( 30 ) 



not, as in the previously detected case of the two tribromotohienes 

 (Dissertation, Lejden 1903) a continuous form; the difference is 

 caused by the lesser degree of form-analogy which these substances 

 possess in proportion to that of the two said tribromotohienes. 



The third example of miscibility, — although partially — , and of 

 form-analogy of position-isomeric benzene-derivatives^) is particularly 

 interesting. 



Mixed crystals were obtained by me from solutions of both com- 

 ponents in acetone -f- ether. 



They possess the form of fig. 1 and often exhibit the structure of 

 a sand time-glass or they are formed of layers. With a larger quantity 

 of the lower-melting deri\'ative, long delicate needles were obtained 

 which are not readily measurable. The melting points lie between 

 ± 75" and 84^°; I will determine again more exactly the mixing limits. 



C. Nitro-2-4-Dibromobenzeiie. 



Structure : CeH, . (N0J(,) . Br(2) . Br(4) ; ra. p. 61°. 6. 



Recrystallised from alcohol, the compound forms large crystals 

 flattened towards a and elongated towards the c-axis. They are of 

 a sulphur colour. 



Triclino-pinacoidal. 



a:b:c = \,\Z01 

 A— 97°13V; 

 B= 113°30\/; 



Cz=z 90°38V; 



1,1698. 

 a= 97°36' 

 /?= 113°37' 

 y= 87°33' 



Fig. 4. 



Forms observed : a = |100j predominant and 

 very lustrous; b = |010| and c = |001}, equally 

 broad, both strongly lustrous; p== |110|, narrow 

 but readily measurable; ö^jlllj, large and 

 yielding good reflexes. 



The compound has been measured previously, 

 by Groth and Bodewig (Berl. Berichte, 7, 1563). 

 My results agree in the main with theirs; in 

 the symbols adopted here, their a- and />-axes 

 have changed places and the agreement with 

 the other derivatives of the series is more 

 conspicuous. 



1) The examples now known are 1-2-3-5-, and l-'i-i-Q-Tribwmobenzene ; 

 1-^-3-b-Tribromo-iS-DinUro- and l-^-i-Q-Tribromo-3-b-Dmitrotohiene\ and 1-2-5-, 

 and 1-^-3-Nitrodibromobenzene, partially miscible. 



