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ReadiJj cleavable, parallel in. 



The optical orientation is that of Fhi,s, in which his forms |010|, 

 {001| and |llT| assume, respecti\cly, in inv project the symbols 

 |001|, |110| and jOlOj. It may be remarked that Fels has incor- 

 rectly stated the structure and also the melting point. Moreover, his 

 angles (11Ï) : (100) and (111) : (010) appear to be > 90\ Perhaps it 

 is owing to this, that the agreement between the calculated and found 

 values is with him so much more uiita\ourable than with me. I have 

 never observed forms |552| and |15 . 15 . 4| 



The sp.gr. ar 8° is 2,368; the equiv. volume: 118,06. 



Topical axes : / -. ip : to = 5,2190 : 3,5005 : 7,0758. 



On comparing the said position-isomeric derivatives, one notices at once 

 not the great similarity between the two compounds, which, although 

 constituting a case of direct-isomorphism, still y^ry closely resembles it. 



Nitro-2-o-Dlbromobenzene. 



Triclino-pinacoidal. 



a:h:c= 1,4909 : 1 : 2,0214. 



.4=91°3V; i?==113°21V; ^=90°27' 



«=90°57\/; ^=113^217,' y=90°2' 



i:\p:a)z= 5,2190 : 3,5005 : 7,0758. 



However : 

 Forms : jlOOj, jOlOj, jOOlj, |T01|, 

 |110|, |113|. 

 Cleavable parallel |110|. 

 Habit tabular towards |001|. 



N itro-2-^-Dlhromobenzene . 

 Triclino-pinacoidal . 



a:h:c= 1,4778 : 1 : 1,9513 

 ^=::90°30'i?=110°37' (7=90"a6V; 

 «=90°45V3'/?=110°367;y=89°59V; 

 X : ip : a> = 5,2565 : 3,5571 : 6,9409. 



However : 

 Forms : |100|, jOlOj, jOOlj, jlTfl, 

 jlllj and llITi 

 Cleavable parallel |100|. 



Habit tabular towards |100|. 



We, .therefore, still notice such a difTerence in habit and cleava- 

 bility that a direct isomorphism, in the ordinary meaning of the 

 word, cannot be supposed to be present. There occurs here a case 

 of isomorphotropism bordering on isomorphism. 



Notwithstanding that difference, both substances can form an 

 interrupted series of mixed crystals, as has been proved by the 

 determination of the binary melting point curve and also crystallo- 

 graphically ^). 



The melting point of the 1-2-3-derivative (53°) is depressed by 

 addition of the 1-2-5-derivative. The melting point line has also 



1) The binary raelling-curve possesses, — as proved by means of more a exact 

 determination, — a eutedic point of 52° G. at 2% of the higher melting com- 

 ponent ; therefore here the already published melting-diagram is eliminated. There 

 is a hiatus in the series of mixed-crystals, from ± i'^/o to circa 48"'o of the 1-2-3-deri- 

 vative. I shall, however point out, that the possibility of such a hiatus thermody- 

 naraically can be proved, — even in the case of directly- isomorphous substances. 



(Added in the English translation). 



