﻿74(3 



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



determined the following axial ratios, &c, in connexion with 

 the subject under discussion. 



Axial Eatios. 



: c=l : 0-7223. 

 : c=l : 0-5514. 

 : 6 : c=0*7761 :1 : 0*6283. 



The following table, giving the topic parameters calculated 

 from the above data and stating the changes which they expe- 

 rience throughout the series, is taken from Nernst (Joe. cit.). 



NHJ 



Crystal System. 



.. Cubic. 

 .. Tetragonal. 

 .. Tetragonal. 

 .. Rhombic. 



V. 



57-51 



N(CH 3 )J . 

 N(C 3 H 5 ) 4 I . 

 N(0 3 H 7 ) 4 I . 



108-70 a 

 162-91 a 

 235-95 a 





NHJ. 



A 



NMe 4 L 



A 



NBt 4 I. 



A 



NPr 4 X 



v. 



57"51 



51-19 



108-70 



54-21 



162-91 



7304 



235-95 



x • 



3-860 



1-459 



5-319 



1-329 



6-648 



— 0-555 



6-093 



*• 



3-860 | 



1-459 



5-319 



1-329 



6-648 



1-103 



7-851 



OJ . 



3-860 



-0-018 



3-842 



-0-156 



3-686 



1-247 



4-933 



The conclusion drawn by Slavik from this table, and 

 repeated in the text-books mentioned, is that on passing from 

 the cubic ammonium iodide to the tetragonal tetramethyl- 

 ammonium iodide one dimension of the crystal structure, 

 that of co, remains sensibly unchanged, and that the main 

 increase in dimensions of the crystal structure occurs in the 

 two directions of % and -\jr. Similarly, in passing from the 

 tetramethyl- to the tetraethyl - ammonium iodide, the 

 dimension w again changes but little and. as before, the chief 

 increase in dimensions of the structure occurs in ^ and i/r. 



This interpretation, which has been so widely adopted as 

 illustrating the value of the topic parameters in elucidating 

 morphotropy, appears to us entirely erroneous for the 

 following reasons : — The method adopted for the description 

 of the tetragonal tetramethylammonium iodide gives to the 

 chief forms present the indices {100} and {111} : the 

 descriptions given by Slavik and by Wagner show that 

 the salt crystallizes in combinations of {100} and {111}, 

 resembling rhombic dodecahedra of the cubic system in that 

 the angle 110 : 11 1 = 41° 23 ; 30". It is clear, therefore, that 

 in accordance with Fedorow's practice of assigning the most 

 simple set of indices to the forms exhibited, the indices 

 {100} and {111} should be changed to {110} and {101} 

 respectively ; when this change is made the axial ratio 

 becomes 



a : c = l : cotan. 44° 23' 30" = 1 : 1-0214. 



