( 200 ) 



The position of the optical elasticity-axis on a was determined bv 

 means of a very thin lamelle, with the aid of a Bkrtrand Quartz- 

 Ocular, and found to be 4° with the side (100) : (010); on b the 

 angle amounts to more than 45° with respect to the side (110) : (010), 

 An axial image could not be observed. 



By means of a Thoulet's solution, the sp. gr. of the crystals was 

 found to be 2,459 at 17°; the equivalent volume is, therefore, 

 170,39, and the topical axial relation becomes: 



X : If' : w = 3,6982 : 6,9496 : 6,6584. 



The close relation of this liicliiiic deriNative witii the two mono- 

 clinic isomers is therefore olixious. That tiiere is still an essential 

 dilference in the nature of the molecular orientation, which in these 

 three cases determines the crystalline structure, is also shown by 

 the fact, that the tirst two derivatives exhibit a decided tendency to 

 a similar and typical twin-formation, which is utterly absent in the 

 latter isomer. 



I will give lateron some information as to the other nitro-products 

 of the isomeric tribromotoluenes, as these have not as yet been 

 obtainable in measurable crystals. 



