Count de Bournon's Description of 
faces of these prisms, a very fine transverse line, A B, on the 
small diagonal of the rhomboidal plane of these surfaces ; this 
line shews the place of union of the two crystals of which the 
made consists. 
Sometimes, the two component crystals of this kind of 
made, belong to the triedral prism which has a secondary plane 
in the place of one of its edges ; it then has the form, either of 
a hexaedral prism, that has four of its sides (two and two in 
opposition) broader than the others, (Fig. 9,6,) or of one that 
has only two opposite sides broader, (Fig. 27 ,) or of a regular 
one, according to the width of the secondary planes : in all 
these forms, the line A B, indicating the place of union of the 
two crystals, is perceptible. 
It is not very common, as I have already observed, to meet 
with specimens of this species, in which the crystals are suffi- 
ciently detached to let their form be distinctly seen. In general, 
the crystals are grouped together in great numbers, and seem 
to penetrate each other, so as to form mamillae, more or 
less round ; or they form a kind of indented cylinders, which 
have some resemblance to the trundle of a mill. In that case, 
the part of the crystals which appears at the surface of these 
aggregations, commonly belongs to one of the sides of their 
prism, either the broad or the narrow one. But, when these 
aggregations form either a kind of cylinders, or of mamillae in 
clusters diverging like a fan, there may be seen, at the two 
edges of the cylinder, or at the summit of the clusters, the 
whole of the equilateral triangular terminal planes, or trapezia, 
of one part of the component crystals. 
The specific gravity of this species of arseniate of copper, is 
nearly the same as that of the preceding one ; I found it to be 
