310 SALTS. 



air, and is converted into nitric acid. The acid then 

 combines with the lime to form the nitrate. The salt 

 is afterward extracted by water. The formation of 

 nitric acid, above mentioned, takes place only in the pre- 

 sence of alkaline substances. In their absence the ni- 

 trogen passes off, combined with hydrogen, as am- 

 monia. Even in the presence of lime, there is reason 

 to believe that ammonia is first formed, and its consti- 

 tuents afterwards converted into nitric acid and water. 



783. NITRATE OF POTASSA. NITRE, OR 



Explain the 



formation of SALTPETRE. This salt is a constituent of 

 certain soils, especially in warm climates. 

 These soils always contain lime, and are said to be 

 never entirely destitute of vegetable or animal matter. 

 It is obvious, therefore, that nitrate of potassa may be 

 formed in them, as the same salt of lime is formed in 

 the nitre beds just described. A small proportion of 

 nitric acid exists in the atmosphere, combined with am- 

 monia. This, also, may be a source of part of the 

 nitric acid of the nitrous soils. Again, it is probable 

 that nitric acid is slowly formed from the atmosphere 

 by the direct combination of its elements in the porous 

 soil. Nitre, on being highly heated, yields a third of 

 its oxygen in the form of gas. 



784. Nitre is obtained from nitrous soils 



Hew ^s nitre 



obtained from, by lixiviation with water, and subsequent 



nitron soils? crystallizatioiL From nitrate o f Iim6j 



it is produced by double decomposition with carbonate 

 of potassa. Carbonate of lime precipitates, while nitrate 

 of lime remains in solution. This may be afterward 

 poured off, evaporated, and crystallized. 



