187 



in accordance with a cubic space-lattice; here in<l< u h cubic 



l.itti.v -corns to be present, but instead of this rather a very 

 plicated arrangement of lower symmetrical individuals. 

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



II (no) 



(100) 



II (no) 



Fig. 146. 



Boracite. 



Plates parallel to (100) and (110). 



towards R on 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. 



In the same way in fig. 14.8 /jj, 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 (110); of analcite, pa- 

 rallel to (100); of fluorspar, parallel to 

 (110); 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 



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



2) N. Von Kokscharow, Materalien zur 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). 



Fig. 147- 

 Boracite. 



