14 
ON THE CRYSTALLISATIONS 
The last of these varieties is hemipyramidal, not only by 
the two forms P — 2 and (P — entering into it with half 
the number of their faces, but also by the difference in the 
extension of the alternating faces of P. There is, how- 
ever, besides this difference in the extension, still another 
existing between those faces, which can be of use, if at- 
tended to, in examining crystals very much engaged and 
grouped with each other. The enlarged faces of P always 
bear distinct stritE, parallel to the edges of combination of 
this form with P+1. Striae in the same direction occur 
also upon the smaller faces of P, but they are generally 
very indistinct, and even, in some cases, not at all observ- 
able. The faces of the other forms are commonly smooth, 
3 
and not streaked at all, only P — 1 and have some- 
times a few horizontal striae. 
The variety in the forms of Copper-pyrites, and at the 
same time the difficulty of recognising them, is greatly in- 
creased by the scarcity of Single Individuals affecting the 
above mentioned forms. More commonly we find two or 
more individuals regularly grouped with each other; so 
that, besides compound forms, we have to consider com- 
pound minerals, in which, however, the composition takes 
place under certain crystallographic laws. 
The regular aggregations or compositions of two indivi- 
duals have received the names of Twin, or Hemitrope 
crystals, according to their being considered as produced by 
two simple crystals ; or by a single one cut in two parts, in 
which the one half has undergone a revolution of 180° in the 
same cutting plane. In general, regular aggregations or com- 
positions may be conceived to arise from several individuals, 
each two of which are in contact with each other in a face 
parallel to a face, or perpendicular to an edge of crystaL 
