Vital Air in the Atmofpherc. 259 



open fields, to be always compofed of from 21 to 22 parts of 

 vitM air, and of from 78 to 79 of azotic gas ; and if at any 

 time, which however very rarely happened, the rcfult varied 

 a' few hundredth parts, the fucceeding experiment, which I 

 could repeat with the greateft facility, and in a few minuter, 

 fi)on detected the error. I was convinced that this fmall dif- 

 ference did not arife from the nature of the air, but from foiiie 

 neghgcnce in the operation. I have often colle6led air in 

 places where a great many perfons were aflembled, or near 

 ponds of ftagnant water, and I always found this air as pure' 

 as the common air. 



It cannot be denied that flagnant v.*ater, the furface of 

 which occupies a confiderable fpace, may produce ini'alu- 

 brious cffetts; but it appears no lefs certain that infalubrlty 

 cannot arife from the difproportion between the vital and 

 nu'phitic air in the atmofphcre ; for the difiercnce was not 

 feniiblc by a hundredth part. It is well known that three 

 different kinds of air are difcngaged from ftagnant water, 

 viz. mephitic air, inflammable air, and carbonic acid gas ; 

 all incapable of maintaining animal life: but we muft fup- 

 pofe that thefe fluids are difengaged in bubbles and iii a very 

 fmall quantity in regard to a great extent of atniofpheric air; 

 that the laft, being more ponderous than common air, mult 

 he immediately precipitated, or combine with the water fuf- 

 pended in it; that the fecond, being lighter, muft rife to the 

 higher regions ; and, in the laft place, that th'? firil being of 

 equal denlity mud rapidly penetrate and lofe itfelf in the 

 imnienfQ fpace. 



It would therefore excite no furprife if all thefe aerial fub- 

 ftanccs (hould alter the quantity of the elaftic portion of the 

 atmofphere, fo that the difference fliould become perceptible 

 by fome hundredths in the proportion between the quantity of 

 vital air and that of the other fluids it is capable of contain* 

 ing; but if this variation in air, collt6^ed in places where 

 emanations of non-reipirable kinds of air are known to exifl, 

 does not rife to a hundredth part, how is it puflible that, at 

 a great difiance from the fphere of the a«Slivity ot thefe partial 

 caules, there fliould be greater variations, which fome philo- 

 fophers pretend to haveobferved not only in different months, 



K k ^ , but 



