138 Defcnpt'ion of the /'hfcniates 



meet with ffcciniens of this fpecles in which the cryftals are 

 fufficienlly detached to let their form be diftinclly feen. In 

 general, the cryftals are grouped togeth.er in sreat numbers, 

 and iccin to penetrate each other. To as to form mamilla*, 

 more or Icfs round ; or they form a kind of indented cyhn- 

 ders, which have foine refemblance to the trundle of a mill. 

 ]n that cafe, the part of the cryftals which appears at the 

 furface of thefe agoregations, comnionlv belongs to one of 

 the fides of their prifm, either the broad or the narrow one. 

 But, when thefe aggregations form cither a kind of cylinders, 

 or of mamillsG in clufters diverging like a fan, there may be 

 feen, at the two edges of the cvlmder, or at the funimit of 

 the clufters, the whole of the equilateral triangular terminal 

 planes, or trapezia, of one part of the component cryftals. 



The fpccific gravity of this fpecies of arfeniate of copper is 

 nearly the fame as that of the preceding one; I found it to 

 be 4,280. Its hardnefs, however, is not fo great; it is with 

 difficulty that it can be made to fcratch calcareous fpar. 



The cryftals of this fpecies, when they have not undergone 

 any change, are tranfparcnt, and of a very beautiful blucifh- 

 green colour, or deep verdigrife; hut their furface eafily be- 

 comes decompofed, and turns black ; the cryftals are then 

 totally opaque. It is indeed very feldom, and only in cavi- 

 ties recently expofed, that cryftals can be found which retain 

 their tranfparency and colour. Yet, as the change they 

 undergo commonly takes place only at the furface, rarely 

 penetrating to any great depth, their original colour may 

 eafily be reftored, merely by fliglitly fcraping the furface with 

 a ftiarp inftrument. 



The above is the only chajige I have had occafion to re- 

 mark in this fpecies. 



Sometimes, but verv rarely, this fpecies is found in the 

 orm of fmall hair pencils, with verv delicate fibres; and as, 

 in the fpccimens in which I obfcrved this variety, the little 

 fibrous tuft had prefervcd its beautiful verdigrife colour, no- 

 thing could exceed the beauty of their appearance. 



\ have likewife obferved this fpecies in a mamillary form, 

 with a compaft texture; but this variety, like the preceding, 

 is extremely rare. 



The matrix of this arfeniate of copper is the fame as that 

 of all the preceding fpecies; and that fpecies which is nioft 

 frequently found with it, is the arfeniate in obtufe oftaedra. 

 It is alio frequently accompanied with that kind of ore which 

 is known by the name of azure copper ore. 



SECTION 



