PROPERTIES OF AIR. 



49 



property a tendency to press against all the sides of any vessel in which 

 it may be enclosed. Of course the larger the vessel containing a given 

 quantity of air, the less actual pressure it will exert on the sides of the vessel. 

 The elasticity of air therefore decreases with increasing expansion, but it 

 gains in elasticity or force when compressed. 



There is a law in Physics which expresses the relation between expansion 

 and elasticity of gases, which may be said to be as follows : 



The elasticity (of a gas) is in inverse ratio to the space it occupies, and 

 therefore by compressing air into a small space we can obtain a great force, 

 as in the air-gun and the pop-gun of our youthful days; 



Fig. 43. Hard boiled egg, divested of its shell, passing through the neck of a glass bottle, under the influence of 



atmospheric pressure. 



In the cut below we can illustrate the principle of the pop-gun. The 

 chamber full of air is closed by a cork and by an air-tight piston (s) at / and /. 

 When the piston is pushed into the chamber the air is compressed between 

 it and the stopper, which at length flies out forcibly with a loud report 



Fig. 44. Ttoe principle of the pop-gun. 



We have said that the tendency of air particles is to fly away from each 

 other, and were it not for the earth's attraction the air might be dispersed. 

 The height of the atmosphere has been variously estimated from a height of 



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