SULPHURET OF LEAD, COPPER, AND ANTIMONY. £33 



thus rendered irregular and granular, and its lustre is much 

 inferior to that of the fracture of the crystals. 



This compact variety in Cornwall is frequently mingled 

 with sulphuvet of zinc, which may easily had to mistakes; 

 and which Mr. Hatchett has very judiciously noticed in the 

 analysis he made of this substance. 



The compact endellion of Brasil and Peru also is inti- 

 mately mixed with yellow sulphuret of copper and iron. 



Description of the crystalline forms of endellion, and obser- 

 vations respecting them. 



The primitive crystal of this substance, as I have already Primitive crys. 

 eaid, is a rectangular tetraedral prism, pi. vii, fig. 1, the 

 height or side of which is to the sides of its terminal faces 

 in the ratio of three to five*. I have not yet seen this crys- 

 tal in its perfect state, that is to say, without the planes of 

 any of the modifications belonging to it; and it cannot be 

 obtained by splitting, the attraction of cohesion, that unites 

 the integrant molecules of this substance, being too strong 

 to be overcome. By means of some of the accidental frac- 

 tures however, that occasionally exist, I have been able to 

 discover the direction of its laminse, and perceive that this 

 direction, as well as that of the secondary faces, agree per- 

 fectly with the results of calculation. 



I do not think however, that this prism is at the same Integrant pa> 

 time the form of the integrant molecule of this substance: tlcle * 

 but hitherto nothing has led me to form any particular 

 opinion with respect to the form of this molecule. 



As the crystals of endellion are frequently loaded with Various alfer- 



facets, which the mineralogist may find embarrassing, I atl . on . s . of lhe 

 , 1 , • n • 1 • » , , • primitive cry s- 



have thought it necessary, tor the ease or the reader, to give ml. 



separately, in fig. 4, the various retrogradationsf experienced 



by the laminae of the crystallization, which I have observed 



. * The method T have-pursred for the determination of the primitive 

 crystal will be seen hereafter. 



+ 1 give the name of retrogradation [recvlem. nt"] to that act of crystal- 

 lization, which has hitherto been known by the name of decrement, an 

 expression that is totally false in many cases, as I have shown in the se- 

 cond volume of my Treatise on Mineralogy, p. 206, in the part relating 

 (0 the theory of crystallization. 



on 



