314 ON THE NATURAL HISTORY 



US remember that even in their time the word " copperas " had 

 escaped from the fetters of etymoloo^y, 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 p37riteG that : — 



*' This mineral is called in some parts of England Copperas stone ; in others- 

 brazil; in others hrass-liitnps ; in others tnst-halls ; in others hoi se- 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 P) rites — 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 

 mavcasite. 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 tw^elve 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 

 Pyvitoliedvon. 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 heniihedral 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 hemihedvism. 



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 



