350 On iJte Proporiwn of 



amounts to the same thing, by finding how much the given 

 voUmie is diminished under a constant pressure, by th«t 

 abstraction of each of its ingredients singly. Thus, if it 

 should appear that, bv extracting the oxygenous gas from 

 any mass of al-mospheric air, the whole was diminished 

 l-5th in bulk, stiil being subject to a pressure of 30 inches 

 of merciirv ; then it ought to be inferred that the oxyge- 

 nous atmosphere presses the earth wiih a force of 6 inches 

 ofmerciuv, &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 given volume of each in a pure 

 slate, subject to the same pressure and temperature. For, 

 the weight of each gas in anv given portion of atmospheric 

 air must be in the compound ratio of its force and specific 

 gravity. 



With respect to the third object, it mav be observed, 

 that those gases which are specifically the heaviest nmst 

 decrease in density the quickest in ascending. If the earth's 

 atmosphere had been a homogeneous elastic lluid of the 

 same weight it is, but ten limes the specific gravitv, it 

 might easily be demonstrated that no sensible portion of it 

 could have arisen to tlie summits of the "iiighest mountains. 

 On the other hand, an atmosphere of hydrogenous gas, of 

 the sanie weight, would support a column of mercury 

 nearly 29 inches on the summit of Mount Blanc. 



The several gases constantly found in every portion of 

 atmospheric air, and in such quantities as are capable of 

 being appreciated, are azotic, oxygenous, aqueous vapour, 

 and carbonic acid. It is probable that hydrogenous gas 

 also is constantly present, but in so small proportion as 

 not to be detected bv any test we are acquainted with : 

 it must therefore be confouiided in the large mass of azotic 

 gas. 



I. Of the JVeight of ike Oxygenous and Azotic Atmospheres. 



Various processes have been used to determine the quan- 

 tity of oxygenous gas. 



1. The mixture of nitrous gas and air over water. 



2. Exposing the air to liquid sulphuret of potash or lime, 

 ■with or v;ithout agitation. 



3. Exploding hydrogen gas and air by electricity. 



4. Exposing the air to a solution of green sulphat or 

 inuriat of iron in water strongly impregnated with ni- 

 trous gas. 



5. Earning phosphorus in the air. 



In 



