1822.] Yelloio Copper Ore, ^ 



having been able to satisfy myself as to the manner in which 

 that octohedron lies fif I may so express myself) in the tetrahe- 

 dron, which is the prevailing form of the crystals ; and I should 

 have found it extremely difficult to satisfy myself on this head, 

 without the assistance of M.Levy, whose mathematical and crys- 

 tallographical acquirements are too well known to need a 

 comment by me. 



The tetrahedron in which pyritous copper most commonly 

 occurs, but which is never to be found, as far as my observation 

 extends, without what may be termed the replacement of its 

 solid angles, is so nearly allied to the regular tetrahedron, that' 

 it is not surprising it should have deceived the eye of the mine- 

 ralogist, even when assisted by the application of the commoh 

 goniometer to its planes, since the two tetrahedrons differ but 

 very little from each other in measurement. 



Fig. I. 



Fig. 2. 



Fig. 3. 



Fig. 1 represents the primary octohedron, which is more acute 

 than the regular octohedron, the measurement of P on P' or 

 P'; on V' being 101° 52', and that of P on V'' or P' on V''' 

 being 126° 30' : these measurements were taken on briUiant 

 planes of cleavage by the reflective goniometer. 



