﻿Topic Parameters and Morplwtropic Relationships. 749 



other two compounds mentioned shows that a very complete 

 correspondence exists. 



(C 2 H 5 ),NBr. (0 2 H 5 ) 4 NL (O a H B ) 4 PI. (C 2 H 6 ) 8 (CH,)NI. 



3ll: 131=52° 52' 111 : 110=51° 54' 311: 131=52° 55' 111 : lll=51°56' 



111:111 76 12 131:100 76 5 111: III 76 7 



311:111 62 54 111:100 64 8 311:111 63 2 



It thus appears that the basal plane (111) on the trigonal 

 compounds corresponds to the pinacoid (001) on the tetra- 

 gonal one, the rhombohedron (100) and the pyramid (311) 

 on the former also corresponds to the pyramid (111) and the 

 prism (110) on the latter. The tetragonal tetraethyl- 

 ammonium iodide is therefore morphotropically closely 

 related with the two trigonal substances, the dimensions of 

 which have been shown to be pseudo-cubic. 



The values for the tetragonal methyltriethylammonium 

 iodide, with a : c=l : 0'5536, are included in the above 

 table because, for the reasons just quoted, this substance 

 must also be referred to the cubic system. 



Without at present entering into detail as to the manner 

 in which the mode of interpreting morphotrophic relation- 

 ships which we have previously developed (Trans. Chem. 

 Soc. Ixxxix. 1906, p. 1675 ; xci. 1907, p. 1150 ; xciii. 1908, 

 p. 1528 ; xcvii. 1910, p. 2308) is applicable to the present 

 case, we may regard the following conclusion as definitely 

 established. The method of representing the morphotropic 

 relationship which exists between ammonium, tetram ethyl- 

 ammonium, tetraethylammonium, and tetrapropylammonium 

 iodide advocated by Slavik and Wagner, and generally 

 adopted in the text-book literature, has been proved to be 

 incorrect and should be abandoned. 



The fact that so many of the alkyl derivatives of the 

 ammonium halides affect types of crystalline symmetry 

 and axial ratios closely related to the cubic system is 

 important ; it seems to be generically connected with the 

 fact that the ammonia in aluminium ammonium alum can 

 be replaced by hydroxylamine, methylamine, ethylamine, 

 and trimethylamine, with retention of the original cubic 

 symmetry. 



The Chemical Laboratory, 

 The University, Cambridge. 



