Mr Brooke on the Arseniaks qf Copper. 133 
but the crystals are most fissile in a direction perpendicular to 
the axis of the rhomboid ; that is, parallel to the terminal planes 
of the hexagonal crystals. 
The replacement of the lateral edges of the primary rhomboid, 
as in Fig. 5., produces the planes of the supposed hexagonal 
prism. 
Several other modifications of the primary rhomboid occur, 
One of which produces the obtuse rhomboid, shewn in Fig. 5., 
a onh measuring about 179° 30'. The planes are brilliant, and 
the edges distinct ; so that I cannot doubt the existence of the 
form. Yet the decrement which would produce it, is so extra- 
ordinary, being several hundred molecules in height and breadth, 
that I suppose it may be produced by some other law than 
that which commonly operates upon those primary forms. 
The primary form of the Third species^ is a right rhom- 
bic prism. Fig. 6., of about 111° 45', M on M', and 68° 15', 
the crystals being usually attached to the matrix by one of the 
obtuse edges of the prism. The dotted lines in Fig. 6., shew 
the position of the most common form of the natural crys- 
tals, Fig. 7. ; by which it appears, that the diedral termina- 
tion of those crystals consists of two of the primary planes. 
There is a sufficiently distinct cleavage parallel to the planes 
M M' and P ; but the cleavage planes are very dull. Count 
Bournon has taken a and Fig. 7., as the primary lateral planes 
of his rhombic prism ; but, as I have not succeeded in cleaving 
the crystal parallel to those planes, or perpendicular to the axis 
of the prism, as constituted by those planes, I prefer taking the 
form I have given as the primary one. If the planes a and b, 
which incline at an angle of about 93°, are the result of a decre- 
ment by one row on the acute angles of the terminal planes, the 
height of the prism. Fig. 6. would be to its terminal edge as 15 
to 19, very nearly. 
The primary form of the Fourth species^ is an oblique rhombic 
prism of 56° and 124° nearly, — oblique from the acute angle of 
the prism, and attached to the matrix, sometimes by the base, 
and sometimes by the obtuse lateral edges of the prism. The 
only cleavage I have observed, is parallel to the terminal planes 
of the prism, in which direction the laminae separate with great 
readiness. Fig. 8, shews the primary form, in which the dotted 
