PHYSIOGRAPHY. CLASS n 



Surface in general smooth, faintly streaked par- a 

 allel to those edges of the icositetrahedron, which 

 meet in the solid angle of three faces, often they 

 appear a little convex. 



Lustre vitreous. Colour white, prevalent, passing 

 into grey, more frequently into reddish-white 

 and flesh-red. Streak white. Transparent ... 

 translucent. 



Brittle. Hardness = 5-5. Sp. Gr. =2-068, crys- 

 tals from the Tyrol. 



Compound Varieties. Massive: composition gra- 

 nular, of various, often considerable sizes of indi- 

 viduals, more or Jess strongly coherent. Faces of 

 composition uneven and rough, and often irregu- 

 larly streaked. 



OBSERVATIONS. 



1. VAUQUEEIN has found a variety of the present spe- 

 cies to consist of 



Silica 58-00. 



Alumina 18-00. 



- Soda 10-00. 



Lime 2-00. 



Water 8-50. 



Its chemical constitution is expressed, according to BERZE- 

 LIUS, by N 3 Si 4 4- 6*A Si 2 + 12 Aq, which corresponds to 



twenty -four solids, such as would arise from laying planes par- 

 allel to those of the dodecahedron through their centres ; each 

 of them on the surface of Fig. 34. Vol. I. being contiguous to 

 the solid angle a, to one of those marked 6, and to two adjacent 

 ones marked c. These solids are symmetrically arranged in 

 respect to the axes of the icositetrahedron ; but each or them 

 possesses a separate optical structure, and a double refraction, 

 as if modified by a succession of strata of variable density. H. 



