182 Mr Haidinger on the changes which take place 



with curved and irregularly formed faces, occur in regular 

 compositions, similar to those of fluor, two of them joining in 

 a twin, which may be supposed in transverse position to each 

 other in reference ta one of the rhombohedral axes of the 

 hexahedron. Each of these groups contains in its interior a 

 six-sided prism, whose smooth surfaces may be relieved from 

 the surrounding homogeneous mass merely by breaking off 

 the latter. The position of this prism is such, that its planes, 

 within the angles different from 120**, agree in position with 

 the prism R + oo, which is the limit of the series of rhombo- 

 hedrons, the hexahedron showing here the properties of this 

 form in regard to the principal axis of the enveloping twin- 

 crystals of variegated copper. There is a face of the hexa- 

 hedron contiguous to every liateral face of the six-sided prism ; 

 nay, it is possible that the existence of the twins depends upon 

 that of the prisms, which might exercise a considerable in- 

 fluence in the deposition of the particles of the species of va- 

 riegated copper. The substance of the prisms themselves is 

 likewise variegated copper. They are divided into thin la- 

 minae parallel to the base of the prisms, having externally a 

 black colour, and scarcely any lustre, but presenting the usual 

 appearance of variegated copper when broken across. 



The original form is generally lost when the decomposition 

 proceeds farther. In this case, what is usually called black 

 copper will remain, a more or less pure peroxide of copper, 

 which forms pulverulent masses. A specimen in the collec- 

 tion in Gratz, from the Bannat, with crystals of the form of 

 copper-glance changed into this substance, is the only one I 

 remember ever to have met with, in which the change has 

 proceeded so far without at the same time altering the form. It 

 is probable that it has taken place immediately, and not pro- 

 ceeded through the stages of variegated copper and copper- 

 pyrites, though both of them, when decomposed, will likewise 

 yield a black powdery residue. 



The prismatic copper-glance is a pure sulphuret of copper, 

 whose composition is expressed in Berzelius**s chemical formula 

 Cu S, the two ingredients, copper and sulphur, being in the 

 ratio of 79-73 and 20.27. Most analyses give a slight quan- 

 tity of iron. 



