139 



found in this way, for instance, l ) that although close relations 

 between such position-isomerides are in general rather rare, in the 

 case of 1 -2-4-6-, and \-2-3-5-tribromo-toluenes a complete isomorphism 

 occurs, which is also preserved in the corresponding dinitro- 

 derivatives of them. 



Although no general rules concerning the relation between che- 

 mical composition and the degree of crystallonomical symmetry 

 have hitherto been found, these, and an exceedingly large number 

 of other phenomena, must convince us of the truth that a rational 

 connection between crystalline form and molecular composition 

 exists beyond all doubt. The discovery of isomorphism by Mitscher- 

 lich was a first step on the way towards the solution of the problem 

 indicated above, as here the "analogy" in chemical composition 

 is expressively pointed out. Isomorphism, however, is only a special 

 case of the much wider conception of morphotropism, i. e. of the 

 causal relations between chemical and crystallographical arrangement, 

 and between the forces which determine the configuration of atoms 

 in space, in connection with those governing the structural arrange- 

 ment of the crystallonomical units. 



16. A highly suggestive theory concerning the problem men- 

 tioned, was developed in 1906 by Barlow and Pope 2 ). It represents 

 a happy completion of Barlow's views on homogeneous configu- 

 rations and on the most closely packing of spheres, by the aid of a new 

 fundamental hypothesis about the relation between the valency of an 

 atom and the space it occupies in such homogeneous assemblages. 

 The domain of each chemical atom is a distinct portion of space, 

 which it occupies by virtue of an influence exerted uniformly in every 

 direction. These spheres of influence are now supposed to have a 

 volume which in every compound is nearly proportional to the valency 

 of the atom, the factor of proportionality being the same for all atoms 

 of the same crystallised substance; and according to the authors, a 



J ) F. M. Jaeger, Kristallografische en Molekulaive Symmetric van Plaatsings- 

 isomere Benzolderivaten, Dissertatie Leiden, (1903); Zeits. f. Kryst. 38, 555, 

 (1904); 39, 170, (1904); 40, 357, (1905). 



2 ) W. Barlow and W. J. Pope, Journ. Chem. Soc. London, 89, 1675, (1906); 

 G. Le Bas, ibid., 91, 112, (1907); W. Barlow and W. J. Pope, Journ. Chem. 

 Soc., 91, 1150, (1907); 93, 1528, (1908); F. M. Jaeger, Zeits. f. Kryst., 44, 61, 

 (1907); W. Barlow and W. J.. Pope, Journ. Chem. Soc., 97. 2308. (1910); W. 

 Barlow, Miner. Mag. 17, 314, (1916); cf. also: W. Barlow, Zeits. f. Kryst., 29, 

 433, (1889); and especially the work of A. E. H. Tutton: Crystalline Structurre 

 and Chemical Constitution, Londen, (1910). 



