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green to a bright perfect green, apparently neither inclining 

 to yellow or blue, passing on to deepish blue green, and 

 thence to an olive colour, being heightened, as it were, with 

 red ; then, the yellow and red prevailing, they are of a brown- 

 ish resin colour : some are very pellucid and transparent, and 

 all so in some degree. The upper figure shows them of 

 their common natural size in a gangue of quartz mixed 

 with oxides of copper and iron, &c. The middle figure is 

 magnified to show their construction more readily; and the 

 right hand geometrical figure shows, the striae. In the left 

 hand bottom figure, the only modification known of this 

 substance, according to count Bournon (to use his own 

 words), (i Four of the eight solid angles of the cube are re- 

 placed by an equal number of equilateral triangular planes, 

 situated in such a manner that every one of the sides of the 

 cube becomes an elongated hexagon, having two angles 

 of 90° each, and four of 133°. Crystals modified in this 

 way are very scarce. I have never seen but one specimen, 

 which is in the collection of sir John St. Aubin. Its 

 crystals are pretty large and well defined." I therefore 

 consider as a great rarity a specimen in my museum, which 

 exposes two crystals thus truncated. It is easily scratched 

 with a pin, but it scratches common calcareous spar. By 

 Chenevix's analysis it was found to contain 



Silica - - - 4 



Arsenic acid - 31 



Oxide of Iron - 45*5 



of Copper 9 



Water - - - 10*5 



100-0 



