108 



0, 



1 



b. 



b. 



there must evidently exist definite connections between them. 

 16. French authors especially l ) have frequently pointed to 

 the fact that for the description of physical relations it is often 

 more desirable to bring to the fore the absence of some symmetry- 

 properties (i. e. ^'ssymetry), rather than to deal with the presence 

 of other symmetry-properties, as we have done in the preceding 

 paragraphs. Indeed, in the course of our considerations we have 

 already been able to draw attention to this fact. 



If in a crystalline medium there is no centre of symmetry, or if 



the principal axis of a crystal 

 be heteropolar, i. e. if no 

 binary-axes, nor asymmetry- 

 plane is perpendicular to 

 it, - then the absence of 

 these symmetry-elements 

 will make it possible that 

 an electric field with a sym- 

 metry C eventually occurs, 

 in which the symmetry 

 centre, the binary axes, and 

 the symmetry-plane per- 

 pendicular to the lines of 

 force, are also lacking. The 

 same is the case if two 



causes are superposed to a resulting cause, which gives an effect 

 in which both components take a part. If the superposition of an 

 electric and a magnetic field occurs in such a way that their axes 

 of isotropy are not parallel, but perpendicular to each other, the 

 only remaining symmetry-element of the resulting cause is a plane 

 passing through the axis of the electric field and perpendicular to 

 the magnetic lines of force. 



The electric current which in this arrangement of both fields is 

 observed in crystallised bismuthum (Hall-effect), may be considered 

 as an effect, the occurrence of which is in full accordance with the 

 absence of definite symmetry-elements in the producing cause 2 ). 

 For such electric current has no plane of symmetry perpendicular 



1 ) Vid. e. g. : L. Pasteur, Deux Lemons sur la Dissymetrie Moleculaive pro/essees 

 devant la Socttti Chimique de Paris, (1860); P. Curie, Journal de Physique, 

 (3), 3, 407, (1894). 



2) Cf. also: H. A. Lorentz, Versl. Kon. Acad. v. Wet. Amsterdam, 19, p. 219,(1884). 



I. 



II 



