Chap. 8.] Compcfition of Atmofpberic Air. 405 



or refpirable air, and that the remaining three-fourths 

 are unfit for refpiration, and equally unfit for combu- 

 tion, fincethe fame fluid which iupports flame is found 

 equally to contribute to the fupport of animal life. 



By the gradual introduction of nitrous air into a clofe 

 veffel filled with atmospheric air, it will be found that 

 about a fourth part of the whole bulk of the air will 

 disappear; the fame quantity is in effedt deftroyed by 

 the combn tion of any inflammable fubftance, and the 

 combuilic-i gradually ceafes in proportion as that fluid 

 is dinr.-in-ird which is necefTary to its fupport. 



The f?.rne quantity is deftroyed by the procefs of re- 

 ipirarion. SPutrefacUon alfo feparaues the. pure airj 

 and the power of (epamting, and alfo of reuniting the 

 two fluids, which 'aft m?.y be done, when both are pro- 

 duced by artificial means, very fj.fl' iently proves them 

 4diflin6t in their nature and properties, and alfo that 

 they are united in the air of our atmofphere. 



Azotic gas being fpedfieally lighter than oxygen, it 

 might naturally be fuppofed, that fince they only exift 

 in the atmofphere in a mixed ftate, and not in a (late of 

 chemical combination, a fpontaneous feparation would 

 take place, and that the azote would occupy the higher 

 regions of the atmofphere; whereas it is found by ex- 

 periments with the eudiometer, that the upper regions 

 of the air actually contain a greater proportion of oxy- 

 gen than thofe nearer the furface of the earth. Whether 

 this is to be attributed to the attraction which azote 

 may have for the earth, or to fome unknown property 

 in the oxygen, we cannot now determine, and can 

 only take the fact as it (lands, without attempting its 

 explanation *. 



* A mixture of empyreal and inflammable airs (the latter of 

 which is much lighter than the former) remaining all night, was 

 found the next morning in the molt perfeft ftate of mixture, 

 and the eleftric fpark puffed through them with the ufual effeft. 

 eriments , vol. vi. p. 2.7. 



D d 3 From 



