( 27 ) 



The ci'jstals, therefore, sliow a decided approach to the mono- 

 clinic system ; on account, liowever, of their optical orientation, they 

 can only be credited with a triclinic symmetry. 



The forms observed ai-e : a = jlOOj, strongly |)redominant and very 

 lustrous; /; = |01ü}, smaller but yielding good i-eflexes ; c = |00J|, 

 narrower than a, bnt very lustrous ; o = jlil}, well developed and 

 very lustrous ; to :=: |ill|, smaller but very distinct ; .v = jillj, very 

 narrow but readily measurable. 



The habit is elongated towards the /^-axis with tlattening towards jJOOj. 



Pleasured : Calculated : 



a:h = (100) : (OlO) =* 90^67; — 



a:c = (100) : (001) =* 69 23 — 

 a\o = (100) : (ill) =* 65 11 



c:o = (OOl) : (111) =* 75 477^ — 



b:o = (OlO) : (iTf) =* 36 6 — 



a: w = (100): (lfl)=r 50 52 50^49' 



c : oj = (001) : ( ill) r= 56 52 56 43 



h : o) = \(dlO) . {Ill) = 46 28 46 35 



: (o = (1ÏÏ) : (111) = 47 13 47 2973 



a:s = (TOO) : (liT) = 49 59 50 4973 



h : s = (OTO) : (Iff) =: 45 48 45 527, 



C : s = (OOf) : (111) = — 56 4 



: s = (111) : (Til) = 63 39 63 5973 



Readily cleavable, parallel |100|. 



The extinction on |100| amounts to about 267-2 in regard to the 

 ó-axis ; in convergent light a hyperbole is visible occupying an eccen- 

 tric position. 



The sp. gr. of the crystals is 2,305 at 87 the equivalent volume 121.47. 



B. Nitro-2-5-Dibroino-Benzene. 



Structure: C„H3 . (NOjr,) . B^o) • Br,,); m.p. : 84°,5. 



This compound has been |)reviously studied crystallographically by 

 G. Fels, (Zeits. f. Kryst. 32, 377). This paper, however, contains 

 several errors, which render a renewed investigation desirable ; more- 

 over, another choice of axial (coordinate) planes is required, which 

 makes the crystals show more analogy with the other triclinic terms 

 of this series. 



The crystals deposited from acetone -}- ligTOïn have the form of 

 small plates flattened towards {00J| (tigs. 2 and 3). They are pale 

 yellow and very lustrous. 



