^53 



now mors fked. 



In the soil, nitric acid is always combined ■\vitli an 

 alkali or alkali-earth, and never exists in the free state in 

 apjireciable quantity. We speak of nitric acid instead of 

 nitrates, because the former is the active ingredient com- 

 mon to all the latter. Before considering its formation 

 and nutritive relations to vegetation, we shall describe 

 those of its compounds which may exist in the soil, viz., 

 the vitrates of potash, coda, lime, magnesia, andiron. 



Nitrate of Potash (K NOJ is the substance com- 

 mercially known as niter or saltpeter. When pure (refin- 

 ed saltpeter), it occurs in colorless prismatic crystals. It 

 is freely soluble in water, and lias a peculiar sharp, cooling 

 taste. Crude saltpeter contains common salt and other 

 impurities. Nitrate of potash is largely procured for in- 

 dustrial uses from certain districts of India (Bengal) and 

 from various caves in tropicil and tempei'ate climates, by^ 

 simply leaching tlie earth with Avater and evaporating the 

 solution thus obtained. It is also made in artificial niter- 

 beds or jDlantations in many European countries. It is 

 likewise prepared artificially from nitrate of sotia and 

 caustic potash, or chloride of potassium. The chief use 

 of the commercial salt is in the manufacture of gunpowder 

 and fireworks. 



Sulphur, charcoal, (which are ingredients of gunpow- 

 der), and other combustible matteis, when heated in con- 

 tact with a nitrate, burn with great intensity at the ex- 

 pense of the oxygen which the nitrate contains in large 

 proportion and readily parts with. 



Nitrate of Soda (Na NO3) occurs in immense quantities 

 in the southern extremity of Peru, province of Tarapaca, 

 as an incrustation or a compact stratum several feet thick, 

 on the ]iampa of Tamarugel, an arid plain situated in a 

 region where rain never falls. The salt is dissolved in hot 

 water, the solution poured off from sand and evaporated to 

 the crystallizing point. The crude salt has in general a 



