ORDER VII. PRISMATIC TOPAZ. 311 



either in crystals, which are generally implanted, and have 

 a smooth surface, or in massive granularly compound va- 

 rieties. Here we meet with the highest degrees of trans- 

 parency and the brightest colours. The crystals of Physa- 

 lite are imbedded, their surface is uneven and rough ; in 

 massive varieties they generally present large individuals, 

 their colour inclines to a pale greenish-grey, and their trans- 

 lucency is very faint. Pycnite has not been found in single 

 crystals, at least not distinctly, but consists only of thin 

 and straight columnar particles of composition, forming 

 larger or smaller masses imbedded in other rocks, and not 

 possessing bright colours or high degrees of transparency. 

 The varieties of the whole species are united by transitions, 

 and render the distinction of any one of these three kinds 

 difficult. The optical observations, however, even in trans- 

 parent varieties, as in those from Brazil and Scotland, in- 

 dicate a difference between some of them which probably 

 may lead to the establishment of several species. 



2. Prismatic Topaz consists, according to BERZELIUS, of 



Topaz, Physalite, Pycnite, 



from Schneckenstein. from Finbo. from Altenberg. 



Alumina 57-45 6774 51-00. 



Silica 34-24 34-36 38-43. 



Fluoric Acid 7'75 7'77 8-84. 



In a strong heat the faces of crystallisation, but not those of 

 cleavage, are covered with small blisters, which, however, im- 

 mediately crack. With borax it melts slowly into a trans- 

 parent glass. Its powder colours the tincture of violetsgreen. 

 Those crystals which possess different faces of crystallisa- 

 tion on opposite ends, acquire different kinds of electricity 

 on being heated. By friction it acquires positive electri- 

 city. 



3. Prismatic Topaz enters into the composition of seve- 

 ral granitic rocks ; thus it forms with rhombohedral Quartz 

 and rhombohedral Tourmaline the Topaz rock of Saxony, 

 and is found crystallised in its drusy cavities. It occurs 

 also in irregular beds, either with quartz and mica, like the 

 variety called Pycnite ; or with prismatic Feld-spar, rhom. 



