vital Atr in the Atmofphere, 251 



of meafunnjr. But it eontains alfo two atirkl fobftances, 

 vi%. vital air, and mephitic air or azotic gas. 



In a former memoir, publiflicd in 1787, I mentioned the , 

 opinion of Mr. Cavendifti, that the vital air at London forms 

 nearly a fifth part of the atmofphere; fo that lOQ parts of at- 

 mofpheric air contain %o of vital air and 80 of mephitic air. 

 Dr. Prieitley thinks that the quantity of vital air is between 

 0-20 and 0*25. Scheele, who made his experiments at Stock- 

 holm during the year 1778, found that the quantity of vital 

 air is between 0*24 and Q*30. Lavoifier and other chemifts 

 at Paris are of opinion that it is nearly 0'28. From fome 

 experiments of Senebier there is reafon to infer that the air 

 of Geneva varies fome hundredth parts, and that its portion 

 of vital air exceeds 0*25. But other obfervations made in 

 Europe, which are entitled to fome confidence, feem to have 

 proved that atmofpheric air contains not more than 30 per 

 cent, of vital air, nor lefs than 20. When I tranfmitted to 

 this Society, in the month of June 1787, my Obfervations on 

 the Vital Air of Plants, I announced that the common air 

 which I had breathed at AltafuUa, my native place, during 

 the four preceding months, was of from 97 to 100 degrees ; 

 inx. that ICO parts of nitrous air and an equal quantity of 

 common air, mixed in the manner of Ingenhoufz, were re- 

 duced 100 or 103 ; confequently, from 97 to 100 parts had 

 difappeared. Since that time I have continued my experi- 

 ments on the fame fubjeft, both by this and other proofs, 

 to afcertain whether this fmall inequality might not. arife 

 from circumftances attending, the operation rather than from 

 the nature of the air. 



Proof hy Nitrous Air, 



The proof by nitrous air is th^t which chemifts have 



chiefly endeavoured to bring to. perfection. Fontana, Prieft- 



ley, Ingenhoufz, and fcveral other chemifts, have made many 



experiments to accomplifti that object; but this method pre- 



' fcnts many difficuhieg to be overcome. 



ift, The water in which the experiment is made is never 

 pure. It contains a greater or lefs quantity of oxygen> azot, 

 and carbonic acid, which it is not eafy to determine. 



li 2 2d, Nitrou* 



