400 Dr. L. Veeard 



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on 



First of all, we shall notice the form of the elementary 

 cell. For the first time during our investigations on the 

 tetragonal crystals, we meet with the case that the ratio 

 d 0Q ijd 100 is not equal to the ratio of the axes cja, but equal to 

 twice this ratio. 



This is a point of great value when we are going to 

 determine the arrangement of the atoms. 



Determination of the Lattice. 



§ 4. The determination of the lattice was, indeed, more 

 difficult than at first expected. It will at once be apparent 

 from a comparison with the lattice found for NH 4 I that 

 we do not derive the lattice of N(CH 3 ) 4 I by a simple 

 substitution with CH 3 . This is evident from the different 

 ratios of the spacings d m and d uo for the two crystals. 



The first question to decide is, What is the size and form 

 of our elementary lattice ? At first it might seem possible 

 that the elementary lattice had the same form as the 

 elementary cell. Let the side of the base of the elementary 



2c 



lattice be a' and its height c' ; then we might have c'/a' — — • 

 © s / a 



This assumption, , however, did not lead to any possible 

 arrangement, and is also very unlikely from a crystallo- 

 graphic point of view : thus a (111) plane of the lattice 

 would correspond to a (221) plane of the crystal. 

 To avoid such difficulties, we must put 



?=J ( 3 > 



Now in the volume of the elementary lattice, which has 

 at least one atom at each corner, there must be at least one 

 molecule. 



If we make the simplest possible assumption, we should 

 have 



a 'r 2d r } w 



and c = a 001 J 



The volume of the elementary lattice, being four times 

 that of the elementary cell, must contain 2 molecules. 



Let us first consider the relative position of the two 

 lattices. Let us, in the usual way, take one corner of 

 one of the lattices as origin of a rectilinear coordinate 

 system with the axes parallel to the sides of the lattice. 



The construction-points of the I lattices must be (000) 



and(-x,— , Z), where Z is a parameter to be determined. 



