220 MR HAIDINGER on the Forms of Crystallisation 



120 and 90. The inclination of a upon b I found to be 

 = 90 29' ; that of c upon c = 120 20' ; and that of b upon c 

 = 119 50'. Though slight, these differences are easily ascer- 

 tained, and their consequences, in the disposition of the crystal- 

 line faces, are so obvious, that they would certainly not have 

 escaped the practised eye of the crystallographers who described 

 them, had not a particular mode of regular composition seemed 

 to establish a kind of symmetry round a rhombohedral axis sup- 

 posed to be perpendicular to the faces of cleavage. 



The system of crystallisation to which the forms of Axoto- 

 mous Lead-baryte belong, is not, therefore, the rhombohedral sys- 

 tem, nor do these forms enter into that class of prismatic forms 

 which exhibit the full number of faces of every simple form in 

 the combinations, but they must be considered as hemiprismatic, 

 the axis of crystallisation which is parallel to* the edges of the 

 prism c, being inclined to the base at an angle of 90 29'. 



There are two observations which can be very easily insti- 

 tuted on almost every group of well-pronounced crystals of the 

 species, and which evidently prove that the forms of these are 

 really hemiprismatic. The first of them refers to oblique trunca- 

 tions of the lateral edges, between b and c, as d,d, in Fig. 2., which 

 are inclined to b at an angle of 156 27', and to c at an angle of 

 143 22'. The other refers to the slightly, but very distinctly 

 marked planes of junction, between two individuals in the regu- 

 lar compositions, in a direction joining alternating angles, like A 

 and B, Fig. 1., in the supposed regular six-sided prism. The re- 

 mainder, ABA', of the terminal face of one of the individuals, is in- 

 clined to the similarly situated face of the other, at an angle of 

 179 10', to which, on the opposite side, corresponds a re-enter- 

 ing angle of 180 50'. Both these facts I had observed separate- 

 ly in numerous specimens, but the smallness of well-pronounced 

 crystals, and the impossibility of distinguishing by the eye an 

 angle of 119 50', from one of 120 20', rendered it very diffi- 

 cult to combine these different observations into one representa- 



