GRAVITY AND LEVITY OF GASES. 



weight, that the pressure it produces is to be 

 referred. 



Whoever reflects on the forced and unnatural 

 means which it is necessary to employ, in order 

 to ascertain the rarity, or the density the 

 levity or gravity of air, will be led to conclude, 

 that it is not by these attributes that its power 

 ought ever to be meted, or measured ; for al- 

 though its expansibility is not only equal to, but 

 greater than its weight its weight is not equal 

 to, but much less than its expansible power. As 

 well might the muscular power of the most 

 gigantic arm, be legitimately estimated, when 

 it is paralysed by the pressure of a weight, or 

 when it is rendered motionless, by being rivetted 

 and fixed to a post, by the strongest chains, 

 as to ascribe to the weight of air the power it 

 possesses. Whenever air is confined in close 

 vessels, whose solid sides have the power to 

 restrain its expansive force ; it may then be 

 said to subsist, in a state of capacity, without 

 power ; of density, and of rarity, without ex- 

 pansibility ; it has then as great a tendency to 

 fall, as it has to rise, and is rendered altogether 

 subservient to the laws of the vessel in which it 

 is inclosed : in cases such as these, it is internal 

 force overcome by external resistance. When 

 ever external resistance is diminished, or re- 

 moved ; whenever air is released from this 



