26o ^ McmfAr on the Quantity of 



but ot different hours of the fame day ? It nmffc certainly be 

 afcribed to the imperfeftion of fiiir inftrunientSj or to fome 

 negligence in the mode of operation, if confiderable inequaH- 

 ties are fometimes obfcrved in the purity of the air around 

 us ; and repeated experiments give me reafon to think that, 

 wherever there is a free communication with the vaft recep- 

 tacle of the atmofphere, the air which we breathe will always 

 be found to confifl of from o*2i to 0*22 of vital air, and from 

 078 to o*79 of azotic gas, provided it be examined with the 

 precautions which I have here pointed out, 



yf eudiometric proofs are not fufficient for explaining the 

 dangerous eflre6ls experienced in the neighbourhood of ftagr 

 nant waters, the caufe might perhaps be found by analyfing 

 the water that floats in the atmofphere. The obfervations of 

 gerthollet prove that volatile alkali is compofed of azot and 

 hydrogen deprived of cajoric, or of that principle which be- 

 fore kept them feparate in an elaftic form, and we know that 

 thcfe two fluids are difengagcd from ftagnant waters : mav 

 we not then prefume that its alkahne portion, always the 

 fame in its moditications. Is in part decompofed j and that a 

 large quantity, not decompofed, either alone or combined 

 with fome other unknown fubftance, is eagerly abforbed by 

 tjie water, as its putric) fmell in evaporating feems to indi- 

 cate; and, confequently, remaining diifolved in the portion 

 of water fufpended in the neighbouring air, it produces cer- 

 tjiin alterations in regard to animal life ? 



It is not impoffible to colie6l fuch a quantity of this water 

 as to be ah!e to examine it : the produ6ls given by the means 

 of analyfis, compared with thofe of other water fufpended in 

 4ip contiguous to running water, might ferve, perhaps, to 

 niake known the eaufe of the infalubrity of ftagnant water, 

 gudiometric qxperirnents have not been able to throw light 

 on this difficulty, and have taught us only that we ought not 

 to afcribe the inlalubrity .of certain places to the exiftence in 

 the atmpfphere of a quantity of aeriform fluids too great for 

 ihat of vital air. 



Bu|; though this proportion does not vary a hundredth part 

 iij the courfe of feveral months, and even years, may it vary 

 a" very fmall part, fuch as a thoufandth part, which after 4, 



