SO PHYSIOGRAPHY. CLASS I. 



a : b : c = 1 : V ' 806 : V 0-107- 



Simple forms. P oo ; P (P) ; (Pr + <*>) 5 (d) 

 = 107 50'; Pr 1 = 121 46'; ?r(o)=83 

 50' ; Pr + OD (p). 



Char of Comb. Prismatic. 



Combinations. 1. P oo. (?r -f oo) 3 . ]?r -f oo. 



2. ?r. (Pr + <z>) 3 . Pr -f cc. Similar to Fig. 9. 



3. Pr. P. (Pr + ce) 3 . Pr + CD. Fig. 16. 



4. P QD. ?r 1. Pr. P. (Pr+ce) 3 . Pr+cc. 

 Cleavage, traces of Pr -|- OD, interrupted by conchoi- 



dal fracture. Imperfect. Fracture conchoidal. 

 Surface generally smooth ; P oo streaked, pa- 

 rallel to its edges of combination with Pr. 



Lustre vitreous, more bright upon Pr -j- cc; the faces 

 Pr 1 and Pr being sometimes dull. Colour 

 white, sometimes yellowish. Streak white. Trans- 

 parent ... semi-transparent. 



Sectile. Hardness = 1-5. Sp. Gr. = 1-562. Taste 

 pungent, alcaline. 



OBSERVATIONS. 



1. The difference existing between this and the preceding 

 species has hitherto remained unnoticed, although it seems 

 that both of them are met with in nature. They may easily 

 be obtained by the assistance of art, as have been the crys- 

 tals of both species described above. If a perfectly satu- 

 rated solution of carbonate of soda be exposed to a further 

 evaporation at a temperature of about 25 40 centigr., 

 about 77 104 Fah., or very slowly cooled, beautiful crys- 

 tals of the prismatic species will be formed, whilst a less 

 saturated solution will produce the hemi-prismatic species 

 at a lower temperature, or if cooled more rapidly. 

 2. The relative chemical proportions of this salt have not 



