798 



above mentioned differences can easily be stated. Anotiier point of 

 divergence of the tigs. 3 and 4, which however does not relate 

 immediately to the difference in period of the symmetry -axis. — is 

 the fact, that in the fig. 4 (also in fig. 2), there are always two 

 spots in the third row from the centre, which spots are missing 

 wholly in fig. 3. We must conclnde from this, that, notwithstanding 

 the misleading analogy of figs. 2 and 4, with respect to fig. 3, the 

 former two do not possess more than a öinnri/ axis of symmetry ; 

 with respect to the fact, that most observers contribute rhombic 

 symmetry to the composing lamellae, it miisi be considered as highly 

 probable, that this binary axis of symmetry corresponds really with 

 a molecular an-angement of rhombic symmetry. In any case with 

 a space-lattice, which is relatively close to a regular arrangement : for, 

 as already mentioned, it was impossible till now, to state goniome- 

 trically any deviation of the \mve regular crystal-form. However, 

 the Röntgenogram shows this deviation with certainty, may it be 

 only by small differences, while a misleading similarity or analogy 

 with true regular symmetry reninins present. This fact proves, that 

 in problems of this kind, in many cases the method of the Röntgen- 

 patterns will be of higher value, than the different methods used 

 up to this date. 



§ 5. The described experiments have thus demonstrated the fact, 

 that by healing to 266° C, simultaneously with its optical isotropy, 

 the boracite shows a slight molecular re-arrangement. The question, 

 if the dimorphism of the boracite is connected with a change in its 

 molecular an-angement, must, after what is found here, doubtlessly be 

 answered in the atïirmative. ') 



It makes only little difference or none in this (piestion, whether the 

 obtained images correspond perhaps only to the exclusive action of 

 one single kind of the atoms, constituting the chemical molecule of 

 boracite. For every kind of atoms of the molecule must be the 

 structure-unit of an individual space-lattice, and all Üio^e intergrown 

 space-lattices must be either congruent, or equal with respect to 



1) Remarkable is also the granular charactei- of the central spot on these 

 photographs: experiment taught us, that this fad is connected with the presence 

 of the two aluminium-screens M and N in the way of the RöNTUEN-rays. Especially 

 remarkable is the more or less regular hexagonal of hexaradiant shape of these 

 spots. Such an hexagonal image was also obtained by means of a thin alumi- 

 niumplate alone, about 1,55 mm. thick. It is not improbable, that this fact is 

 connected in some respect with the octahedral crystalform of the aluminium: 

 perhaps the hexagonal image corresponds to six octahedron- faces as direchons of 

 preference for the RöNTGEN-rays. 



