105 



b 



roperties (\. e. dfosymetry), rather than to deal with the presence 



other symmetry-properties, as we have done in tin- preceding 



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



sady been able to draw attention to this fact. 



1 1 in a crystalline medium there is no centre of symmetry, or if 

 principal axis of a crystal be heteropolar, i. e. if no binary-axes, 

 a symmetry-plane be perpendicular to it, -- then the absence 



these symmetry-elements 



11 make it possible that 



electric field with a sym- 

 ictry C eventually occurs, 

 which the symmetry- 

 mtre, the binary-axes, and 

 le symmetry-plane perpen- 

 icular to the lines of force, 

 re also lacking. The same 



the case if two causes are 

 iperposed to a resulting 

 luse which gives an effect 



which both components 



ce a part. If the super- 

 Fig. QQ. 

 sition of an electric and 



magnetic field occurs in such a way that their axes of isotropy 

 not parallel, but perpendicular to each other, the only remaining 

 ictry-element of the resulting cause is a plane passing through 

 le axis of the electric field, and perpendicular to the magnetic 

 les of force. 



The electric current, which in this arrangement of both fields is 

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

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

 bsence of definite symmetry-elements in the producing cause, 

 "or such electric current has no plane of symmetry perpendicular 

 its direction ; therefore at least in one of its causes must lack that 

 letry-element too. Now in the above mentioned superposition, 

 conditions have become such as to make the occurrence of the 

 electric current possible: neither the electric field alone, nor the 

 magnetic field alone, can be the cause of the Hall-phenomenon 

 in a direction perpendicular to the plane of the current and the 

 magnetic lines of force ; but */ both be combined, the symmetry-centre 

 of the magnetic field, as well as the symmetry-planes of the electric 



I. 



II. 



