604 MANUAL OF NATURAL HISTORY. 



or " Apatite/' hexagonal, sp. gr. 3*1, H. = 5, 

 greenish or bluish, brittle, in crystalline 

 rocks ; 4, as Fluoride of Calcium or " Fluor- 

 spar/' octahedral, sp. gr. 3-1, H. = 4, bright 

 coloured, brittle, in veins intersecting gneiss, 

 mica and clay slate, and some secondary 

 rocks ; 5, as Silicate combined with magnesia, 

 in oblique rhombic prisms, sp. gr. 3-1, H. = 

 5-6; when dark coloured, it is Hornblende; 

 with felspar, it is Greenstone; with "albite," 

 Biorite; when fibrous, Asbestus or Amiaib- 

 thus; with very fine fibres, Mountain-leather ; 

 when spongy and elastic, Mountain-cork. 

 Salts of calcia tinge flame brownish-red ; 

 when soluble they throw down a white pre- 

 cipitate with oxalate of ammonia. Symb. Ca. 

 5. Family. — Magnesiides. Minerals containing 

 compounds of Magnesium. Occur 1, rarely as 

 Oxide or "Periclase," cubical and octohedral, 

 sp. gr. 3-67, H = 6, grayish; 2, as Sulphate 

 or " Epsomite/' in four-sided prisms, sp. gr. 

 1*75, H. = 2-2, soluble, white, taste bitter, in 

 mineral springs, as an efflorescence on rocks, 

 &c. ; 3, as Carbonate or " Magnesite/' rhom- 

 bohedral, sp. gr. 2-9, H. = 3-4, grayish, in 

 magnesian rocks ; 4, with Carbonate of Cal- 

 cia as "Dolomite/' or "Magnesian-limestone/' 

 rhombohedral. sp.gr. 2*9, H. = 3*5-4, generally 

 whitish, brittle, often forming beds ; 5, as 

 Borate or " Boracite/' hemihedral, sp. gr. 2*9, 

 H. === 7, whitish, pyro-electric, in beds of 



