﻿Topic Parameters and Morphotropic Relationships. 747 



This mode of interpreting the goniometrical data, which is 

 a more rational one than that applied by Slavik and Wagner, 

 shows that the salt is markedly pseudo-cubic. So that in 

 passing from the cubic ammonium iodide to the pseudo- 

 cubic tetragonal tetramethylammoninm iodide, tlie three 

 equal topic parameters of the first, ^57*5 = 3*9, change 

 almost equally to approximately y / 108 , 7 = 4'8. This necessary 

 alteration in the table of topic parameters reproduced 

 above entirely destroys the sequence as between ammonium 

 iodide, tetramethylammonium iodide, and tetraethylam- 

 monium iodide ; the table in question is thus valueless as an 

 illustration of the use of these parameters in exhibiting 

 morphotropic relationships. 



A broad view of the morphotropic relationships holding 

 between the ammonium halides and their tetraalkyl derivatives 

 is obtained by considering the axial ratios and crystal systems 

 presented by a number of related compounds which exhibit 

 high crystalline symmetry. 



KI Cubic. 



NH 4 I „ 



(CH 3 ) 4 NC1 Tetragonal. a : c=l : 0-7107. 



(CH 3 ) 4 NBr „ a : c=l : 0*7131. 



(CH 3 ) 4 M , „ a : c=l : 0*7223. 



(UH 3 ) 4 SbI Hexagonal. a : c = l : 1*422. 



(C a H 5 ) 4 NI Tetragonal. a : c = l : 0*5544. 



(C 2 H 5 ) 4 PI Trigonal. a : c=l : 1*4719. 



(C 2 H 5 ) 4 NBr „ a : c = l : 1*4656. 



(CH 8 )H 8 NI Tetragonal. a : c = l : 1*467. 



(C 2 H 5 ) 3 HNC1 Hexagonal. a : c = l : 0-8451. 



(C 2 H 5 ) 3 HNBr ... „ a : c=l : 0'8746. 



(C 2 H 5 ) 3 (CH 3 )NI . . Tetragonal. a : c = l : 0*5536. 



The table shows that on replacing the metallic atom in 

 potassium iodide by the ammonium radicle, NH 4 , the system 

 retains its cubic symmetry ; on replacing each of the four 

 hydrogen atoms in the ammonium halide by methyl, pseudo- 

 cubic symmetry results in the manner described in detail 

 above. The general applicability of the principle thus stated 

 is demonstrated by the similar axial ratios exhibited by 

 tetramethylammonium chloride, bromide, and iodide : simi- 

 larly, on replacing one of the hydrogen atoms in the 

 ammonium iodide molecule by a methyl group, so as to 



