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ATMOLOGY 



CHAPTER III. 

 THE RELATION OF VAPOUR AND AIR. 



Sect. 1. The Boylean Law of the Airs Elasticity. 



IN the Sixth Book (Chap. iv. Sect. 1.) we have 

 already seen how the conception on the laws 

 of fluid equilibrium were, by Pascal and others, 

 extended to air, as well as water. But though air 

 presses and is pressed as water is, pressure pro- 

 duces upon air an effect which it does not, in any 

 obvious degree, produces upon water. Air which 

 is pressed, is also compressed, or made to occupy 

 a smaller space; and is consequently also made 

 more dense, or condensed; and on the other hand 

 when the pressure upon a portion of air is dimin- 

 ished, the air expands or is rarefied. These general 

 facts are evident. They are expressed in a general 

 way by saying that air is an elastic fluid, yielding 

 in a certain degree to pressure, and recovering its 

 previous dimensions when the pressure is removed. 

 But when men had reached this point, the ques- 

 tions obviously offered themselves, in what degree 

 and according to what law air yields to pressure ; 

 when it is compressed, what relation does the 



