186 



Fig. 144. 

 Leucite. 



lamellae disappear abruptly at about 500 C., an re-appear on cooling. 



Of bomcite in fig. 145 a plate parallel to (111) in its condition 



after heating and subsequent cooling has been reproduced (Mallard), 



which shows the lamel- 

 lar structure quite 

 clearly. At 265 C. 

 the birefringence dis- 

 appears suddenly, and 

 re-appears without re- 

 tardation if the crys- 

 tal be cooled down 

 below that tempera- 

 ture. The optical be- 

 haviour of plates cut 



parallel to (110) and (100) is schematically shown in fig. 146, while 

 in fig. 147 "A. pseudo-rhombicdodecahedron of boracite is reproduced, 

 and the arrangement of the component rhombic individuals is 

 indicated by the direction of their axial plance. Every face of the 

 rhombicdodecahedron is the base of a 

 rhombic pyramid, with its top lying 

 in the centre of the crystal; the biaxial 

 individuals have their optical axial 

 plane parallel to the longer diagonal 

 of each rhomboid. By means of Ront- 

 g en-rays, patterns for plates parallel 

 to (100), (1 10), and (1 1 1) were obtained 

 by us *) at room-temperature, which were 

 in accordance with the symmetry of a 

 cubic space-lattice, but also others 

 which, when parallel to (100), only 

 manifested a binary axis with two perpendicular planes of sym- 

 metry; the last plate however, when heated to 300 C., and then 

 passed by a pencil of Rontgen-rays, gave a pattern, the symmetry 

 of which was that of a true cubic crystal-plate. 



Although definite conclusions cannot yet be drawn from these 

 results, the last mentioned experiment nevertheless seems to support 

 the explanation given by Mallard. In the case of leucite we were 

 not able to obtain R on t gen-patterns at all, whose symmetry was 



i) H. Haga and F. M. Jaeger, Proceed. Kon. Akad. van Wet. Amsterdam, 

 vol. 16, (1914). 



Fig. 145- 



Boracite. 



Plate parallel to (111). 



