183 



of particles arranged in rhomboids of nearly 60, tlwn the 

 binary axis of the rhombic individual perpendicular to the plane of 

 tlu> layrr i> at the same time an axis of apparently three/old symmetry. 

 Thus, if the space-lattice be turned round this axis of apj; 

 symmetry through 120 or 240, the space-lattice in its new posi- 

 tions will coincide, not completely, it is true, but in any case very 

 neatly, with the space-lattice in its original position. 



If therefore such layers of molecular dimensions are successively 

 subjected to all symmetrical operations which correspond to the 

 presence of one or more symmetry-elements of apparent symmetry 

 in the space-lattice under consideration, a complex pseudo-symmetri- 

 cal crystal may be formed, exhibiting rotatory power or not, accor- 

 ding to the way in which the superimposed layers are arranged. *) 



As the cross-laid lamellae become thinner, 

 and their compound twinning and mutual 

 penetration more perfect, the mimetic crystals 

 will approach more closely to a true higher 

 symmetrical individual ; and if the dimensions 

 of the transverse lamellae become submi- 

 croscopically small, the crystal is no longer 

 distinguishable from a homogeneous one by 

 any existing physical means. Mallard then 

 says that the substance under consideration 

 is dimorphic; the symmetry of both poly- 

 morphic forms being that of the composing lamellae on the one 

 hand, and that of the higher symmetrical individual now obtained 

 on the other. The higher symmetrical modification thus appears as 

 an extremely perfect and regular aggregate of submicroscopical 

 individuals of the lower symmetrical form, in an analogous way 

 to that in which the mixed-crystals are built up from their isomor- 

 phous components by the intercalation of alternating layers of 

 molecular dimensions. 2 ) The suggestive idea of Mallard therefore 

 should have the great advantage of reuniting under the same point 

 of view three different phenomena exhibited by crystals : their optical 

 rotatory power, their optical anomalies, and their eventual polymor- 



Fig 142. 



1) If, for instance, the angle between the consecutive lamellae be 90 or 180, 

 no rotation of the plane of polarisation of the emergent ray will occur. 



2) See however in this respect: L. Vegard and H. Schjelderup, Phys. Zeits. 

 18. 93. (1917). 



