314
ON THE NATURAL HISTORY
us remember that even in their time the word "copperas" had
escaped from the fetters of etymology, and that the houses were
in no way connected with the copper industry.
Bishop Watson in his famous Chemical Essays, published in
1781, remarks in reference to pyrites that :—
" This mineral is called in some parts of England Copperas stone ; in others
brazil; in others brass-lumps; in others rust-balls ; in others horse-gold; in
others marcasite; though naturalists are now, I think, agreed to give that name
to such mineral bodies as are angular and crystallized, especially with a cubic
form. The scientific name is Pyrites—fiery."
Much of the pyrites occurring as nodules in clays, in shales
and in the Chalk belongs to the kind which is now known as
marcasite. In fact iron disulphide is a dimorphous substance, and
forms two distinct mineral species, differing widely from each
other in crystallographic characters. Common iron-pyrites
crystallizes in the cubic system, generally in cubes,
or hexahedra, less commonly in regular octahedra, and
very characteristically in the form known as the
pentagonal dodecahedron. That special dodecahedron, which is
bounded by twelve regular pentagons is a well-known geometri-
cal form, being one of the five Platonic solids; but the pentagon
of the crystal is not a regular figure, only four of its sides being
equal, whilst the fifth is either longer or shorter than the
others. This solid, with its twelve irregular pentagons, is so
characteristic of pyrites that it was called by Haidinger the
Pyritohedron. It is a hemihedral form of the four-faced cube, or
tetrakis-hexahedron, from which it may be derived by develop-
ment of the alternate faces. Another hemihedral form sometimes
seen in pyrites is the dyakis-dodecahedron, a solid bounded by
24 equal and similar trapeziums. This form, known also as the
diplohedron, is one of the semi-forms derivable from the six-
faced octahedron. It is very notable that both the pentagonal
dodecahedron and the diplohedron are parallel-sided hemihedral
forms, that is to say, each face has an opposite corresponding face
in parallel position, and this is so characteristic of pyrites as to
be called pyritohedral hemihedrism.
The crystals of iron pyrites are in many cases bold, sharply-
defined solids, which readily attract attention, and were recog-
nized by certain old writers as "figured mundics."
But it sometimes happens that iron disulphide assumes
crystalline forms totally different from those of the cubic system