the Arseniate s of Copper , and of Iron. 
* 9 ' 
Species II. Cupreous Arseniate of Iron. 
The crystals of this species are of uncommon brilliancy, and 
are perfectly transparent. Their form is a rhomboidal tetra- 
edral prism, having two of its edges very obtuse, and the other 
two very acute : but, ov/ing to the minute size of these crystals, 
I have not yet been able to determine the measure of their 
angles. The prism is terminated, at each of its extremities, by a 
tetraedral pyramid, which is pretty sharp ; and its planes, which 
are scalene triangles, unite by pairs, forming elongated ridges, 
which join the acute edges of the prism : in the other direction, 
they unite, also by pairs, so as to form a ridge which is less 
elongated, and joins the obtuse edges. Very often, the obtuse 
edges of the prism are replaced by planes (of greater or less 
extent) equally inclined upon the adjacent ones. (Fig. 31.) 
Sometimes the acute edges are also replaced in the same man- 
ner, but always by planes of less extent. (Fig. 32.) 
The above are the only varieties I have observed of this 
arseniate. Its crystals seldom occur singly, being generally 
grouped together, in a very irregular manner; sometimes, how- 
ever, they are so united as to assume a mamillated form, 
having the pyramids of the crystals which compose the mamillas 
all placed upon the surface thereof. 
The specific gravity of this arseniate is 3,400. 
Its hardness is rather greater than that of the simple arse- 
niate of iron : it scratches calcareous spar with greater facility, 
but does not scratch fluor spar, or heavy spar. 
