308 PHYSIOGRAPHY. CLASS n. 



perature of about 14 wedg and yields with oxygen, car- 

 bonic acid gas. It is not acted upon by acids or alcalis. 



3. The rocks hitherto considered as the matrix of octahe- 

 dral Diamond, are secondary ones, as several kinds of sand- 

 stone, consisting of aggregated quartz-pebbles. Octahedral 

 Diamond is also found in strata of ironshot sand and clay, 

 and in the loose sand of plains and rivers. In a speci- 

 men from Brazil, in the possession of Mr HEULAND, it is 

 associated with Skorodite, and imbedded in a compact vanety 

 of prismatic Iron-ore. 



4. The octahedral Diamond was first discovered in the 

 East Indies, where it has been worked for many centu- 

 ries, and in Brazil. They are found in various places on the 

 eastern coast of the British peninsula in India, but parti- 

 cularly between Golconda and Masulipatam, also near Pan- 

 na in Bundelcurid. There occur some likewise in the pe- 

 ninsula of Malacca, and the isle of Borneo. In Brazil they 

 occur in the district of Serro do Frio in the capitania of 

 Minas Geraes, and were first discovered in the Riacho 

 Fundo, then in llio do Peixe, and also in the Terra de St 

 Antonio. 



5. Octahedral Diamond is the most valued of all the 

 gems, and is used as an ornamental stone. But it is em- 

 ployed likewise for cutting glass, and for engraving, cutting 

 and polishing other hard stones and the octahedral Diamond 

 itself. 



GENUS IV. TOPAZ. 

 1. PRISMATIC TOPAZ.- 



Prismatic Topaz. JAM. Syst. Vol. I. p. 78. Man. p. 188. 

 Topaz. PHILL. p. 84. Topas. Phisalit. Picnit, cder 

 Schorlartiger Beril. WERN. Hoffm. H. B. I. S. 577- 

 620. IV. 114. Topas. HAUSM. II. S. C48. Topas. 

 LEONH. S. 405. Topaze. Pycnite. HATJY. Traite, T. 

 II. p. 504. T. III. p. 236. Silice fluatee alumineuse. 

 . Topaze. Tabl. camp. p. 17- Alumine fluate'e siliceuse 

 ou Topaze. Traite', 2de Ed. T. II. p. 131. 



