Sulphuret of Lead
- Div. 1. Crystallized; var. Cubo octaëdron compressed and mackled.
We have exhibited the primitive cubic crystals of this substance, tab. 24, It is not rare to see the corners truncated, which are the faces of the octaëdron; thus it passes to what has been called the cubo-octaedron—see the description, and tab. 99. It has been lately observed, that Sulphuret of Lead, or Galæna has rarely been found in six-sided tables; and those w ho have the few so found, think much of them on that account; and it is certainly a curiosity, when we see a substance so decidedly fracturing in cubes to find it forming the octaedron, which it often approaches, and sometimes does most perfectly—that a combination of these forms should be deposited, and at the same time with two of the faces much broader than the rest, or other twelve; viz, six hex angular faces of the octaëdron, and six rectangular faces of the cube—see the middle geometrical figure: that these should again be so deposited in crystallizing, as for two to meet with two of the broader faces as a basis to each, of the nature of that of the macklc, tab. 33; and like that they will meet at opposite angles, as if turned on an axis. Thus the six-sided faces of the octaëdral formation, and the right-angled ones of the cube formation, may meet opposite, as at the right hand figure, or alternate, as on the left hand figure.
The upper figure is from a specimen in the Marchioness of Bath’s cabinet, and contains most of these modifications. The lower one is from a specimen in my own collection. They both came from Derbyshire some time since, and are the only British ones we know of.