( 79 ) 



When reciystallised from ether, very strongly refractive, hexagonal ly 

 bounded little plates are obtained. On investigation it appears that 

 these are twin crystals: jOOlj is the twin-plane with a twin-axis 

 standing normally on it. 



In addition to c = |001|, /> = jOlOj, r' = |101| and 7^' = |lll|, I 

 noticed a form q:=\011\; c is strongly developed, b and q are 

 narrow, r' and u' are equally broad and well-formed. The planes of 

 cleavage are the same as above. 



There were also measured : 



q:h = (01ï)(2) : (010)(,) z=z 23°58' ; calculated : 23°42'. 

 h:h = (010)(,) : (010)^2) = — 19°23' ; calculated : — 19°28'. 

 r:r = (lOT)r,^ : (10l>2) = 21°45' ; calciilate'd : 22°2'. 

 c:q = (001) : (Oil) = 58°21' ; calculated : 58°9'. 



The nature of the twin-formation has, therefore, been sufficiently 

 explained. 



On jOOlj, the direction of the optical elasticity -axis is orientated 

 nearly perpendicularly on the sides (001) : (010) ; an axial image 

 could not be observed. 



The specific gravity of the crystals was determined by means of 

 a Thoulet's solution; <:/= 1,343, at 16'' and consequently the topical 

 axes, calculated according to the formule: 



V 



c sin ^ sin y sin A 



v., 



\xp 



V 



OJ 



ac sin /? sin 7 sin A 

 a sin /? sin y sin A 



Vs 



M 



in which F=:— , if J/ represents the molecular 



weight, are equal to: 



X : ip : fo = 4,8513 : 5,7458 : 7,8145. 



] j "i Z).) During a cold winter night, I once obtained 



from a solution of the compound in benzene a 

 second, less stable modification. 



There were formed transparent hexagonal bounded 

 little plates; the crystals were single and not twin 

 crystals as in the former case. The melting point 

 was situated at J 15°; the little crystals, however, 

 soon became opaque so that on warming a mole- 

 cular transformation probably takes place. But I 

 have only once succeeded in obtaining this modification, 



Fig. 2. 



