SULPHURET OF LEAD, COPPER, AND ANTIMONY. «J5 1 



II. 



Memoir on the triple Sulphuret of Lead, Copper, and Anti- 

 mony, or Evdellion. By M. le Comte de Bourn on, 

 F. It. and L. S. 



(Continued from page 237. ) 

 Determination of the primitive crystal of endeUion. 



JT ROM the direction of the laminae of crystallization of Primitive "crys- 

 the endellion, with which some accidental fractures had tal ascertained, 

 made me perfectly acquainted, as well as from the general 

 aspect of the crystals of this substance, I could not doubt, 

 that the form of its primitive crystal was a rectangular te- 

 traedral prism; and the similitude of the retrogradation of 

 the crystalline lamina? along the edges of the terminal faces, 

 indicated by the equality of the inclinations of the faces 

 belonging to them, made me presume, that these faces must 

 be squares. Accordingly it appeared to me, that the pri- 

 mitive crystal could only be a cube, or a rectangular te- 

 traedral prism with square bases, the altitude of which 

 would be greater or less than the side of the terminal faces. 



Now remarking, that, with very few exceptions, in all Where a pri- 

 substances, that have a perfectly symmetrical solid for their ™ ime crystal 



. . . i i"i i • a is symmetrica', 



primitive crystal, as the cube, rhomboid, octaedron, regu- the secondary 



lar tetraedron, &c, the secondary forms, produced by the c| 'vstalsareso 



. . i-ii i i • t likewise. 



various retrogradations to which they may be subjected, re- 

 tain the same symmetry ; 1 observe, that, in the crystals be- 

 longing to the secondary faces of the endellion, there is no 

 symmetiy between the planes that supply the place of the 

 edges of the terminal faces, and those of the longitudinal 

 edges of the prism, either with respect to the number of 

 these planes, or to their inclination; whence I am naturally 

 led to infer, that the rectangular tetraedral prism, the pri- 

 mitive crystal of endellion, is not a cube. Of this I was 

 fully convinced, when I presented my first paper on this 

 substance to the Royal Society ; and it was this, that then 

 prevented me from giving the dimensions of its rectangular 



tetraedral 



