sioii figures on jllOj could be obtained, liaviug tiie sliape of l<ites or 

 long hexagons; they appeared symmetrical with respect to a horizontal 

 and to a vertical plane. From this and the holohedrical development 

 of the crystals, it must be concluded that they can not be considered 

 to have tetragonal-trapezoliedrical symmetry, but that they must be 

 described as of ditetragonal-bipyramidal symmetry. 



On the rotation in solution and its dispersion, the data of tlie 

 previous paper can be consulted. 



The RöNTGENOgram obtained of a plate perpendicular to the c-axis 

 was too imperfect, to make a good reproduction possible. Thus on 

 Plate 7 in ü we have given its stereographical projection ; it appears 

 to possess all the symmetry-elements of a ditetragonal-bipyramidal 

 crystal, and inter alia the four vertical symmetry-planes and the 

 quaternary axis can be easily ilistinguished. In reality the photo for 

 the laevogyratory salt, notwithstanding its imperfection, appeared to 

 be identical with that of the dextrogyratory salt. In all cases studied 

 up till now, loe have found the RimTGV.NO(/ram.s- of the dextro- and 

 laevoi^yvatory crystals always identical, just as the theory of the 

 phenomenon postulates : so in the cases of quartz, cinnabar, etc. 

 However we found in these investigations some quite remarkable 

 facts, which are already partially described in tliese Proceedings 

 (March 1915), and which can lead to a perhaps justifiable doubt about the 

 correctness of the suppositions accepted hitherto about the explanation 

 of the symmetry-properties of the RöNTGENOgrams, notwilhslauding 

 the above-mentioned agreement of facts and theory in the case of 

 the optically active crystals. 



In any case it appeared not to be possible to prove in this way 

 the presence of enantiomorphous forms. 



All experiments made with the purpose of obtaining limiting 

 crystalfaces, which could demonstrate the hemiliedrical character of 

 the crystals, either by crystallisation from neutral or alkaline or 

 acid solutions, either by addition of other salts to the aqueous 

 solutions, — were without any otiier result, than that of always 

 giving holohedrical crystal-forms. In connection with the above- 

 mentioned experience, we have no reason to suppose the occurrence 

 of hemiliedrical crystals in this case. 



The optical rotation of the crystals must thus be ascribed wholly 

 to the optically active molecules themselves, which here build up 

 the iiolohedrical molecular configuration of the crystals. In the same 

 way, as e.g. sodiumchlorate is a salt, whose //«active molecules are 

 arranged in a hemiliedrical space-lattice, which causes the rotatory 

 power of the crystals, — in the same way we must suppose the 



