Results of Crystal Analysis. 401 



The (xy) coordinates of the second point are found from the 

 condition that ^100— 2 a '- 



With regard to Z, two values might naturally suggest 

 themselves : 



Z = | and Z = 0. 



The first value is excluded because it would give a wrong- 

 spacing of the (001) face, and the second value is not 

 possible because it would lead to a wrong spacing of the 

 (101) face. 



We are then led to consider Z as a parameter io be 

 determined. 



In order to preserve the tetragonal symmetry, the 

 N lattices must be placed in such a way that each of them 

 may be made to cover one of the I lattices by a translation 

 parallel to the c axis. 



Consequently, the construction-points of the two N lattices 

 will be 



(0,0, -ZO and g,|,Z-Z/'). 



If the arrangement is to give a bipolar tetragonal axis, 

 the following relation must hold : 



Z^-Z/'^Z, (5) 



where I is a parameter ; and putting 



z+i=l-i 



we get for the construction-points of the I and N lattices : 

 For the I lattice : (000), (a/2, a/2, c/2-(l +l),) (&) 

 „ N „ (00 - Z), (a/2, a/2, c/2-l . j* 



The arrangement will be more easily understood when we 

 introduce the conception of molecules. 



To each of the two I lattices corresponds one N lattice, 

 the position of which is found by a translation along the 

 c axis a distance I. In this way the atoms are naturally 

 divided up into pairs consisting of one I and one N atom. 



The line of length Z, connecting the two atoms, we might 

 call the molecular axis. 



Now four C atoms and twelve H atoms must be placed in 

 tetragonal arrangement round this axis, and we get a kind 

 of molecular element. 



The axis of each molecular element is unipolar, or the 

 molecule has no symmetry-plane perpendicular to the c axis. 



If, then, the crystal as a whole is going to have a bipolar 



