272 



be given, if the apparently holohedral crystalform of an optically 

 active compound really showed corrosion-figures agreeing with 

 holohedral, but not with hemihedral symmetry; this demonstration 

 was, however, not given at that time, because in all cases in 

 which apparently holohedral crystalforms were mentioned, no 

 corrosion-figures could be obtained suitable for the purpose. 



27. Experience has since shown, that hemihedrism could 

 really be found in a large number of cases, even in many of those 

 formerly looked upon as exceptions. It is a wellknown fact, that 

 hemihedral crystalline substances under special circumstances will 

 show the development of such hemihedral forms, which will remain 

 latent under other circumstances of crystallisation. Calcium-bimalate- 

 crystals do not ordinarily show hemihedral faces; but if some free 

 nitric acid be added to the saturated solution, they then constantly 

 appear. According to Pasteur 1 ) also the ammonium-salt, if crys- 

 tallised from pure water, does not exhibit hemihedral forms ; however, 

 these appear, if the substance be previously melted and then 

 recrystallised from the same solvent. Indeed, very minute amounts 

 of impurities or other substances purposely added to the solutions, 

 can influence to a rather high degree this tendency towards the 

 manifestation of hemihedral faces in such crystals. 



Although many examples given by Waldenhave 

 thus lost their importance as arguments in this 

 principal question, however, the problem can by 

 no means be considered to be as yet finally settled. 

 In a crystallographical investigation of an 

 optically active ketone, lupeon: C 3l H i8 (mpt: 

 170 C.), prepared in Van Romburgh's laboratory 

 by N. Cohen, the present author 2 ) found a sub- 

 stance which crystallises readily from acetone in 

 very beautiful, apparently holohedral rhombic 

 Crystafforms and crystals (fig. 16?). The corrosion-figures obtained 

 corrosion-figures on the faces of the prism, showed undeniably the 

 of Lupeon. existence of a horizontal plane of symmetry, and 

 they were, moreover, situated on adjacent prismfaces in such a 

 way, that the existence of two vertical planes of symmetry in the 

 crystals could not be doubted. The corrosion-figures were, therefore, 



*) L. Pasteur, Jahresber. f. Chemie, (1852), p. 176; (1853), p. 410. 

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

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



