6 
ON THE CRYSTALLISATIONS 
The regular form most commonly exhibited by the crys- 
tals of coppei'-py rites, is tliat represented in Fig. 5. The 
re-entering angles shew that it belongs to a compound, and 
not to a simple mineral. It is the same twin-crystal, which, 
from its great resemblance, has been confounded with those 
of the octahedron in Spinel and other minerals. The forms 
of the individuals can be obtained from the variety Fig. 5., 
by laying a plane through the re-entering angles, and turn- 
ing one of the parts thus detached in this plane under an 
angle of 180°. The result of the operation is the form 
Fig. 6. 
This form is not contained under planes altogether equal 
and similar to each other, and therefore is not a simple one. 
It is a combination of four different simple forms, to one of 
which belong the octagonal, to another the oblong rect- 
angidar, to a third the irregular hexagonal, and to a fourth 
the trapezoidal faces. Each of these forms is produced in 
succession, by enlarging the homologous faces, till the rest 
disappear. Thus the irregular hexagonal faces yield that 
four-sided pyramid, which by measurement is found to have 
the angle at its basis = 108° 40', and for the present spe- 
cies bears the designation P. The trapezoidal faces are 
parallel to the faces of cleavage ; their intersections with 
the faces of P are parallel to each other, and to the ter-, 
minal edges of that pyramid, which results from the en- 
largement of the trapezoidal faces themselves ; the axis of 
the pyramid therefore, as we have seen above, is equal to 
the axis of P multiplied by ; and the pyramid itself, 
P-f-1. The rectangular faces likewise produce a four-sided 
pyramid, which, since it appears with parallel edges of 
combination in the place of the terminal edges of P, is to 
this pyramid in the same ratio, in which P is to P-f 1. Its 
axis will therefore be equal to the axis of P divided by 
and itself the member P— 1 of the same series, to which 
