( 82 ) 



First of all the proportion a : h is the same in both the «- and ^- 

 modification ; a proof that these ciyslallographical forms are closely 

 related to each other as regards their internal structure. I have 

 noticed more than once this equality of two parameters in difierent 

 modifications of a same compound ; 1 met lately with a striking instance 

 in the case of the red a- and the less stable yellow /3-modification 

 of the 1-^A-Dinitrodiethi/laniUne notAvithstanding the difference in 

 the degree of symmetry. From a crystallographical point of view 

 such modifications must always be dependent on each other, although 

 that dependence may not always be immediately noticed. 



But then the very close relationship between the less stable ^- 

 modification and the crystalline form of the iso-imide, as is plainly 

 shown from the analogous values for the angle ^ and the proportion 

 6 : c is surprising in a high degree. 



Accordi])g to Drs. Hoogewerff and van Dorp ^) the isomerism of 

 imides and iso-imides is based on a difference in the way of combi- 

 nation of the N-atom on the one side and the 0-atom on the other: 



CO C = N.CH, .CeH, 



C,H,<^ \n.CH,.C«H, and C«H,<^ ^0 

 CO CO 



Benzylphtalimide. Benzylphtal-iso-imide. 



The heteromorphous ^^-modification of the first substance now 

 appears to be extremely closely related to the crystalline form of 

 the first one owing to desmotropical change. Both phenomena, 

 dimorphism and desmotropism therefore cause, respectively, an ana- 

 logous change in the crystal-symmetr}' of «-benzylphthalimide. 



The presumption raised by me some time ago that crystallographic 

 polymorphism in a number of organic compounds might be caused 

 by a chemical isomerism ^), which would then account for the tem- 

 porary existence of more stable atom-configurations is again a little 

 more justified by the fact that above connection has been found. 



1) loco cit. 



~) Kristallografische en Molekulaire Symmetrie van plaatsings-isomere Benzol- 

 derivaten. Proefschrift, Leiden, 1904, pag. 120, 121. Z. f. Kryst. 38. 600. (1904). 



