134 Mr Brooke on the Arsemates Copper. 
lines mark the modification Fig. 9. This appears something 
like a triedral prism, where only one summit is seen ; but, in 
those crystals where both summits are visible, the triangular ter- 
minations are reversed with respect to each other, as they appear 
in the figure. Fig. 10. contains some modifications wliich oc- 
cur in most of the crystals I have seen, and shews, by dotted 
lines, the direction in which the crystals are sometimes elongated, 
when attached to the matrix at 5. 
M on M', measures about 56® 
M on P, ~ - 95“ 
P on a, - - 80® 30^ 
Pon^/, - - 125° 
If the prism a, be the result of a decrement, by one row on the 
acute angle of the terminal plane adjacent to the acute solid an- 
gle of the prism, the lateral edge will be to the terminal edge as 
5 to 2 ; and the small plane would result from a decrement, by 
two rows in height, and three in breadth, on the other acute angle 
of the summit. 
The F^th species of Count Bournon belongs, I believe, to the 
'Thirds which I have observed to pass into the fibrous variety, 
through a series of prisms successively diminishing in size. 
There being a difference in the results of the analysis of these 
substances, as given by Klaproth, Vauquelin, and Chenevix, I 
have been induced to examine them again, and I am inclined to 
believe, that their composition may be stated as follows, in atoms 
or proportions : 
Obtuse octahedron, 1 Oxide of copper, 1 Arsenic acid, 5 Water. 
Acute rhomboid, 2 1 — 3 
Right rhombic prism, 4 2 3 
Oblig[ue rhombic prism, 2 1 2 — 
There is some uncertainty in the results obtained, by dissolving 
the mineral in dilute nitric acid, and apparently saturating th(g 
solution with carbonate of potash, which has probably occasioned 
the results of the published analyses to vary from each other : 
For, if the carbonate of potash be added, until a precipitate of 
carbonate of copper begins to appear, which is not re-dissolved, 
and the arsenic acid be then precipitated by an excess of nitrate 
of lead, a portion of arsenic acid sometimes remains in the solu-^ 
tion ; and when the excess of lead has been precipitated by std^ 
