402 Dr. L. Vegard on 



axis, the two systems of molecules which correspond to the 

 two component elementary lattices must have their axes 

 turned in opposite directions. This statement is equivalent 

 to the condition expressed in equation (5). 



Now we might naturally introduce the middle point 

 between the I and N atoms on the molecular axis as point 

 of reference for the determination of the position of the 

 molecular elements. Choosing one such point as the origin 

 of our coordinate system, the construction-points of the two 

 molecular systems would be 



(000) and (a/2, a/2, c/2-Z ). . . . ( 7) 



Thus the arrangement of the molecules involves the 

 determination of the parameter l . 



To this parameter is added those necessary for the deter- 

 mination of the molecular element itself. As the molecular 

 axes of the two groups of molecules have opposite directions, 

 each group of molecules must give tetragonal symmetry ; 

 and as each group only forms one simple eiementary lattice, 



Fig. 2 a. Fig. 2 b. 



I I 



2* 



/ e © 



2« 



© 



© 



•2 



® 9/ 



°2 



@ C atoms 

 • H )> 



the C and H atoms must be arranged in such a way that each 

 molecular element shows tetragonal symmetry with respect to 

 the molecular axis. Further, in order to preserve the highest 

 degree of tetragonal symmetry of the crystal, the planes 

 through the axis and parallel to the sides of the latrice 

 must be planes of symmetry. This condition considerably 

 diminishes the possible arrangements of the atoms. 



Fig. 2 a shows the most general arrangement of the 

 C atoms round the tetragonal axis. In fig. 2 b the atoms 



