under a pressure of one atmosphere. If, 



