the Arseniates of Copper , and of Iron. 187 
4,280. Its hardness, however, is not so great; it is with 
difficulty that it can be made to scratch calcareous spar. 
The crystals of this species, when they have not undergone 
any change, are transparent, and of a very beautiful bluish 
green colour, or deep verdigrise; but their surface easily 
becomes decomposed, and turns black ; the crystals are then 
totally opaque. It is indeed very seldom, and only in cavities 
recently exposed, that crystals can be found which retain their 
transparency and colour. Yet, as the change they undergo 
commonly takes place only at the surface, rarely penetrating 
to any great depth, their original colour may easily be re- 
stored, merely by slightly scraping the surface with a sharp 
instrument. 
The above is the only change I have had occasion to remark 
in this species. 
Sometimes, but very rarely, this species is found in the form 
of small hair pencils, with very delicate fibres; and as, in the spe- 
cimens in which I observed this variety, the little fibrous tuft 
had preserved its beautiful verdigrise colour, nothing could 
exceed the beauty of their appearance. 
I have likewise observed this species in a mamillary form, 
with a compact texture ; but this variety, like the preceding, is 
extremely rare. 
The matrix of this arseniate of copper is the same as that 
of all the preceding species ; and that species which is most 
frequently found with it, is the arseniate in obtuse octaedra. 
It is also frequently accompanied with that kind of ore which 
is known by the name of azure copper ore. 
Bb 2 
