100 FIRST YEAR COURSE IN GENERAL SCIENCE 



stroke of the piston a portion of the air in the receiver is 



removed. 



A space from which all air has been removed is a vacuum. 



With an air pump it is impossible to produce a perfect 



vacuum, but from experiments 

 with a partial vacuum much 

 can be learned about the be- 

 ... .1 havior of the atmosphere. 



109. The Direction of 

 Atmospheric Pressure. It 

 is easy to understand, without 

 an experiment, that air exerts 

 pressure downward, because 

 all matter has weight. The 

 air pump makes it possible to 

 show that air also exerts pres- 

 sure upward and in a hori- 

 zontal direction . Suppose 

 that a thin sheet of rubber is 

 tied closely over the mouth 

 of a bottle filled with air. 

 The molecules of air in the 

 bottle are in the same rapid 

 motion as those outside, and 

 the rubber remains flat, be- 

 cause the pressure above and 

 below it is the same. 



If now the bottle is placed 

 under the receiver of an air 

 pump, and some of the air 

 is removed from around the 

 bottle, the rubber bulges up- 

 ward. The air in the bottle 

 is pressing in every direction, 

 but the rubber is the only 



FIG. 47. APPARATUS TO BE USED 

 WITH AN AIR PUMP 



1. The glass dish A is placed 

 on the plate of an air pump and 

 one double stroke of the piston is 

 made. What is the effect on the 

 rubber cover? Why? 2. What is 

 the effect of continued pumping? 

 3. If B is placed on the plate while 

 the mercury is at 1, and the pump 

 is worked, the mercury falls to 2 in 

 the tube. Explain its position at 

 1, and at 2. 



