126 Mr HAIDINGER'S Mimralogical Account of 



of 100 and 80, and both its diagonals." Though, in many va- 

 rieties, the cleavage parallel to the long diagonal of that prism 

 may in fact be obtained, it is always less distinct than that pa- 

 rallel to the short diagonal, and often not at all observable. It 

 is important to attend to this difference in the perfection of 

 cleavage ; the more so, because the cleavage parallel to the short 

 diagonal of P + oo 99 40', is at the same time parallel to the 



y 



long diagonal of another prism (Pr + oo) s 76 36' (the sup- 

 plement of which is 103 24'), which occurs very frequently in 

 the same mineral, and might be, or has actually been, mistaken 

 for it, in a more superficial examination of the crystalline forms 

 of the species. 



The most remarkable peculiarity in the series of crystalliza- 

 tion of this species, is its hemi-prismatic character, the faces of 

 those forms which assume it being inclined to each other. I 

 have much pleasure in adding here, that the observation of this 

 peculiar character, which I gave an account of from a rather li- 

 mited number of crystals, has since been repeated, and perfect- 

 ly confirmed, by Dr CHARLES HARTMANN of Blankenburg. The 

 faces marked c, if sufficiently enlarged, would give rise to a form 

 resembling a tetrahedron, like Fig. 11, the planes of which are 

 equal and similar scalene triangles. Among the remaining spe- 

 cies whose forms belong to the prismatic system, only the sul- 

 phates of zinc, of magnesia, and of nickel, are known to possess 

 an analogous formation. This was first placed beyond a doubt 

 by Professor MITSCHERLICH, who observed the fact, that the 

 faces * and t, Fig. 12, appear only contiguous to the alternating 

 faces of / ; although the alternating enlargement of these same 

 faces, represented in Fig. 13, had been previously noticed in the 

 sulphate of magnesia by mineralogists, so far back as the time of 

 ROME' DE L'IsLE and LINN^US. Large crystals of this salt ge- 

 nerally shew the hemi-prismatic character much more distinctly 

 than small one^s. 



