COMPOUNDS OF POTASSIUM AND SODIUM. 229 



mostly of the nitrates of lime and magnesia which form in 

 these beds. Kef use animal and vegetable matter putrefied 

 in contact with calcareous soils produces nitrate of lime, 

 which affords the nitre by reaction with carbonate of pot- 

 ash. Old plaster lixiviated affords about 5 per cent. This 

 last method is much used in France. The nitric acid of 

 the cavern nitrates comes from the atmosphere, which also 

 consists of nitrogen and oxygen ; but the combination takes 

 place through the agency of a peculiar kind of microscopic 

 plant. 



Nitratine. — Soda Nitre. Sodium Nitrate. Cubic Nitre. 



Ehombohedral ; R : 72=106° 33'. Also in crusts or efflo- 

 rescences, of white, grayish and brownish colors. Taste 

 cooling. Soluble and very deliquescent. 



Composition. Na. 2 3 N= Nitrogen pentoxide 63*5, soda 

 36'5 = 100. Burns vividly on coal, with a yellow light. 



Biff. It resembles nitre (saltpetre), but deliquesces, and 

 gives a deep yellow light when burning. 



Obs. In the district of Tarapaca, Northern Chili, the 

 dry Pampa for an extent of forty leagues is covered with 

 beds of this salt, mixed with gypsum, common salt, glauber 

 salt, and remains of recent shells. 



It is used extensively in the manufacture of nitric acid. 

 It is also used in making nitre by replacing the sodium by 

 potassium. In 1866, one million quintals of this salt were 

 exported from Chili. 



Natron. — Hydrous Sodium Carbonate. Carbonate of Soda. 



Monoclinic. Generally in white efflorescent crusts, some- 

 times yelloAvish or grayish. Taste alkaline. Effloresces on 

 exposure, and the surface becomes white and pulverulent. 



Composition. Na 2 3 C + 10aq = Carbon dioxide 26*7, soda 

 18*8, water 54 '5 = 100. Effervesces strongly with acids. 



Biff. Distinguished from other soda salts by efferves- 

 cing, and from trona, by efflorescing on exposure. 



Obs. This salt is found in solution in certain waters, 

 from which it is crystallized in efflorescences by evapora- 

 tion. Abundant in the soda lakes of Egypt ; also in lakes 

 at Debreczin, in Hungary ; in Mexico, north of Zacatecas, 

 and elsewhere. Sparingly dissolved in the Seltzer and 

 Carlsbad waters. 



