of the red oxyd of Copper and arseniated Iron. 35 



THIRD MODIFICATION. 



This modification is the result of a decrease on each of the soHd 

 angles of the octohedron, and on its edges, which replaces each of 

 the solid angles by four planes inclined on the axis, and placed on 

 the edges of the octohedron. 



Fig. 61, which represents this modification uncombined with any- 

 other, is here delineated in order to shew it in that state, but I have 

 not so seen it. It is however probable that it may hereafter be 

 discovered. 



This modification is extremely rare. The crystals on which I 

 have hitherto observed it scarcely exceed half a line in length. It 

 is perhaps, therefore, impossible to determine with accuracy the 

 admeasurement of the angles. 



FOURTH MODIFICATION. 



This modification is the result of a decrease along the edges of the 

 octohedron, as well as the second modification, but with this differ- 

 ence, that, in this, each edge is replaced by two planes inclined on 

 the axis passing through the middle of the edge (fig. 64.) 



The planes 4, 4, on fig. 67, may be considered as resulting from 

 an after-deposition of those planes on a crystal formed as fig. 39. I 

 possess a specimen on which there are many of these singular 

 crystals. This species of deposition may frequently be observed. I 

 have repeatedly noticed it on the plane 2 of fig. 39, not exceeding 

 one half the length or breadth of the plane ; and again, in distinct 

 laminse so disposed as to produce a triangular pyramid on each face 

 of the primitive crystal, which respectively formed the base, as in 

 the instance of fig. 70, but much more acute. 



E 2 



