97 



The same is the case if a crystal of quartz be compressed homo- 

 leously parallel to the direction of one of its heteropolar binary 

 ces: the direction of the binary axis remains heteropolar as before, 

 that an electric potential-difference can eventually occur at both 

 ends. Similar symmetry-relations occur if a planparallel crystal- 

 late, cut perpendicular to a binary axis, be compressed in the 

 irection of the ternary axis of the quartz-crystal. In the two cases 

 e considered, this dielectric polarisation could really be detected 

 experiment, because its magnitude was sufficient to be measured. 

 That such phenomena now really can occur in a crystalline 

 ledium which has the symmetry of a sub-group of that group, to 

 rtiich the proper symmetry of the phenomenon under investigation 

 *longs, is elucidated by the fact that the symmetry of a crystalline 

 ledium is in reality ^.minimum symmetry, namely the lowest degree of 

 letry, beneath which the symmetry of any physical phenomenon 

 whatever observed in the crystal, can never sink. For many phe- 

 jmena occurring in the crystal the special symmetry in truth appears 

 be much higher than that attributed to the medium itself accor- 

 ig to its cohesion and molecular structure, i. e. than that of the 

 "crystal-class", to which it belongs. These higher symmetries of 

 le phenomena observed are such that certain symmetry-elements 

 /hich are characteristic of these phenomena under all circumstances, 

 re added to those of the crystal-class to which the crystal belongs. 

 13. As an illustration of this we wish to consider the symmetry 

 rtiich a crystal will manifest with respect to the diffraction of 

 ^6 nt gen-rays, if a planparallel plate cut from it in some known 

 lirection be traversed by a narrow pencil of such rays perpendicular 

 its surface. This highly important phenomenon was discovered 

 Von Laue *) some years ago, and has since been studied by 

 sveral investigators in different ways, and with particular success 

 yy Bragg Sr. and Jr 2 ). However we will not consider these 



experiment until now. The existence of the so-called "rotatory coefficients" 

 the equations of Stokes' theory, could never be demonstrated up to this 



date; cf. C. Soret, Journ. de Physique (2) 2. 241. (1893); Archives d. Sc. phys. 



et nat. de Geneve, (3). 29. 355. (1893); ibid. 32. 631. (1894). 



1) M. Von Laue, Friedrich and Knipping, Sitz. Bayr. Akad. d. Wiss. Mun- 

 chen, (1912), p. 303. 



2) W. H. and W. L. Bragg, Proceed. Roy. Soc. London, 89. A. 277, 477. 

 (1913); Zeits. f. anorg. Chemie 90. 255. (1914). 



For the special questions dealt with here, see the papers of: G. Friedel, 

 Compt. rend, de 1'acad. d. Sc. Paris, 157. 1533. (1913); F.M.Jaeger and 



7 



