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SECTION XXVI. 



Air is one of the mojl changeable fub-* 

 fiances in nature^ and appearing 

 under very different forms and 

 qualities from a variety of caufes. 



THE air of our atmofphere is 

 feldom during a whole day of the 

 fame quality. Its degree of whole- 

 fomenefs is perhaps not lefs fubjecl: 

 to variations than its weight and its 

 degree of heat and cold. The ba- 

 rometer indicates the firft, and the 

 thermometer the other. But thofe 

 two inftruments feem to have no 

 relation to the more or lefs purity 

 of the atmofphere, or the more or 

 lefs fitnefs of the air for the ufe of 

 refpiration. 



plant during the night, when the plants yield but a 

 very fmall quantity of bad air. So that it rather ap- 

 pears to be changed by the plant in a kind of ample 

 explofive air, or a true fulminating air, the only yet 

 discovered, as far as I know. 



The 



