the Arseni ate s of Copper , and of Iron. 181 
small mamillas is covered with little rough points : these are the 
diedral apices, which terminate the little crystals supposed to 
contribute to their formation. 
This hematitic variety is found with the same diversity of 
colours as the preceding, or amianthiform variety. 
Fourth Species. Arseniate of Copper in the form of a triedral 
prism. 
The primitive form of this species is a triedral prism, the bases 
of which are equilateral triangles; (Fig. g. Plate XIV.) this prism 
is often considerably lengthened, in a direction parallel to one 
of its bases. (Fig. 10.) This form is one of the most rare in 
crystallography. The crystals have all their sides smooth and 
brilliant ; yet there are observable in some of them, when exa- 
mined with a magnifying glass, transverse stride on the sides 
of the prism, all of which are parallel to the edges of the bases. 
It is, therefore, chiefly on the planes of the bases, that the 
crystalline laminae appear laid upon one another, to produce 
either the increase or the modification of the primitive crystal. 
As the crystals of this species are seldom sufficiently detached 
to be easily perceived, and indeed are very frequently so small 
as to escape the observation of the naked eye, I think it neces- 
sary to describe here all the various forms in which I have seen 
them, with the progress observed in their passage from one form 
to the other, however small the difference between these forms 
may appear. Such a description will lead to a better know- 
ledge, not only of the primitive crystal itself, but also of those 
forms of it which seem to be the most distant from its ori- 
ginal one. 
