271 



because there are a much greater number of valid cases, and 

 because the absolute proof of the presence or absence of hemi- 

 hedrism in many crystallographical researches was, or could not, 

 be given as a consequence of the rudimentary development of 

 the crystals, etc. Of course, the first argument is of no significance, 

 where just the special question is put, whether the connection of 

 both phenomena be a necessary one? 



With respect to the absence of rotatory power in many hexa- 

 gonal and tetragonal crystals of these optically active substances, 

 he also agrees with the explanation of this given by Mallard 1 ) 

 and Wyrouboff 2 ), who, from their observations of crystals 

 exhibiting rotatory power, had already concluded that all these 

 crystals were really pseudosymmetrical intercalations, and warned 

 in 1894 against the dogmatic view, that an obligatory connection 

 between optical activity and hemihedrism should really exist. In 1896 

 and 1897 Walden 3 ) again drew attention to the fact that the 

 number of cases in which a rotatory effect in solution was not 

 combined with the occurence of hemihedral crystalforms, increased 

 more and more, their number gradually approaching that of the 

 cases in which such hemihedrism was concisely determined. 



He was, therefore, convinced that optical activity in solution or 

 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 

 going parallel to each other. 



On the other hand, Traube 4 ) brought forward, that the excep- 

 tions mentioned by Walden 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 methods of research, as: pyro-electrical observations, 

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

 vincing proof of the incorrectness of Pasteur's conclusion could 



!) E. Mallard, TraiU de Cristallographique, II. p. 330, (1884). 



2 ) G. Wyrouboff, Ann. de Chim. et Phys., (6), 8, 416, (1886); (7), 1, 10, (1894); 

 A. Descloizeaux, Ann. des Mines, (5). 14, 18. 



3) P. Walden, Ber. d. d. Chem. Ges., 29, 1692, (1896); 30, 98, (1897). 

 *) H. Traube, Ber. d. d. Chem. Ges., 29, 2446, (1896); 80, 288, (1897). 



