138 



that under such conditions most crystallographers felt inclined 

 rather to adopt the simpler views of Bravais, than the much more 

 complicated doctrine of the regular arrangements in space discussed 

 in the above l ). 



15. The problem of giving a rational deduction of the crys- 

 talline structure from the chemical composition of the crystal has 

 puzzled investigators ever since early times. It has been more in 

 particular the deduction of the observed symmetry, which originally 

 occupied most workers in this field, and in this respect the endea- 

 vours of Groth 2 ) in 1870 are worthy to be remembered, who tried 

 to find out what change the symmetry of the crystalline substance 

 undergoes when one of its hydrogen-atoms is replaced by another 

 univalent atom or radical. The phenomena observed in such cases 

 are named morphotropism. 



Some frequently occurring facts could be stated in these inves- 

 tigations, although it appeared impossible to find here absolute 

 regularities, because it very soon became clear that the "morphotropic 

 action" of any substitute does not only depend on its proper 

 chemical character, but also on that of the substituted molecule. 



Thus, Groth was able to draw attention to the fact that the sub- 

 stitution of a hydrogen-atom in organic molecules by a hydroxyl- 

 or nitro-group, has commonly as a consequence only a rather slight 

 and one-sided change of the crystallographical parameters of the 

 substituted substance, while the original symmetry of it is usually 

 preserved. The substitution of hydrogen-atoms in such molecules 

 by halogen-atoms or by alkyl-groups, is, however, in almost all cases 

 followed by a striking change of the crystallographical symmetry 

 of the original substance. These rules are valid in most cases, but 

 the number of exceptions is by no means negligible. 



Other observers tried to trace the connection between chemical 

 constitution and crystallographical symmetry by the comparative 

 study of the crystal-forms of position-isomerides. The present author 



1) Cf. also: G. Friedel, Bull, de la Soc. Miner., 31, 326, (1907); G. W. 

 Wulff, Zeits.' f. Kryst., 47, 607, (1910). 



W. J. Sollas, Proc. Royal Soc., 63, 270, (1898); 67, 493. (1900); 69,294, 

 (1902; 80, A, 267, (1908); Brit. Assoc. Rep., (1907), p. 481. 



G. Cesaro, Bull. Acad. R. beige, (1901), p. 303; P. Groth, Zeits. f. Kryst., 

 54, 65, 498, (1914); E. Riecke, Phys. Zeits., (1900), /, 277; Ann. der Phys., 

 3, 545, (1900). 



2 ) P. Groth, Ber. d. d. Chem. Ges. 3, 449, (1870)., 



