86 Mr HAIDINGER on the Parasitic Formation 



neous 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-j-oo , which is the 

 limit of the series of rhombohedrons, the hexahedron shewing 

 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 hexahedron contiguous to every lateral face of the 

 six-sided prisms ; nay, it is possible that the existence of the 

 twins depends upon that of the prisms, which might exercise a 

 considerable influence in the deposition of the particles of the 

 species of variegated copper. The substance of the prisms 

 themselves is likewise variegated copper ; they are divided into 

 thin laminae parallel to the base of the prisms, having external- 

 ly a black colour, and scarce 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, 

 in pulverulent masses. A specimen in the collection 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 tune altering the form. It is probable 

 that it has taken place immediately, and not proceeded 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 formu- 

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

 ratio of 79-73 and 20-27. Most analyses give a slight quantity 

 of iron. 



According to the analysis by Mr RICHARD PHILLIPS, of a 



