ORDER IV. PRISMATIC NITRE-SALT. 35 



Lustre vitreous. Colour white. Streak white. 



Transparent ... semi-transparent. 

 Sectile. Hardness = 2-0. Sp. Gr. = 1-9369. 



HASSENFRATZ. Taste saline and cool. 



Compound Varieties. Twin-crystals, very fre- 

 quent, compound parallel to one or both the faces 

 of P + OD, variously repeated, similar to Fig. 39. 

 In crusts and flakes : composition sometimes co- 

 lumnar. 



OBSERVATIONS. 



1. The prismatic Nitre-salt, as it is found at the Pulo di 

 Molfetta, in Apulia, consists, according to KLAPROTII, of 



Nitrate of Potash 42-55. 



Sulphate \ f 25-45. 



Muriate I of Lime < 0-20. 



Carbonate J 130-40. 



The composition of the crystals is expressed by K N 2 = 

 55-28 of Potash and 44-72 Nitric acid. It dissolves very 

 easily in water, is not altered on being exposed to air, and 

 detonates with combustible substances. 



2. More commonly this salt occurs in thin crusts on the 

 surface of the earth, sometimes upon limestone, chalk, or 

 calcareous tufa ; also in caves contained in limestone, and 

 mixed up with, or in small veins traversing, sandstone. 



3. Spain, Italy, and Hungary, afford considerable quan- 

 tities of this salt. In still greater proportions, and in a high 

 state of purity, it is met with in India, and in several 

 caverns in limestone, and in the sandstone of the United 

 States of North America. 



4. Its chief employment is in the composition of gun- 

 powder. Besides this, it is used in medicine, for obtain- 

 ing nitric acid, and for several other purposes. In some 

 countries, as in the East Indies, in Spain, and in Hun- 

 gary, it is collected for use ; the greater part of that, which 



