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XL. Results of Crystal Analysis. — IV. 

 By L. Vegard, Dr. phi!., University of Christiania *. 



[Plate IX.] 



Part I. — The Structure of Ammonium Iodide and 

 Tetramethylammonium Iodide. 



§ 1. rpHE substances NHJ and N(GH 3 ) 4 I are the first 

 A. members of a series which show very interesting 

 morphotropic relations, and have attracted much attention 

 from crystallographersf . 



The crystals o£ NHJ. belong to the cubic system, When 

 the four hydrogen atoms, however, are replaced by four 

 alkyle groups (four CH 3 groups, say) the crystal becomes 

 tetragonal, with the ratio cja decreasing with increasing 

 number of C atoms. 



It would be natural to suppose the tetragonal structure 

 to be produced by some tetragonal arrangement of the 

 carbon atoms; and an attempt to explain the morphotropic 

 relation between NH 4 I and N(CH 3 ) 4 I has been made by 

 P. Groth t 



According to his assumption, the NH 4 I crystals should 

 have the following structure : — The I atoms should be 

 arranged in a simple cubic lattice, which is centred with 

 a similar lattice of the N atoms. The hydrogen atoms 

 should be placed on the cube-diagonals through the N atoms, 

 in such a way that the H atoms are placed in the corners of 

 a tetrahedron with the N atoms at the centre. 



The structure of X(CH 3 ) 4 I he simply derives from that 

 of its parent substance, by supposing the H atoms replaced 

 by the group CH 3 in such a way that the C atoms take up a 

 similar position to that previously occupied by the H atoms, 

 and the H atoms of the CH 3 group are supposed to be placed 

 in the corner of a tetrahedron with the C atom at its centre. 



Although these hypothetical lattices of Groth show several 

 striking points of similarity with those actually found, it 

 will be seen that the morphotropic relation between the 

 two substances is not quite so simple as that supposed bv 

 Groth. 



The topic parameters, as calculated by Groth, for the 



* Communicated by the Author, 

 t P. Groth, Cliemische Crystalluyraphie. 



+ See Stefan Kreutz, Element der Theorie der KrystalUtrulxtur, p. 145 

 (1915). 



