the Arseniates of Copper , and of Iron. 177 
It is also less hard ; it easily scratches gypsum, but not cal- 
careous spar. 
When its crystals are very thin, they are very transparent ; 
but their transparency is diminished when they have any degree 
of thickness. 
When exposed to fire, this species decrepitates very strongly. 
This arseniate, the matrix of which is generally quartzous, 
is occasionally found mixed with some other arseniates of cop- 
per, and particularly with the acute octaedron in the capillary 
or fibrous state. (Spec. 3. Var. 1 and 2.) But the ore most 
commonly found with it, is the red copper ore, which is fre- 
quently very abundant. 
I have never observed in this species any appearance of 
decomposition. 
Third Species. Arseniate of Copper in the form of an acute 
octaedron. 
The most simple form in which this third species is found, 
is likewise an octaedron ; but this octaedron, instead of being 
obtuse, like that of the first species, is slightly acute. It has, 
like that, in each of its pyramids, two opposite planes more 
inclined than the other two. The more inclined planes meet 
at the apex, in an angle of 84° ; and at the base, in one of g6 ° : 
the other two meet at the apex, in an angle of 68° ; and at the 
base, in one of 112. 0 (Fig. 4.) 
In this octaedron it sometimes happens, that the planes which 
compose its pyramids tend to one and the same point, in order 
to form the apex; but it is much more common to find it 
extended in a line parallel to the less inclined planes of the 
mdccci. A a 
