MANUAL OF NATURAL HISTORY. 611 



colour various, in rhombic or hexagonal scales 

 or plates, or " Mica ;" 7, with silica, boracic 

 acid, and various bases or "Tourmaline/' 

 rhombohedral, sp. gr. 3 — 3 '3, H. = 7 — 7*5, 

 generally dark-coloured, pyro-electric, in pri- 

 mary rocks ; 8, as Fluosilieate or " Topaz " 

 in right rhombic prisms, sp. gr. 34 — 3*6, 

 H. = 8, yellow or blue, in cavities and veins 

 of granite rocks ; 9, with silica, sulphuric 

 acid, soda, calcia and oxide of iron, or 

 " Lapis lazuli," or when powdered, " Ultra- 

 marine/' in dodecahedrons, sp. gr. 2*3 — 2*4, 

 H. = 5'5, in granite or crystalline limestone ; 

 1 0, with magnesia or " Spinel/' octahedral, 

 sp. gr. 3*5 — 4*9, H. = 8, colour shades of red, 

 among granitic and micaceous debris. Alu- 

 mina forms the chief part of all clays, and is 

 the basis of the manufacture of porcelain. 

 Compounds of alumina, if heated to redness 

 before the blowpipe, then moistened with 

 solution of protonitrate of cobalt, and again 

 strongly heated, yield a sky-blue unfused 

 mass, the colour becoming more distinct on 

 cooling, and appearing violet by candle-light. 

 Symb. Al. 

 1 2. Family. — Glucinides. Minerals containing com- 

 pounds of Glucinum. Occur 1, as Silicate, 

 coloured with oxide of chromium or u Eme- 

 rald/' or with oxide of iron or " Beryl/' in 

 hexagonal prisms, sp. gr. 2 7, H. = 7*5 — 8, 

 green or blue, brittle, in dolomite, granite 



