569 



f. Altont the hiimhenjitc and its normal and abnormal images we 

 have said already something in the foregoing paper. In fig. 7 — 9 

 on Plate IV we reproduce here again the more exact stereographieal 

 projections of' the normal RöNTOEN-patterns of this mineral, with 

 indication of the corresponding indices of the reticular planes. 



§ 6. Of the I'lwinbic-pip'am'uhil class, to which thus belong the 

 liemimorpliic crystals of the system, — we investigated here the 

 slruvlte{-iniHjnesium-ammo7iiiwi-oi-thü-ph()Spk(iti'\ Q!i]\^)MgFO ^-\-QW^O)), 

 and the calamine: Zn, (OH)3SiOs. 



/. From a big, brownish yellow and only little translucid crystal 

 of stvuvite from Homburg, three plates were cut parallel to the three 

 pinacoïdes JlOOj, jOlOj and iOOlj, whose thickness was from J, ■20 

 to 1,20 mm. The time of exposition was two and a half hours. 



The three very beautiful RöNTGEN-pat terns are reproduced in the 

 fig. 9, 1Ü and 11 on Plate III, and as stereographieal projections in 

 lig. 10 — 12 on Plate IV. Also in these images two planes of sym- 

 metry perpendicular to each other are evidently manifested ; notwith- 

 standing the polarity of the c-axis is very strongly revealed in the 

 crystals themselves, the result is also in this case in full agreement 

 with the theoretical prediction. 



g. However in the case of calamine we obtained for crystal-plates 

 parallel to {lOOj and JOlOj, cut from a very beautiful crystal'), 

 always abnormal patterns, from which one parallel to jOIOj is re- 

 produced partially as a stereographieal projection in fig. 13 of 

 Plate IV^; the image parallel to jIOOj was quite analogous to that 

 parallel to iOlOj, but it was loo bad to allow in any way a repi-o- 

 duction of it. Both patterns contained moreover such a great number 

 of very small and feeble spots, that also in the projection of fig. 13 

 on Plate IV, only the most important spots could be reproduced. 



The RöNTGENOgram of the calamine parallel to |001| however 

 was very beautifully regular (fig. 14 on Plate IV) and (fig. 12 on 

 Plate 111) ; quite in concordance with the theory, it is symmetrical 

 with respect to two planes perpendicular to each other. Why 

 it is the images parallel to jlOOj and |010!, — (corresponding with 

 those sections, that in the crystal itself do not possess the horizontal 

 plane of symmetry), — where the plane of symmetry parallel 

 to the c-axis is suppressed, can hardly be understood at this moment. 



§ 7. Fiiuxlly we used t'vo\u ihe eixatah of the rhomlnc-lnsphenodical 



') Por this very beautiful crystal we are much indebted to our colleague, Prof. 

 MoLENGRAAFF at Delft, whom we render our best thanks here once more. 



