571 



planes" is by no means diniinislied by flie particular rirenmstanoe, 

 that tlie considered molecular layers are just those, which play the 

 role of directions of perfect cleavability in the crystals. (Thus being 

 perpendicular to the direction of minimal cohesion '). 



/. The RoNTGEN-pal terns of d-sodium-ammonium-tartrate, as well 

 the normal as the abnormal ones, and all particulars observed in 

 that case, have been discussed already in detail in our last paper. 

 We can here therefore refer to the resp. figures; only it may be 

 remembered here once more, that the patteins parallel to all three 

 pinacoidal faces, in the nornuxl case ap[)eared to be symmetrical 

 with respect to two perpendicular planes. 



k. From big, colourless and perfectly transparent crystals of 

 laevogyratory nsparagine, crystal-plates paialkil to jlOOj, {OlOj and 

 |Ü01| were cut. The plate parallel to jJOOj had a thickness of 

 1,21 mm., that parallel to jOlOj, of 1,06 mm., and that parallel to 

 {001 1, of 1.22 mm. 



The obtained RöNTGEN-patterns were all too feeble to make a 

 direct reproduction possible. But in tig. 18 and 19 on Plate IV 

 two of their stereographical projections are drawn. Also these images 

 evidently are symmetrical with respect to two perpendicular planes. 

 The third pattern was too disturbed to allow any valuable judgment 

 about this question. 



^ 8. From these researches, which will be still completed, it 

 becomes clear even now, that in ordinary cases also with optically 

 biaxial crystals, the theoretical predictions are in full concordance 

 with experience. 



The repeatedly observed suppression of one of the two expected 

 planes of symmetry in the RöNTGEN-patterns, must be considered 

 also in these cases as a peculiar "abnormality", which undoubtedly 

 is caused by internal disturbances of molecular dimensions, whose 

 true nature however at this moment cannot yet be more sharply 

 defined. 



University- Laboratories for Physics and for 

 Inorganic and Physical Chemistry. 



Groningen, August 1915. 



1) In this connection a remark made a short time ago by P. Ehrenfest (these 

 Proceed. 18. 180. (1915) is of interest, considering the possibility of cleavage 

 along planes, which are "vicinal" wilh respect to such directions of perfect 

 cleavability. 



37 



Proceedings Royal Acad. Amsterdam. Vol XVDI 



