420 Dr. L. Vegard on 



direction of the C axis, in spite of the fact that the molecules 

 in the two cases are arranged in quite different lattices. 



The analyses of these two substances have given the 

 somewhat startling result that a simple relation between 

 the topic parameters, such as that given in Table L, does 

 not necessarily involve any very simple relation between 

 the lattices. 



In fact, the morphotropic relation might be most easily 

 explained in the way proposed by Groth, by supposing that 

 the I and N lattices were arranged in the same way in the 

 two crystals. If the H atoms were substituted by C atoms 

 and arranged tetragonal ly in planes perpendicular to the 

 C axis, we should expect the lattice to expand in the direction 

 perpendicular to the C axis, while in the direction of this 

 axis the dimensions should be left unaltered. 



Contrary to this supposition, the analysis has shown that 

 the morphotropic relation is not explained by substitution 

 alone ; but the I and N lattices are not merely quite 

 differently arranged, but the structure in the two cases 

 contains a different number of elementary I and N lattices. 



The NH 4 I lattice is composed of 4 1, 4 N, and 12 H 

 lattices ; while the number of elementary lattices of 

 N(CH 3 )iI is 



2 1, 2 N, 8 C, 24 H. 



What determines the arrangement, we cannot at present 

 tell ; but generally there will be a tendency to form closely- 

 packed systems, or that arrangement will be chosen which, 

 under the conditions present, gives a minimum of potential 

 energy due to the mutual attraction of the atoms. 



Thus it is quite possible that a simple substitution (if such 

 a lattice might be formed) would give a larger volume 

 of the lattice and not give the observed simple relation 

 between the topic parameters. 



It was found that, in quite a formal way, we could imagine 

 the N(CH 3 ) 4 I crystals composed of two molecular lattices ; 

 but in this case we must not put too much in the name — 

 molecule. It may not be so that the affinity forces are 

 mainly engaged to form the molecular element. Thus the 

 groups of (CH 3 ) 4 which we have previously considered 

 (h'g. 6) may be kept together by forces of the same nature 

 and order of size as those acting between the atoms of a 

 single molecular element. This group, however, does not 

 belong to a single molecular element, but is equally related 

 to four different molecules. 



