2^% Memoir on the Quantity of 



2d, Nitrous air is not always of the fame purity. 

 3d, The nitrous air is in part abforbed by the nitrous acid 

 which is produced. 



Proof by Injiammahle Air. 



It is then demonflrated that the eudiometric proof made 

 with nitrous air is imperfe^l. The fecond proof, of^vvhich I 

 murt fpeak, pra<^ifed by the means of inflammable gas, from 

 its being an aerial body like nitrous gas, will be fubje£t alfo 

 to the fame imperfection. For this reafon I have not only 

 given over ufing it, but, in confidering the difcovery of Mr. 

 Cavendifl), that a quantity of mephitic air may unite with 

 vital air in the ftate of ignition, I mud obferve that, in the 

 proof by inflammable air, which is burned with the air fub- 

 je6led to examination, the whole portion of vital air con- 

 tained in the latter may not only dilappear, but a certain 

 quantity of mephitic air be abforbed, unlefs the two airs are 

 entirely free from it, which cannot be known without great 

 difficulty; and it will be neceflary to calculate how much 

 inflammable and azotic gas the refiduum contains, to be able 

 to afcertain, with fufficient exadnefs, the quantity of vital air 

 which has difappeared, and which is the point to be deter- 

 mined. 



Proof hy Phofphorus* 



It is therefore better that the fubftance employed for deter- 

 mining the purity of the air fhould not be gafeous, nor in a 

 ftate of combufl:ion. For this reafon phofphorus, recom- 

 mended by M. Achard as proper for eudiometric proofs, 

 though a folid matter, may be liable to the fame inconveni- 

 ence; for not only the vital will lofe its elaftic form, but a 

 part of its mephitic air will be abforbed. 



Proof by a Mixture of Iron and Sulphur. 

 The proof which feems to be fubjed to no error, is that 

 by a moiltened mixture of fulphur and filings of iron. At 

 firfl I employed this method as much as that by fulphuret, 

 judging, with other philofophers, that they were equally 

 proper. It is true, indeed, that both thefe fubftances abforb 

 only that portion of vital air which is contained in the atmo- 

 fph eric air, leaving the mephitic air untouched; and thus, 



by 



