[ 404 ] [BookV. 



CH*P. VIIL 



OF ATMOSPHERIC AIR. 



Atmbfpbtrt compofed chiefly of Two Kinds of Air : Contains alfo Fixed 



Air, and occajionally ether Subflances Effefls of this Mixiurc on 



Metals and Purple Dyes. Means of purifying the Atmofphere from 



fixed Air, and putrid Vapours. Effects of Moijiure contained in 



Air. The Hydrometer. Cold in the higher Regions cf the Atmt* 



fphere. ~-Caufe. 



WHATEVER has been hitherto dated relative 

 to the different fpecies of claflic fluids is chiefly 

 important, becaufe the knowledge of thcfe fluids is 

 neceflary to enable us to comprehend the nature of 

 that atmofphere in which we exift, and which is in- 

 deed of itfelf one of the principal agents of our exift- 

 cnce. 



In treating feparately of the different kinds of air, it 

 was neceflary in fome meafure to anticipate the. prefer^ 

 fubjecl:, and to intimate that the air of our atmofphere 

 is not, as was formerly fuppofed, a fimple homogenous 

 fluid, but that in reality it is compofed of two different 

 fluids, which have been defcribed under the appellations 

 of azotic and oxygen gas, or phlogifticated and vital air. 



In one hundred parts of atmofpheric air there are 

 contained about feventy-two parts of azotic gas to 

 twenty-feven of oxygen, befides one part of carbonic 

 acid gas 1 or fixed air, which is generally found united 

 with them, or to fpeak in round numbers, in order to 

 .be better underftood, we may fay that the air of our at- 

 mofphere contains rather better than one-fourth of pure 



or 



