( 198 ) 



The investigation, however, soon showed that the symmetry is 

 not tetragonal, and I first thought I hiid before me a monoelinic 

 compound, — certainly with plenty of geometrical anomalies, — but still 

 reasonably conforming to all the requirements of the monoclinic 

 symmetry. This would also have agreed with everything that I 

 have formerly communicated as to the formrelation of tetragonal 

 and monoclinic crystals. 



I hesitated a long time before I could give up this last idea, 

 particularly because the measurement of a very large number of 

 ci'ystals taught me, tliat in this case just as with the two former 

 compounds, the peculiar softness of the substance causes geometrical 

 disturbances of the crystals during the crystallisation, and because 

 the axial elements of this compound when assuming the monoclinic 

 symmetry exhibited an analogy bordering on isomorphy ^vith those 

 of the two investigated isomei-s. 



The deviations of the angles from the values required for the 

 monoclinic symmetry appeared however, to be so systematical, that 

 there could be no longer a doubt as to the tricUnic symmetry of 

 these remarkable crystals, even though they were found to represent 

 a monocJinic limit-form. 



Afterwards, I have also been able to find exceedingly small devia- 

 tions in the optical orientation of the directions of extinction which 

 again corroborated my belief, that triclinic crystals of a pseudo- 

 monoclinic form are present here. This makes us feel convinced that 

 even if the morphotropical force of the .nngJe (NO,) group is gene- 

 rally feeble tivo such groups may cause a comparatively strong 

 deformation of the crystal-molecule ; and also that the spacial structure 

 of the benzene derivatives is not fully explained by the usual formu- 

 lation of these substances. For the ordinary schematical manner of 

 writing alone does not explain why a compound of the type 1 should 

 have another symmetry than one of the type II, — even though, owing 

 to the occurrence of the pseudo-monoclinic limit- form, the close 

 relation of -the two kinds of symmetry is in each case very plainly 

 perceptible : 



CHj CH3 



h/\h (N0o)/\(N02) 



Bi\ /Br BiN^^Br 



Br Br 



II 



