NITRIC ACID. 173 



through the tubes is known by the quantity of water 

 that has flowed out from the cask. The materials em- 

 ployed in the tubes are pumice stone drenched with oil 

 of vitriol, in the first, to absorb the water ; and caus- 

 tic potassa, in the second, to retain the carbonic acid. 

 The method for determining the amount of ammonia 

 in the atmosphere is essentially the same, muriatic acid 

 being used as the absorbant. 



422. PROPORTIONAL COMPOSITION OF 



What are the 



proportions of THE AIR. The proportions of the four 

 fousfttwnteof constituents of the air above mentioned, 

 the atmo- as obtained by the method just described, 

 are, about 21 per cent, of oxygen, 79 of 

 nitrogen, arWth of carbonic acid, and t?4fofcrvtb of 

 ammonia. The proportion of aqueous vapor is ex- 

 tremely variable. That of carbonic acid and ammo- 

 nia is also variable to a considerable extent. 



NITRIC ACID. 



What is nitric 423. DESCRIPTION. Nitric acid is a thin. 

 acid? colorless, and intensely acid fluid. It cor- 



rodes metals instantaneously, with the evolution of deep 

 red vapor. It is composed of nitrogen and oxygen, in 

 the proportion of one atom of the former to five of the 

 latter. It contains, in addition, water, with which it 

 is chemically combined. It is possible to make it an- 

 hydrous, or free from water, but such an acid is never 

 used. 



How is nitric 424. PREPARATION. Nitric acid exists 

 add prepared? m a dormant state in ordinary saltpetre. 



