192 



in accordance with a cubic space-lattice; here, indeed, no such cubic 

 lattice seems to be present, but instead of this a very complicated 

 arrangement of lower symmetrical individuals. 



As far as we can at present judge, the behaviour of leucite-crystals 

 towards Ron t gen-radiation can surely not be explained by the 

 mere assumption of internal stresses which are related to the limiting 

 facets and edges of the crystals, as was occasionally done. 



In the same way in fig. 150 755 several other crystal-sections 

 are drawn, as they appear between crossed nicols. Here the 

 optical properties of plates cut from crystals of garnet, parallel to 

 1 10} ; of anakite, parallel to [WO] ; of fluorspar, parallel to 1 10} ; 

 of perowskite, parallel to 100} and 111}; of apophyllite, parallel 

 to 001 } and at 45 to the planes of the nicols; and of rutile, parallel 

 to 001} are expressed, as observed by Mallard and others. 



In particular the figure relating to perowskite: CaTi0 3 , parallel 

 to 100}, is very instructive. There is not the least doubt in this 

 case that we have here to deal with a pseudo-cubic crystal being 

 in reality a very complex twin of differently oriented lamellae. 

 According to Baumhauer 1 ) and Von Kokscharow 2 ), the true 

 symmetry should be orthorhombic, with a twinning chiefly occurring 

 with respect to a face of the prism 110} and of the pyramid 111}. 



In several of these cases, Brauns and others have tried to demon- 

 strate, that the optical anomalies are caused by internal stresses, 

 as a consequence of isomorphous admixture, etc.; cf. : Brauns, loco 

 cit., p. 358, (1891). The truth may lie in the middle also in this matter, 

 - the two views separately being perhaps both too exclusive and 

 one-sided. It is quite possible and even probable, that in many cases 

 internal stresses are in fact the direct cause of the optical anomalies ; 

 but our experience in the study of the anomalous crystals by means 

 of Rontgen-rays seems to indicate, that there is in many cases a 

 greater probability of the correctness of Mallard's view. 



In the course of our studies on the symmetry of the Rontgen- 



!) H. Baumhauer, Zeits. f. Kryst., 4, 187, (1880). 



2 ) N. Von Kokscharow, Matevalien zuv Mineralogie Russlands, Bnd. 6, 

 p. 388, (1871); 7, p. 375, (1875); 8, p. 39, (1878); Neues Jahrb. f. Miner., (1878), 

 p. 38; A. Ben Saude, Ueber den Perowskit, Gottingen, (1882); C. Klein, Neues 

 Jahrb. f. Miner., (1884) /, p. 245; A. Des Cloizeaux, Neues Jahrb. f. Miner., 

 (1875), p. 279; (1877), p. 160, 499; (1878), p. 43; Ann. des Mines, (5), 14, 417, 

 (1858); Pogg. Ann., 126, 420, (1865). 



