C 91 3 



turning the hand, the air in the meafure may be 

 made to afcend into the tube, within which it will 

 caufe a portion of the water to defcend, exaftly 

 correfponding to the quantity of air that was con- 

 tained in the meafure. Now mark the fpace 

 which the air occupies, by applying the graduated 

 fcale to the outfide of the tube; next take the 

 fame meafure full of nitrous air, and introduce it 

 alio in the fame manner into the tube; gently 

 agitate the tube, and, after waiting fome little 

 time, apply the fcale again, and obferve what di- 

 minution has taken place; for if two meafures of 

 the fame air had been introduced, they would 

 occupy of courfe twice as much fpace within the 

 tube as one would have done. But this will by 

 no means be the cafe in the inftance before us ; 

 for a meafure of common air, and one of nitrous 

 air, will not occupy as much fpace as two mea- 

 fures of common air, or two of nitrous would, but 

 will fall confiderably fhort of it; and it is in 

 proportion to this diminution that the purity of 

 the air is afcertained, the greater diminution de- 

 noting the greater purity of the air. Thus, if one 

 meafure of nitrous air be added to one of any 

 other kind of air, and the diminution be i T %%, 

 the meafure of the teft will be 1.26. The ni- 

 trous acid being compofed of nitrous air and 

 oxygen air, whenever thefe two airs meet, they 

 put off their aeriform Mate, and become liquid 

 nitrous air; which, as it mixes with the water, 

 no longer impedes the afcent of that fluid in the 

 tube. 



The 



