99 



themselves, and by the particular choice of the symmetry primarily 

 given to a certain phenomenon which is considered as the starting- 

 point for the definition of the others related to it. 



12. The symmetry of the "image" / in any point P of a phy- 

 sical system determines the maximum symmetry compatible with the 

 occurrence of the phenomenon considered in P. The phenomenon, 

 namely, can occur in a medium, if its symmetry is the same, or if 

 it is that of a sub-group of the symmetry characteristic for the 

 phenomenon in question. 



If we have a crystal of turmaline, whose symmetry with respect 

 to the cohesion-phenomena (which are closely related to its internal 

 structure), is that of the group Cf, and if this crystal be heated 

 uniformity to a certain temperature, the symmetry of the crystal 

 is of course by this scalar change altered in no respect; it remains, 

 as before, C%. But C% is a sub-group of C ; and therefore the 

 possibility exists that a dielectric polarisation, the symmetry of which 

 is precisely C , will occur in the heated crystal: as Curie expressed 

 it in his way: "c'est la dissymetrie, qui produit le phenomene". 

 Nevertheless nothing has yet been said about the true magnitude 

 of the expected phenomenon, nor about the real necessity of its 

 occurrence. It is possible that the effect is, for instance, so extremely 

 small, that it cannot be tested by any experimental method now 

 available *) . 



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

 geneously parallel to the direction of one of its heteropolar binary 

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

 so that an electric potential-difference can eventually occur at both 

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

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

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

 here considered, this dielectric polarisation could really be detected 

 by experiment, because its magnitude was sufficient to be measured. 



In this connection attention may be drawn upon the remarkable 

 fact, that the piezo-electrical polarisation produced by homogeneous 



1 ) It is a curious fact, for instance, that the theory of Stokes on the con- 

 ductivity of heat in certain crystals, as scheelite, etc., could not be verified 

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

 in the equations of Stokes' theory, could not be demonstrated up to the present; 

 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). . 



