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LXXIX. On Topic Parameters and Morphotropic Rela- 

 tionships. By Willtam Barlow, F.R.S., and William 

 Jackson Pope, F.R.S.* 



^F^OPIC parameters were introduced by Becke (Anz. d. 

 1 Kais. Akad. d. Wiss., Wien, xxx. 1893, 204) for 

 defining, as between crystallographically related substances, 

 the lineal dimensions of the corresponding crystal structures ; 

 in a substance crystallizing in one of the rectangular systems, 

 the topic parameters, %, -v/r and co, are calculated as 



% = </(a 2 V/c) ; ifr=x/ a > « = + <>, 



a, b, and c being the crystallographic axial ratios and V the 

 molecular volume of the substance. 



The topic parameters of a set of isomorphously related 

 substances define the actual changes in dimensions of the 

 point system which attend the passage from one crystalline 

 substance to others isomorphous with it. During a number 

 of years past, however, it has been customary to calculate 

 and record the topic parameters of series of substances the 

 members of which exhibit any chemical or morphotropic 

 relationship ; so far as we are aware, no fact or conclusion 

 of importance has resulted from the application of the topic 

 parameters to cases of morphotropic relationship as distinct 

 from those of isomorphism. 



That topic parameters, as hitherto applied to the quanti- 

 tative description of morphotropy, are without physical 

 significance is well illustrated by a consideration of the 

 particular case generally chosen in explanation of the subject. 

 The instance in question refers to the morphotropic relation- 

 ship between ammonium iodide and its tetramethyl, tetra- 

 ethyl, and tetrapropyl derivatives, and derives authority 

 from the important position assigned to it in a number of 

 our most valued text-books (Groth, ' Einleitung in die 

 chemische Krystallographie/ 1904. p. 32; Groth, ' Ghemische 

 Krystallographie,' i. 1906, p. 171 ; Nernst, ' Theoretische 

 Chemie/ Siebente Aufl., 1913, p. 373 ; Roscoe and Schor- 

 lemmer, ' Treatise on Chemistry,' vol. ii. 1907, p. 2*0; Barker, 

 in same, vol. ii. 1913, p. 220); more recently Groth has again 

 referred to the importance of this example [Zeits.f. Kryst. 

 liv. 1914, p. 68). The data were collected by Slavik (Zeits. 

 /. Kryst. xxxvi. 1902, p. 268), and his interpretation of the 

 meaning of the topic parameters was confirmed and amplified 

 by Wagner (Zeits. f. Kryst. xliii. 1907, p. 148). Slavik 



* ComniunioattHt bv the Authors. 



