44 FLUID PRESSURE 



Experiment 40. Make a common popgun, using a piece of 



elder (removing the pith) and two good cork stoppers. Fit the stop- 



a b pers as in Fig. 28, and push 



i -=i upon the one at b until a 



flies out. Explain. How 

 FIG. 29 



much air is in c (Fig. 29) 



as compared with c (Fig. 28) ? What is its condition in c (Fig. 29) ? 



54. Buoyancy in Gases. Gases, like solids and 

 liquids, vary much in specific gravity. If it were not 

 for their tendency to diffuse, all heavy gases would 

 sink to the ground and all that are lighter than air 

 would rise. In a general way, gases do this, but of 

 course they soon diffuse and lose their purity. If a gas 

 can, however, be kept in a very light covering (e.g. a 

 soap bubble), it will rise or fall in the air, according as 

 it is lighter or heavier than air. Thus large amounts of 

 hydrogen gas (only -^ as heavy as air) may be put into 

 a silk covering, and the whole will be so much lighter 

 than the air that it will rise. In this way balloons are 

 made. When large enough they may carry up heavy 

 loads; but since the air becomes rarer as we go up 

 from the earth, there is a limit to the height that a 

 balloon may reach. 



QUESTIONS 



1. What shape does a liquid body assume when left to itself? 

 (Think of liquids that are freely falling.) What becomes of a 

 gas when left free in space ? Explain the difference. 



2. What is meant by expansion ? Give examples. 



3. What is meant by a pressure of one atmosphere ? What is 

 a rarefied gas ? 



4. What is an air pump? Explain its action. By what force is the 

 gas removed from the receiver? Can it be entirely removed? Why? 



