TAB. XCIX. 



FERRUM sulphureum. 

 Sulphur et of Iron; Iron Pyrites, 



Class 3. Metals. Order 1. Homogeneous. 

 Gen. 6. Iron. Spec. 6. Sulphuret. 



Div. 1. Crystallized. Far. Octaedral, &c. 

 Syn. Fer sulfure octaedre. Haily, v. 4. 69. 



Octaedral Pyrites is not so common as cubical Pyrites ; 

 we have it however along with various substances, as calca- 

 reous spar, limestone, coal, &c. The present figures are 

 designed to show this modification from the cube passing 

 into what Haiiy calls the cubo-octaedre, thence into the 

 perfect octaedron. At the commencement of this change 

 the corners of the cube are replaced by triangular faces — see 

 the left hand figure — which, as the modification goes on, 

 become planes of six sides each — see the middle figure — and 

 at last the primitive faces are lost. These six-sided planes 

 are reduced again to triangular ones, forming the octaedron. 

 I am indebted to the late Lady Elizabeth Noel of Bath 

 for the upper and right hand specimens. The first is the cast 

 of a shell of the Trochus genus ; and it should seem that the 

 crystals are on the cast in place of the shell, as the rock is 

 about the thickness of the shell from the cast, and is a mould 

 of the outside of the shell. On the left hand side remains 

 a bit of the rock, and on the right hand are exhibited the 

 thickness and calcareous remains of the shells, sufficient to 

 indicate the species to a concholog-ist, which appears to be 

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