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12 ON THE CRYSTALLISATIONS 
larity of the series of crystallisation of copper-pyrites, 
the alternating faces of P, and of other forms, to be 
much more extended than the rest of the faces of the 
same, which even sometimes entirely disappear. This 
last is indeed the case with the forms to which belong 
the small triangular faces a, a\ situated between the 
faces of P — GO , P— 1, and the enlarged of P. If all 
the faces of this form would appear, the result would 
be a scalene eight-sided pyramid, fig. 20 ; but instead 
of sixteen, we find only eight faces, and if we enlarge 
these, till they limit the space by themselves, the re- 
sultant form is contained under eight equal and simi- 
lar scalene triangles, as represented fig. 21, which 
nevertheless is not a scalene four-sided pyramid, but 
must be considered as lialfoi the eight-sided. This 
form is produced at once from the scalene eight-sided 
pyramid by enlarging the alternating pairs of the 
faces till the rest disappear. The dimensions could 
not be determined, on account of the want of parallel 
edges of combination between the half of the eight- 
sided pyramid, and other forms ; besides the small- 
ness of the crystals would not allow any application 
of a goniometer, 
15. P— X. P— .1. P. P-hl. (P+oo)^ fig. 22. The pre- 
sent variety differs from that of fig. 6. only by its 
having eight rhombic faces disposed vertically round 
the axis. 
Suppose fig. 28. to be the projection of fig. 22, 
upon a horizontal plane *. 
If the faces a', &c. are rhombs, 'OP' must be 
= P'Q =: QF' =: P' O, and OP = PQ. 
* The letters signify the same in both the figures. 
