OttDER II. PRISMATIC ZINC-BARYTE. 109 



(Pr + oo) 3 . Pr + oo. P. Fig. 37. A I- 

 tenberg near Aix la Chapelle. 



Cleavage, Pr and (1?r -j- oc) 5 , both easily obtain- 

 ed, the latter highly perfect. Traces of P x. 

 Fracture uneven. Surface of Pr -f oo and 

 (Pr -j- oo) 3 streaked parallel to their common 

 intersections. The rest of the faces generally 

 smooth : sometimes rounded, particularly P QD. 



Lustre vitreous, inclining to pearly upon Pr + oc, 

 sometimes to adamantine upon curved faces of 

 crystallisation. Colour white, prevalent. Occa- 

 sionally blue, green, yellow or brown. Streak 

 white. Transparent ... translucent. 



Brittle. Hardness 5-0. Sp. Gr. = 3-379, 

 crystals from Rossegg in Carinthia. 



Compound Varieties. Globular, botryoidal 

 shapes : surface drusy, composition columnar. 

 Massive : composition either granular or columnar; 

 the former of them often impalpable, and strongly 

 coherent, and then the fracture becomes uneven ; 

 the latter straight and divergent. 



OBSERVATIONS. 



1. Among the forms quoted above, only Var. 4. Fig. 37. 

 has been observed on both ends, which have presented that 

 remarkable dissimilarity of configuration. In the others, 

 the figures to which they are referred, suppose the two 

 apices to be similar. The species of prismatic Zinc-ba- 

 ryte affords another example of the co-existence of a dif- 

 ferent configuration of the opposite summits, with dif- 

 ferent kinds of electricity. The latter has been long 

 ago observed; it is excited by the common changes of 



