246 OntheProportwnof 



the weight of the atmosphere of that gas 

 solely, the different atmospheres not gravitat- 

 ing one upon another. AVhence the first object 

 will be obtained by ascertaining what share of 

 elastic force is due to each gas in a given 

 volume of the compound atmosphere ; or, 

 which amounts to the same thing, by finding 

 how much the given volume is diminished 

 under a const-ant . pifessure, by the abstraction 

 of each of its ingredients singly. Thus, if it 

 should appear that ,by extracting the oxygenous 

 gas from any mass of atmospheric air, the 

 whole was diminished f in bulk, still being 

 subject to a pressure of 30 inches of.mercury ^ 

 tjie^ it ought to be inferred that the oxygenous 

 atnrosphere presses the earth with a force of 6 

 inches of mercury, &c. 



In order to ascertain the second point, it 

 will be further necessary to obtain the specific 

 gravity of each gas ; that is, the relative 

 weights of a ^iv.en volume of each in a pure 

 state, subject to the same pressure and tempe- 

 rature. For, the weight of each gas in any 

 given portion of atmospheric air, must be in 

 the compound ratio of its force and specific 

 gravity. 



With respect to the third object, it may be 

 observed,- that those gases which are speci- 

 ficallv the heaviest must decrease in density ^the 



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