the Arseniates of Copper , and of Iron . 175 
These two varieties are the only ones I have observed in the 
form of the crystals of this species, although I have had the 
opportunity of examining a great number of specimens. 
This arseniate is very light; its average specific gravity, 
taken on six pieces perfectly pure, was 2,881. Its hardness is 
likewise very inconsiderable ; it easily scratches calcareous spar, 
but makes no impression on fluor spar. 
It is seldom perfectly transparent ; but has generally a cloudy 
aspect. 
The usual colour of this species (for this character is as essen- 
tial in metallic substances as it is immaterial in stones) is a 
beautiful deep sky-blue ; sometimes, but very rarely, it inclines 
more or less to Prussian blue. It is frequently of a very fine 
grass-green; the crystals have then a much more beautiful 
transparency. I have seen some which were of a fine apple- 
green ; others white, having a slight blue cast. In one piece, 
the crystals of which were of a green colour, and less transpa- 
rent than they commonly are, I discovered, on breaking them, 
that the colour of their central part, for about half their thick- 
ness, was blue. From the observations made by Mr. Che- 
nevix, in his analysis of these arseniates, it appears that the 
variation in their colour, principally depends on the quan- 
tity of water, which enters as a constituent part into their 
formation. 
This species is found mixed with all the other kinds of arse- 
nical copper ore ; but that which most commonly accompanies 
it, is the prismatic triedral species. 
I have never discovered in this species, any thing which 
could induce me to suppose it susceptible of decomposition, or 
even of change. 
