258 



in the liquid state in general, and the hemihedrism of the crys- 

 talline forms are phenomena of a different order, which have no 

 direct causal relation, although they often appear combined and 

 parallel to each other. 



On the other hand, Traube 1 ) brought forward that the excep- 

 tions mentioned by Wai den are undoubtedly partially explained 

 by the fact that the corresponding crystallographical data are 

 very incomplete and rudimentary; the cause of this being, that 

 the most suitable circumstances were not found for the develop- 

 ment of hemihedral faces in the crystals, or that the investigation 

 was only applied to the external form, without the use of other 

 physical methodes of research, as: pyro -electrical observations, 

 corrosion-figures, etc. He emphasizes the fact that only then a con- 

 vincing proof of the incorrectness of Pas teurs's conclusion could 

 be given, if the apparently holohedral crystal-form 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 crystal-forms were mentioned, no 

 corrosion-figures could be obtained suitable for the purpose. 



27. Experience has since shown that really hemihedrism 

 could be found in a large number of cases, even in many of 

 those formerly looked upon as exceptions. It is a well-known fact 

 that hemihedral crystalline substances under special circumstances 

 will show the development of hemihedral faces, which will remain 

 latent under other circumstances of crystallisation. Calcium-bimalate- 

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

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

 constantly. According to Pasteur 2 ) also the ammonium-salt, if 

 crystallised from pure water, does not exhibit hemihedral faces; 

 however these re-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 Wai den have thus lost 

 their importance as arguments in this principal question however 



1) H. Traube, Ber. d. d. Chem. Ges. 29. 2446. (1896); 30. 288. (1897). 



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



