ORDER VII. KHOMBOHEDRAL TOURMALINE. 349 



to the optical researches of Dr BREWSTER, tetrahedral Bo- 

 racite possesses one axis of double refraction, positive like 

 that of Quartz, and coinciding with one of its principal 

 rhombohedral axes. This is one of the remarkable exceptions 

 to the otherwise almost general law, that bodies, whose forms 

 are tessular, do not possess any double refraction. Our pre- 

 sent knowledge of the connexion of these two phenomena 

 is as yet insufficient to account for the anomaly of this case. 



2. Tetrahedral Boracite consists, according to PFAFF, of 



Boracic Acid 54-55. 



Magnesia 30-68. 



Oxide of Iron 0-57. 



Silica 2-27- 



Before the blowpipe upon charcoal it intumesces, and melts 

 into a glassy globule, which becomes white and opake on 

 cooling. It becomes electric by heat, four alternating termi- 

 nal points of its rhombohedral axes being positive, and 

 those which are opposite to them negative. 



3. The varieties of tetrahedral Boracite are found in 

 crystals terminated on all sides, imbedded in compound va- 

 rieties of prismatoidal, in a few instances also of prismatic 

 Gypsum-haloide, and have been hitherto found only at 

 lAineburg in Brunswick, and at Segeberg in Holstein. 



GENUS X. TOURMALINE. 

 1. RHOMBOHEDRAL TOURMALINE. 



Rhomboidal Tourmaline. JAM. Syst. Vol. I. p. 104. 

 Man. p. 221. Tourmaline. PHILL. p. 139. Tur- 

 malin. Schorl. WERX. Hoffm. H. B. I. S. 627. 647. 

 Schorl. Aphrit. HAUSM. II. S. 640. 642. Turmalin. 

 LEONH. S. 397- Tourmaline. Tourmaline apyre. 

 HAIJY. Traite, T. III. p. 31. T. IV. p. 401. Tour, 

 maline. Tabl. comp. p. 38. Traite', 2de Ed. T. III. p. 14. 



Fundamental form. Rhombohedron. R = 133 

 26'. Vol. I. Fig. 7. HAUY. 



