ATMOSPHEllIC ATH AS THE FOOD OP PLANTS. 71 



unites with great avidity to all basic bodies, forming a 

 long list of nitrates. 



It is volatile, and evaporates on exposure to air, though 

 not so rapidly as water. 



Nitric acid has a strong affinity for water; hence its 

 vapors, when they escape into moist air, condense the 

 moisture, making therewith a visible cloud or fume. For 

 the same reason the commercial acid is always more or less 

 dilute, it being difficult or costly to remove the water en- 

 tirely. 



Nitric acid, as it occurs in commerce, is made by heat- 

 ing together sulphuric acid and jiitrate of soda, when 

 nitric acid distils off, and sulphate of soda remains behind. 



Nitrate of Sti^yhuric Bisulphate of Nitric 



soda. acid. soda. acid. 



NO3 Na + H,S O, = HNa SO, + NO3 H 



Nitrate of soda is formed in nature, and exists in im- 

 mense accumulations in the southern part of Peru, (see 

 p. 252.) 



AnIiydrouM I\itric Acid, NoOs, is what is commonly under- 

 stood as existing in conibiuation witli bases in the nitrates. It is a 

 crystallized body, but is not an acid until it unites with the elements of 

 water. 



Nitrate of Ammonia, NH3 NO^H, or NH, NO3, may 



be easily })repared by adding to nitric acid, ammonia in 

 slight excess, and evaporating the solution. The salt read- 

 ily crystallizes in long, flexible needles, or as a fibrous 

 mass. It gathers moisture from the air, and dissolves in 

 about half its weight of water. 



If nitrate of ammonia be mixed with potash, soda, or 

 lime, or with the carbonates of these bases, an exchange 

 of acids and bases takes place, the result of which is ni- 

 trate of potash, soda, or lime, on the one hand, and free 

 ammonia or carbonate of ammonia on the other. 



,^'itrou!« Oxide, N^O.— When nitrate of ammonia is heated, it 



