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P R E F 



I 



^HE common air, that element 

 in which we live, that invi- 

 fible fluid which furrounds the 

 whole earth, has never been fo 

 much the object of contemplation 

 as it has in our days: it never en- 

 gaged fo much the attention of the 

 learned as it has of late years. This 

 fluid, diffufed every where, the 

 breath of 'life ', deferves fo much the 

 more the attention and inveftigation 

 of philofophers, as it is the only 

 fubftance without which we can 

 fcarce fubiift alive a fingle moment, 

 and whofe good or bad qualities 

 have the greateft influence upon 



our 



