( m ) 
AiiT. XXIII. — 0« the Arseniates of Copper. By H. J. Brooke, 
Esq. F. R. S. Lond. M. G S. &c. &c. Communicated by the 
Author. 
HE following communication is occasioned by a recent exa- 
mination of the crystalline forms of the Arseniates of Copper. This 
examination has led me to differ from the Count de Bournon, in 
regard to the primary forms of the several varieties of that sub- 
stance, and has induced me to adopt such others as appear more 
compatible with the secondary forms under which those varie- 
ties usually present themselves. 
The Count has divided the arseniates of copper into five spe- 
cies. 
1. An obtuse octahedron, with a rectangular base. 
2 . A hexagonal prism, 
3. A right rhombic prism of 91” and 86°. 
4. A right prism, whose bases are equilateral triangles. 
5. Fibrous. 
The First species may possibly be an octahedron, with a rect- 
angular base. There are, however, striae or two planes of each 
pyramid of some of the crystals, as in Plate VI. Fig. 1., which seem 
to indicate an oblique rhombic prism as the primary form ; in 
which case, the octahedron may be conceived to be produced by a 
truncation of two of the solid angles of the prism, as in Fig. % 
I have not been able to cleave the crystals in any other direction 
than parallel to one plane of the pyramid ; nor have I seen any 
variety of form among those I have examined, from which the 
primary form might, with greater probability, be inferred. The 
natural planes of the crystals do not afford sufficiently good re- 
flections to give their inclination with accuracy. They may be 
said, however, to measure, over the edges of the base, about 73° 
30', and 61° 30', which would give the ratios of the axis of the 
octahedron, and the edges of the rectangular base, nearly as 3, 
4, and 5. 
The primary form of the Second species appears to be an acute 
rhomboid of nearly 68° 53', by measurement ; but, by inference, 
from C on P, Fig. 4., measuring 107° 30', its angle would be 
68° 38'. The form under which the crystals occur, is that mo- 
dification in which the summits of the rhomboid are very deeply 
truncated, as shewn by the dotted lines in Fig. 3., and Fig. 4. 
There are cleavages parallel to the primary planes P, P' P'" ; 
