GRAVITY AND LEVITY OF OASES. 251 



s.ures which we employ, give us the relative, but 

 not the absolute weight; they prove that differ- 

 ent gases, when they are of different densities, 

 press as much as they are pressed, and resist 

 as much as they are resisted ; that they are 

 neither heavy nor light, but subsist in a state 

 of equilibrium, or of balance; that, on the con- 

 trary, when they are of different degrees of 

 density, or of rarity, they are either heavy or 

 light, according to the. quality of the medium in 

 which they are situated.* 



* Sucb is the expansive force which the air exerts, when 

 external resistance is diminished and removed, that in making 

 experiments to ascertain the degree of density, which any 

 given quantity of it possesses, with relation to the rarity of 

 the medium of air by which it is surrounded or the rarity of 

 the one, with relation to the density of the other ; that it is 

 necessary to inclose it in vessels of copper, or some other 

 metallic substance ; as bottles made of glass, unless they are 

 very strong, are found generally to crack, and break. It is 

 more especially the case, when a large volume of air is con- 

 densed into a smaller volume, the condensation which is 

 produced, must not be considered the cause of Us expansi- 

 bility ; it only proves, that the expansibility of air may be 

 pvercome, by an external force : but it does not prove, that 

 pressure is able to generate expansibility in bodies, which are 

 not expansible. Pressure can no more create, or impart, 

 expansibility to air, than it does to lead, or to water ; air is 

 expansible ab initio, and which pressure can neither create, 

 nor destroy. 



