ORDER IV. OCTAHEDRAL ALUM-SALT. 51 



dodecahedron sometimes faintly streaked, parallel 



to its edges of combination with the octahedron. 

 Lustre vitreous. Colour white. Streak white. 



Transparent ... translucent. 

 Not very brittle. Hardness = 2-0 ... 2-5. 



Spec. Grav. = 1-753, of a transparent variety. 



Taste sweetish astringent. 



Compound Varieties. Twin-crystals : face of 

 composition parallel, axis of revolution perpendi- 

 cular to one of the faces of the octahedron. Sta- 

 lactitic and other imitative forms : composition co- 

 lumnar, if they are very delicate the lustre becomes 

 pearly. Massive : composition either columnar or 

 granular, often impalpable. Mealy efflorescence. 



OBSERVATIONS. 



1. The Character of Combinations is given on the autho- 

 rity of Count BOURNON and M. BEUDANT. According to 

 BEUDANT, certain combinations obtained by dissolving Alum 

 in muriatic-acid, and allowing it to crystallise, contain the 

 faces of a pentagonal dodecahedron, which is the half of a 

 hexahedral trigonal-icositetrahedrou. Count BOURN ON like- 

 wise quotes this variety as having been obtained by himself. 



2. KLAPROTH having analyzed a variety from Freien- 

 walde hi Silesia, called natural alum, but which probably 

 does not belong to this species, obtained the following result : 



Alumina 15-25. 



Potash 0-25. 



Oxide of Iron 7-50. 



Sulphuric Acid and Water 77*00. 



The crystallized varieties to which the above description 

 refers, are K S 2 + 2 Al S* + 48 Aq, or 9'94 Potash, 

 10'82 Alumine, 33*77 Sulphuric acid, and 45-47 Water. 

 It is pretty easily soluble in water, melts before the blow- 



