135 



to tind out what change the symmetry of the crystalline -ub-i 



r^iM", when one of its hydrogen-atoms is replaced by another 



inivairnt atom or radical. The phenomena observed in Mich cases 

 in- n. nurd morphotropism. 



Some frequently occurring facts could be stated in these inves- 

 ti.ua tions, although it appeared impossible to find here absolute 



2gularities, because it very soon became clear that the "morphotropic 

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



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

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

 mit nt ion of a hydrogen-atom in organic molecules by a hydroxyl- 

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



id one-sided change of the crystdllographical parameters of the 



ibstituted substance, while the original symmetry of it is usually 

 reserved. The substitution of hydrogen-atoms in such molecules 

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

 followed by a striking change of the crystallographical symmetry 

 )f 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 

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

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

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



:curs, which is preserved in the corresponding ^m^o-derivatives 

 of them also. 



Although no general rules concerning the relation between che- 

 mical composition and the degree of crystallonomical symmetry 

 lave hitherto been found, these, and an exceedingly great number 

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



mnection between crystalline form and molecular composition 



syond all doubt exists. 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 to. Isomorphism however is only a special 

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



i) F. M. Jaeger, Kristallografische en Molekulaire Symmetric van Plaat 

 isomere Benzolderivaten, Dissertatie Leiden, (1903); Zeits. f. Kryst. 88. 555. 

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



