88 PHYSIOGRAPHY. 



Olivenite Opal. 



metallic globule is formed, which, in the process of cooling, becomes 

 covered with a red coating of sub-oxide of copper. In some varieties, 

 a scoria is formed round the metallic globule. It is soluble in nitric acid. 



2. Analysis. 



By KLAPROTH. By CHENEVIX. By KOBELL. 



var. Wood-Copper. 



Oxide of copper - - 50 62 - 50 00 - 56-43 

 Arsenic acid - - 45-00 - 2900 - 36-71 



Phosphoric acid - - 00 - 0-00 - 3-36 



Water - - 3-50 - 21-00 - 350 



3. It is found in veins, chiefly consisting of various ores of copper, and 

 of Quartz. 



4. It occurs in the copper-mines near Redruth in Cornwall, and in the 

 Tynehead mine near Alston-moor in Cumberland. 



OLIVINE. (See Peridot.} 

 OMPHAZITE. (See Pyroxene.) 



OPAL. Uncleavable Quartz. MOHS. 



Regular forms, and cleavage, unknown. 



Fracture conchoidal, of various degrees of perfection, 

 sometimes highly perfect. 



Lustre vitreous, in some varieties inclining to resinous. 

 Color white, yellow, red, brown, green, grey ; none of 

 them lively, except some red and green ones, and generally 

 pale ; dark colors, owing to foreign admixtures. Streak 

 white. Transparent . . . translucent, sometimes only on the 

 edges, or even opake, if the colors be dark. Lively play 

 of light observable in some varieties ; others exhibit differ- 

 ent colors by reflected and refracted light. * 



* The play of light appears to depend upon openings in the interior of 

 the mass of Opal, which are not fissures, but of an uniform shape, and re- 

 flecting the tints of NEWTON'S scale. In some varieties of Hydrophane 

 they are so large, that these colors cannot be any longer reflected by the 

 included air ; but they appear when filled with water, and of still higher 

 tints, if filled with fluids possessing a high refractive power. 



