4:U 



and fnrtlierinore, as we already mentioned, tlie accidental situation 

 of the pliospliorescent screen will play in this question also a more 

 or less important role. 



§ 4. The obtained results were 'so surprising, that we thought it 

 necessary to repeat the experiments of radiation through the plates 

 parallel to |100| and jOlOj, also with cordierites of other localities. 



h. From a beaiiiifiil, pink cordierite of Mount Ihiiy, Madagascar, 

 which had no geometrically definite boundaries, two planeparallel 

 plates were cut after |10ü| and |010} and about 1 m.m. thick. The 

 plate parallel to JOJOj was distinctly dichi'oitic : for vibrations in 

 the directions of the axial plane it was lilac-white, for those perpen- 

 dicular to it intensively pink. On |1Ü0| the colour for vibrations 

 perpendicular to the c-axis was pink ; for those parallel to it, the 

 plate was almost ^vhite. 



In the same way two such plates were cut from a single, short 

 prismatic, chalcopyrite-covered covdierite-crystai oï Bod en ma is ; it was 

 fixed upon an aggregate of chalcopyrite and sphalerite. The mentioned 

 plates were from 1,0 to J,l m.m. thick, and showed no distinct di- 

 chroism: the plate parallel to {lOOj showed hardly any difference of 

 colour for two perpendicular directions; that parallel to jOlOj was for 

 vibrations parallel to the c-axis yellowish-white, for those perpen- 

 dicular to it however pirdv coloured. 



In a quite analogous way as described before, RöNTGENOgrams of 

 these four cryslal-j>l;itcs weie obtained. The fig. 6 and 7 give the photo- 

 graphs for the crystal from Bodeninais, the figures 8 and 9 those 

 for the crystal of Mount Ihity ^). 



F'rom these RöNTGEN-patterns it can in the first instance immedia- 

 tely be seen, tJiat also inith these crystalplates all radiograms are 

 only symmetrical with respect to one single vertical plane, and 

 that in these minerals also binary axes perpendicular to \100\ or \0\0\ 

 appear to he absent. In connection with the results obtained with 

 the other cordierite-plates, it is hardly possible to give any other 

 explanation of this, than that the absence of both horizontal binary 



1) The cordierites of Ibity are somewhat richer in SiO-j, Al^Os, and MgO, than 

 those of Bodenmais, but their content of iron-oxides is less ; the following analysis 

 may give some idea of this : 

 Ibity : 49.05 o/o SiO. ; 83 08 o/o Al^Og ; 11 .04 " 'o MgO ; 5.2 o/,, FeO + FcoOg ; 



1.64% HgO. 

 Bodenmais liS.bS^/o SiOa ; 31.47 'Vu AI.O3 ; 10.68% MgO; 4.90% FeO; 

 1,85% FeA; 0.09% GaO ; 1.96%, H.O. 

 Vide also : Wulfing and Oppenheimep., Sitz. B Heidelb. Akad. d. Wiss. Abt. A. 

 NO. 10. (1914). 



