﻿748 Mr. W. Barlow and Prof. W. J. Pope on 



produce the tetragonal methylammonium iodide, the axial 

 ratio, c/a — 1*467, indicates that pseudo-cubic symmetry 

 again results, this value of c/a being comparable to that of 

 2c/a in tetramethylammonium iodide. 



On substituting an atom of antimony for one of nitrogen 

 in tetramethylammonium iodide, so as to obtain tetramethyl- 

 stibonium iodide, (CH 3 ) 4 SbI, the tetragonal symmetry 

 changes to hexagonal with the axial ratio, a : c=l : 1*422. 

 An hexagonal system showing an axial ratio of this order 

 of magnitude is closely related to the cubic system for the 

 following reasons : — If a cube diagonal, which is a trigonal 

 axis of symmetry, is taken as the vertical axis-c of a system 

 of crystallographic coordinates of a trigonal or hexagonal 

 kind, one of the two obvious horizontal translations is twice 

 the distance between a cube corner and the centre of an 

 opposite cube face ; taking the cube edge as of unit length, 

 the diagonal is \/3 and the distance from the cube corner 

 to the centre of an opposing face is V(3/2). The axial ratio 

 corresponding to this mode of referring a cube to a set of 

 trigonal coordinates is, therefore, a : c= V(3/2) : V3 = l * 

 1*4142. The trigonal or hexagonal tetramethylstibonium 

 iodide, tetraethyl-phosphonium iodide and -ammonium 

 bromide obviously approximate closely to these dimensions. 



The hexagonal axial ratio alternative to the value, 

 a : c=\ : 1*4142, is a : c = l : 1*6330 (Trans. Ohem. Soc. 

 xci. 1907, p. 1157) and the values recorded for triethyl- 

 ammonium chloride and bromide are rather more than 

 one-half of this, namely, a : c = l : 0*8165 ; these two sub- 

 stances may therefore also be regarded as morphologically 

 related in a simple manner to the cubic ammonium halides. 



The axial ratio, a : c=l : 0*5544, of the tetragonal 

 tetraethyl-ammonium iodide is also, though somewhat less 

 distinctly, that of a pseudo-cubic substance, because on 

 multiplying unit length along the axis-c by two, it assumes 

 the pseudo-cubic form, a : c=l : T1088. Ample justifi- 

 cation can be given for altering this axial ratio in this 

 manner; this substance is of the same type as the hexagonal 

 tetramethylstibonium iodide and tetraethylammonium bro- 

 mide which have already been shown to be morphologically 

 closely related to the cubic system. 



A comparison of the measured angles on tetraethyl- 

 ammonium iodide with principal measured angles on the 



