142 PHYSIOGRAPHY. 



Common Salt. 



Fracture conchoidal. Surface generally smooth. 



Lustre vitreous, somewhat inclining to resinous. Color 

 generally white, passing into yellow, flesh-red and ash-grey. 

 Sometimes beautifully violet, Berlin or azure-blue. Streak 

 white. If scratched with the nail, it does not -yield any 

 powder, but receives an impression, and becomes a little 

 shining. Transparent . . . translucent. 



Rather brittle. Hardness = 2-0. Sp. gr. = 2-257, a 

 yellowish-white transparent variety. Taste saline. 



Compound Varieties. Dentiform and some other imi- 

 tative shapes, rare. Commonly massive. Composition 

 granular or columnar, the latter in most cases parallel, 

 sometimes curved. Size of the component individuals va- 

 rious. Faces of composition rough. 



1. Several varieties in the geological relations, and some differences in 

 the chemical composition, have given rise to the subdivision of Common 

 Salt into subspecies in the older books. Thus the varieties found in beds 

 have been called Rock-salt ; such as are found at the bottom of salt 

 lakes, or on their shores, Sea-salt ; and the former again have been di- 

 vided into foliated and fibrous Rock-salt, according to their granular or 

 columnar mode of composition. 



Common salt is very easily soluble in water. It remains unaltered, if 

 exposed to the dry atmosphere, and decrepitates upon glowing charcoal, 

 or before the blow-pipe. It crystallizes, both from solutions in water, 

 and from fusion. It undergoes a remarkable change if exposed to a 

 moist atmosphere, in consequence of the absorption of w"ater. The solu- 

 tion of a mass of a cubical shape begins regularly at its edges, and trans- 

 forms the cube first into a cube with its edges bevelled, and finally into 

 the icositelrahedron with triangular faces, (fig. 124. Part I.) In the lat- 

 ter form, Ihe mass of the salt diminishes in size, till at last it is entirely 

 dissolved. 2. Analysis. 



By HEJVRY. [Rock-salt variety.] 

 Muriate of soda . . . 983 25 



Sulphate of lime . . . 6-50 ' 



Muriate of magnesia . . . - 19 



Muriate of lime . . . 0-06 



Undissolved matter . 10-00 



