ATMOLOGY. 



CHAPTER III. 

 THE RELATION OF VAPOR AXD AIR. 



Sect. i. The Hot/lean Law of tlic Air's Elasticity. 



JN the Sixth Book (Chap. iv. Sect, i.) we have already seen how the 

 conception on the laws of fluid equilibrium was, by Pascal and 

 others, extended to air, as well as water. Bnt though air presses and 

 is pressed as water presses and is pressed, pressure produces upon air 

 an effect which it does not, in any obvious degree, produce 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 

 diminished, the air expands or is rarefied. These broad facts are evi- 

 dent. 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 questions obviously 

 offered themselves, in what degree and according to what law air 

 yields to pressure ; when it is compressed, what relation does the 

 density bear to the pressure ? The use which had been made of tube- 

 containing columns of mercury, by which the pressure of portions of 

 air was varied and measured, suggested obvious modes of devising ex- 

 periments by which this question might be answered. Such experi- 

 ments accordingly were made by Boyle about 1650 ; and the result at 

 which he arrived was, that when air is thus compressed, the density is 

 as the pressure. Thus if the pressure of the atmosphere in its common 

 state be equivalent to 30 inches of mercury, as shown by the barome- 

 ter ; if air included in a tube be pressed by 30 additional inches of 



