436 



bj tlie different influences of tlie factors accompanying tlie formation 

 of the crystals. But from our experiments it follows moreover, that 

 the wternal arrangement also, the molecular structure itself, must he 

 considered as being variable uuth those external factors ; thus to the 

 different localities, where minei-als are found not only the especial 

 differences in habitus of (he crystals must correspond, but also some 

 variations of its internal structure. With respect to the great signi- 

 ficance of this conclusion for the (|uestion about the constancv of 

 mineral-species in general and about the relations between the 

 external forces during the crystallisationprocess and the internal 

 crystalline structure, — we must remark, that the correctness of 

 our view will be established oJily satisfactorily by a great 

 number of such experiments, to be made with minerals of very 

 different origin and accurately known chemical composition. For 

 especially of many silicates, and also of cordiei-i(e, it is known, that 

 they can be altered under the influence of chemical reagents') ; and 

 it is very well explicable, if such differences in internal structure, 

 as we have stated here, were dependent upon such differences in 

 chemical composition, instead of being attributed to the variation of 

 physical factors, which may have had a \ariation of the external 

 forms as a consequence, however in the case of cordiriete, these 

 variations in chemical coinj)Osition are only small. Only numerous 

 experimenis in the direction indicated above, will enable us to decide 

 in the alternative. 



§ 6. We have tried to prove the hemimorphy of the cordierite, 

 just as it follows doubtless from the described experiments, by 

 verifying it again by means of the now usual physical methods. 



In the first instance we tried') to reach our purpose by the aid 

 of the wellknown method of corrosion-figures. The plates of cordierite, 

 having been carefully cleaned by benz.ene, afterwards by alcohol 

 and ether, were submitted during a short moment to the action of 

 a very dilute solution of hydrofluoric acid ; later we made again 

 such experiments by means of gaseous hydrofluoric acid and with 

 dilute potassiumhydrate-solutions. In the last mentioned case, we 

 were unable to get any well-shaped corrosion-figures ; in the expe- 

 riments with hydrofluoric acid however, we always got, even after 



^) Vide in this respect the paper of VVulfing and Oppenheimer, just published 

 in: Sitz. B. Heidelb. Akad. d. Wiss., Abt. A. N". 10. (1914), p. 5 and 6; 

 L. Oppenheimer, Inaug. Diss. Heidelberg, 1914. 



-) In these experiments Dr. A. Simek has willingly given us his esteemed assist, 

 ance. 



