the native Carbonate of' Soda from Fezzan. 327 



Lustre vitreous. Colour white, when pure. Streak white. 

 Semitransparent. (Even very small crystals possess lower de- 

 grees of transparency than crystals of Glauber-salt of the same 

 size.) 



Sectile. Hardness = 1.0 ... 1.5. Sp. gr. = 1.423. Taste 

 pungent, alkaline. 



Compound Varieties. — Several imitative shapes: composi- 

 tion columnar. Massive : composition granular. Individuals 

 large, generally obtained by the assistance of art, as also the 

 crystals themselves. It occurs in nature in a decomposed 

 state, reduced to powder by the loss of its water. 



3. Prismatic Natron-salt. 

 Prismatic. P - 141° 48', 52° &, 145° 52' Fig. 10. Ap. 



a : b : c == 1 : V 0.806 : */ 0.107. 

 Simple forms. P — od ; P (P) ; (Pr + oo f (d) = 107° 50' ; 

 Pr — 1 = 121° 4& ; Pr (o) = 83° 50' ; Pr + oo (p). 

 Combinations. 1 . Pr . (Pr + oo ) 5 . Pr -f- oo . Fig. 1 1 . 



2. Pr . P . (Pr + co ) 3 . Pr + oo . Fig. 12. 

 v Cleavage very imperfect ; traces parallel to p ; much inter- 

 rupted by fracture, which is small conchoidal. Surface ge- 

 nerally smooth. P — oo streaked parallel to its edges of com- 



bination with Pr. 



Lustre vitreous, more bright upon p ; the horizontal prisms 

 being sometimes dull. Colour white ; sometimes yellowish. 

 Streak white. Transparent . . . semitransparent. 



Sectile. Hardness = 1.5. Sp. gr. = 1.562. Taste pun- 

 gent, alkaline. 



Observations. 



As it has always been the custom in mineralogy to quote 

 Pliny when treating of soda, it may be observed here, that the 

 Nitrum of the ancients, generally allowed to be our soda, which 

 was found in the vicinity of Naucratis and Memphis, in Egypt, 

 may be Trona, because lapidescit ibi in acervis : multique 

 sunt tumuli ea de causa saxei ; * in the same way, as we find 



* Plin- Hist. Nat. lib- xxxi. cap- x. vol iii. p. 9RS- Elzev- 163.V 



