296 PHYSIOGRAPHY. CLASS n. 



Fundamental form. Hexahedron. Vol. I. Fig. 1. 



Simple forms. 6 (P). Vol. I. Fig. 2. ; I) (o). 

 Vol. I. Fig. 31. C 2 (r). Vol. I. Fig. 34. 



Char, of Comb. Tessular. Combinations 1. O. D. 

 %>. O. D. Cs. Irregular forms, grains. 



Cleavage, octahedron difficult. Fracture conchoi- 

 dal. Surface smooth, the icositetrahedron some- 

 times striated parallel to the edges of combina- 

 tion with the octahedron. 



Lustre vitreous. Colour red, passing into blue 

 and green, also into yellow, brown, and black. 

 Sometimes nearly white. Streak white. Trans- 

 parent ... translucent, only on the edges if the 

 colour be very dark. 



Hardness = 8-0. Sp. Gr. = 3-523, of a transpa- 

 rent variety, between cochineal- and carmine-red. 



Compound Varieties. Twin-crystals, face of com- 

 position parallel, axis of revolution perpendicular 

 to a face of the octahedron. Fig. 156. Sometimes 

 parallel to several faces of the octahedron. 



OBSERVATIONS. 



1. The varieties called Salamstcin by WERNER cannot 

 be received in the species of dodecahedral Corundum, as 

 their forms belong to the rhombohedral system, but it is 

 necessary to include in it the varieties called Zeilanite or 

 Pleonastc, which differ only in regard to colour, and perhaps 

 to specific gravity, owing to an admixture of oxide of iron. 



2. The contents .of three varieties of dodecahedral Co- 

 rundum have been found as follows : 



Blue Spinelle Red Spindle. Pkonaste 



from Aker. from Ceylon. 



Alumina 72-25 74'50 68-QO, 



