MR HAIDINGER on the Forms of Crystallisation 



The perfectly hemiprismatic appearance of a crystal similar 

 to the last of these figures, would be alone sufficient for ex- 

 cluding the forms of Axotomous Lead-baryte from the rhom- 

 bohedral system, even though the measures of the angles should 

 have been found to approach still nearer to 120 and 90. But 

 one and the same individual seldom presents more than one or 

 two of its six sides to the observer, being in most cases joined to 

 other individuals, according to the law of regular composition 

 mentioned in the beginning of this paper. 



The planes of composition pass through a line nearly per- 

 jtendicular to two sides of the six-sided tabular crystals, like AB, 

 Fig. 8. They are parallel to one face of (p-f- co ) 3 , rr 119 40', a 

 prism which is likewise found in the crystals of this species. 

 Upon this supposition, the angles are = 90 20', and 89 40'. Of 

 the individuals AA'BCDE, and AA"BC'D'E', which meet in 

 the plane of composition passing through A B, nothing will re- 

 main but the rhomb-like trapezium AA'BA", the angles of which 

 are A = 60 20' ; A' and A" each = 119 50', and B = 60. If 

 a third individual A'A"CGFC' joins the regular composition of 

 the preceding .two, being applied to BMA', the remainder of 

 BAA', Fig. 8. in the line A'M, there will also arise a face of 

 composition between A'MA" and BMA", and the angles of the 

 remaining triangular figure A'A"B will be exactly =. 60. In 

 the compound crystals of Axotomous Lead-baryte, each of the 

 edges A'M, A"M, and BM is = 179 10'. 



The regular composition of three individuals of the variety 

 Fig. 5., if they terminate at the faces of composition, wiU produce 

 a form like Fig. 10., but the compound takes the appearance of 

 Fie. 11. whenever the substance of the individuals reaches to the 

 opposite side of the compound crystalline group, and thus pro- 

 duces faces of crystallisation on the other side, similarly situated, 

 and parallel to those considered above. If, in the compound 



