34 



FLUID PRESSURE 



that you observe. Try to pull the bottle off the tongue. Try to 

 find an explanation for these things. 



Experiment 31. Blow into a paper bag until it is well filled 

 out ; then, without crushing it, open the end a little way so that 

 the air inside may be under equal pressure with that outside. 

 Now putting it to the lips, draw out some of the air and note 

 what happens to the paper bag. Explain. 



In these cases the air was partly removed. A space 

 containing no air or other matter is called a vacuum. 

 A space from which the air has been partly removed is 

 called a partial vacuum ; the air remaining in it is said 

 to be rarefied. In practice, no perfect vacuum can be 

 produced, but air has been rarefied to one-millionth of 

 its usual density. 



45. Some Effects of Atmospheric Pressure. To under- 

 stand the effects of atmospheric pressure, it must be kept 

 in mind that whenever a partial vacuum is 

 formed in any space, the pressure within the 

 space is less than that of the air around it; 

 also that the atmosphere, being a fluid, exerts 

 force equally in every direction and can push 

 itself into any opening, whatever its shape 

 or size. In other words, wherever a partial 

 vacuum exists, the atmosphere tends to enter or 

 to force something else in. The effort of air to 

 do this is seen in many common happenings, 

 some of them useful and some annoying. 



Experiment 32. Dip the end of a clean straw or 

 other tube into clean water. With the other end at 

 the lips, draw the air from the straw. What condition 

 do you tend to cause within the tube ? Do you succeed in causing 

 that condition ? Why ? Describe and explain all that you observe. 



FIG. 18 



