262 



binary axis of the second order in the case of the optically inactive 

 crystals. The hemihedrism of the optically active forms is notwith- 

 standing the enormous rotatory power, here only betrayed by 

 the occurrence of a trigonal bipyramid; not even a single "trape- 

 zohedral" facet wa's ever met with in these salts. 



Pope and Peachy 1 ) have studied the crystal-forms of the right- 

 and left-handed components of tetrahydroquinaldine-hydrochloride: 

 C 1/0 H IS N, HCl + H 2 0; although the molecular rotation for sodium- 

 light in alcoholic solution was almost 141, in aqueous solution 

 122, the crystals of both antipodes (rhombic) not only showed 

 no trace of hemihedrism, but the corrosion-figures on (001), ob- 

 tained by means of alcohol + water, were in full agreement with 

 holohedral. symmetry. If however in the molecule : 



CH CH = C CH 2 CH 2 



CH CH = C NH -- CH.(CH 3 ), 



whose molecular rotation for so^'wm-light was about 85 at 20 C., 

 the hydrogen-atom of the NH-growp were substituted by a benzoyl- 

 radical, the rotation gets the opposite algebraic sign and is 

 simultaneously increased enormously, its value being now 814; 

 and at the same time the hemihedrism of the crystalline forms 

 now appears clearly manifested. 



From these data it appears that the connection between mole- 

 cular dissymmetry and crystallographical hemihedrism, as supposed 

 by Pasteur, may be hidden in many cases to such an extent, 

 that it may be doubtful as to whether it be present in such cases 

 at all. Surely in most cases a parallelism between both phenomena 

 is present: where the dissymmetry of the molecule is betrayed 

 by the rotatory power of the molecules, there is exhibited in 

 most cases also a non-superposable hemihedrism of the crystalline 

 forms. But this need not always be the case. The greater the 

 chemical contrast between the substitutes round the asymmetric 

 atom present in the molecule is, the greater chance there will 

 evidently be, that crystallographical hemihedrism will appear to 

 accompany the rotatory power of the substance. For the magni- 

 tude of the rotatory power of the molecule, this chemical contrast 

 between the substitutes is only of secondary significance: here 

 it is the non-superposable stereometrical arrangement and the 



W. J. Pope and S. J. Peachy, Journ. Chem. Soc. London 75. 1066. (1899). 



