570 



class, l)esi(les the already' formerly discussed crystals of d-sodium- 

 aminonhiiii-tivtfiilc, moreover : those of l-aspnragine : {C,Hg(),N,+H,0), 

 and of zbic-siilpliiilc: ZnSO^ + 1H^0\ of these compounds both the 

 first named are optically active in soiu'ions, while the zwc-siilpluite 

 does not change the plane of |)olarisation of the ligh*, when passing 

 throuoji its solution. 



h. Ziiic-siiJplifite. Kroin a heauliful crystal three rather thick plates 

 were cut: that |)ai'allel to jlOOj had a thickness of 2Jlmra., that 

 parallel to jOlOi of 3.30 mm., and that parallel to jOOlj of 3.10 mm. 

 Even with a time of exposition of two and a half hours, the patterns 

 parallel to jiOOj and jOOJj were too feeble, to allow of any repro- 

 duction; but in fig. 15 and 16 their stereographical projections are 

 drawn. 



These images are agaiu symmetrical with respect to two perpen- 

 dicular planes However the pattern obtained with a plate parallel 

 to jOlOj appeared to be ulirdys ahnorinni; the respective photo is 

 already reproduced in the foregoing paper on Plate V, while here 

 in fig. 17 on Plate IV its stereographical projection is represented. 



This last fact is indeed of high importance for our problem : for 

 it may be supposed with good reason, that in cases, where such 

 crystal-plates are prepared by d'avagc. along planes of very perfect 

 cleavability, all chance to get a faiüty orientation of the plate is 

 altogether eliminated. Now in our case the very perfect orientation 

 of this plate obtained by such cleavage, could moreover be very 

 rigorously tested, because of the fact that the 6-axis, being the 

 direction through which radiation here takes place, is at the same 

 time the first bisectrix of the crystal. Indeed the interference-image 

 in convergent polarized light appealed after measuring with the 

 microscope, to be accurately centred, so that no deviation between 

 the /^axis and the normal on the plate could be found by any 

 means. And while now the orientation of the perfectly clear and 

 lustrous plate could hardly show any error exceeding a few minutes, 

 the image was in two repeated experiments, absolutely abnormal 

 in the way indicated here: evidently only the plane of symmetry 

 parallel to the plane of the optical axes has remained. 



Tiiis fact must convince us in a striking way, that the abnor- 

 malities occurring in the case of such crystals cannot have their 

 origin in a faulty orienttUion of the crystal-sections. Indeed, they 

 must be caused by internal disturbances of the molecular structure, 

 which evidently, as here with the zinc-sulphate, cannot even be 

 discerned b}' the usual optical means. At the same time it appears 

 furthermore by this fact, that the probability of such "internal vicinal 



