of Copper and of Iron, 1 1 



the name of ivood tin, to which feme pieces of this arfeniate 

 of copper have a very screat refemblance. \ et it iometimes 

 happens, as in many aL'gregate pyrites of a globular form, 

 th;U the furface of' the fmall maniillae is covered with little 

 rnucrh points : thefe are the diedral apices, which terminate 

 thelittle cryftals fiippofed to contribute to their formation. 



This hematitic variety is found with the fame diverfity ot 

 colours as the preceding, or amianthiform variety. 



Fourth Specks.— Jrfeniaie of Copper in the Form of a triedral 

 Prifn. 

 The primitive form of this fpecies is a triedral prifm, the 

 l)afes of which are equilateral triangles, (fig. 9.): thi3 

 prifm is often confiderably lengthened in a direction pa- 

 rallel to one of its bafes. (Fig. ic.) This form is one of the 

 moll rare in crvftallographv.^ The cryftals have all their fides 

 fmooth and brilliant; yet' there are obfervable in fome ot 

 them, when examined' with a magnifying glafs, tranlverfe 

 ftria? on the fides of the prifm, all of which are parallel to 

 the edges of the bafes. It is therefore chiefly on the planes 

 of the bafes that the cryftalline laminae appear laid upon one 

 another, to produce either the increafe or the modification 

 of the primitive crvftal. 



As the ciyftals' of this fpecies are fcldom fufficienlly de- 

 tached to be eafily perceived, and indeed are very frequently 

 to fmall as to efcape the obfervation of the naked eye, I think 

 it neceirary to defcribc here all the various forms in which I 

 have feen them, with the progrefs obferved in their, palfage 

 irom one form to the other, however fmall the clifierence 

 between thefe forms niav appear. Such a defcription will 

 lead to a better knowledge, not only of the primitive cryUal 

 itfelf, but alto of thofe forms of it which fecm to be the moft 

 diitant from its original one. 



Very frequently the triedral prifm pafles to a tetraedral 

 modification bv the fimple replacing of one of its edges by a 

 plane equally inclined on the achacent ones. This plane i» 

 cither very narrow, (fig. 11.) or of a more coufiderable width. 

 (Fig. 12.) Sometimes the width of the plane is fuch, that it 

 reduces the primitive adjacent planes to extreme narrownels. 

 (Fig. 13.) In this lall cafe, the cryltal appears under the 

 form of a rectangular plate or lamina, having two of its nar- 

 row oppofite fides or planes inclined, in one and the fame 

 direction, on one of the two broad planes. It fometiioes 

 happens, in this variety, that the two broad oppofite planes 

 approach more or Ids to a fquare form. (Fig. 14.) 1 have 

 alfo feen fome cryflals in which the two other edges of the 



prifm 



