259 



tin- problem can by no means be considered t<> be as yet finally 

 settled. 



In a crystallographical investigation of an optically a 

 return-, lupeon: C^H^O (mpt: 170 C), prepared in Van Kom- 

 b urgh's laboratory by Cohen, the present author ') found a 

 substance which crystallises readily from acetone in very beau- 

 tiful, apparently holohedral rhombic crystals (fig. 167). The cor- 

 rosion-figures obtained on the facets of the prism, showed un- 

 deniably the existence of a horizontal plane of symmetry, and 

 they were, moreover, situated on adjacent prism- 

 faces in such a way that the existence of two 

 vertical planes of symmetry in the crystals could 

 not be doubted. The corrosion-figures weje there- 

 fore in full agreement with the apparently holo- 

 hedral symmetry of the crystals, as inferred from 

 their geometrical form. Groth points 2 ) to the 

 desirability of confirming this result by etching 

 two adjacent pyramid-facets. In any case, if hemi- 

 hedrism be supposed here too, it appears to be 

 so feebly expressed that no convincing proof of 

 its existence could hitherto be given; the optical 

 rotation in chloroform-solution however amounts 

 here to: [x] D = -f 63, 1. This rather great rotation 

 surely bears no proportion to the imperceptible degree of hemi- 

 hedral development. 



In an analogous way it was found 3 ) that cholestenon : CM H 44 0, 

 (mpt: 78 C.), prepared by Win da us, cristallises from ethyl- 

 acetate in every case in the prismatic class of the monoclinic 

 system. Here also there can be no doubt about the holohedrism 

 of the crystal-form. It is possible that in molecules of such a 

 magnitude the influence of the asymmetric carbon-atom is almost 

 effaced, and that therefore its presence is by no means revealrd 

 in the crystalline form of the compound. 



28. With respect to the manifestation of hemihedrism, the 

 question may be asked whether the two factors differentiated in 



Fig. 167. 



Crystal-form and 



corrosion-figures 



of Lupeon. 



1) F. M. Jaeger, Zeits. f. Kryst. u. Miner. 44. 568. (1908); Cf. N. H. Cohen, 

 Rec. de. Trav. d. Chim. d. Pays-Bas. 28. 368. (1909). 



2) P. Groth, Chemische Krystallographie /// p. 526. (1910). 



) F. M. Jaeger, Groth's Chem. Krystall. ///. 536. (1910); A. Windaus, 

 Ber. d. d. Chem. Ges. 89. 2008. (1906). 



