( 437 ) 



On acconnt of its great solubility in most solvents it is very difficult 

 to obtain lliis compound in a properly ci'ystallised form. The greatest 

 success is met with by very slow evaporation of its solution in dilute 

 alcohol when it crystallises in i)ale yellow, very thin laminae with 

 extended hexagonal peri[>hery ; the crystals are very transparent and 

 give sharp signal-images. 



The symmetry is inonodmo-pvismatk' . In the choice of the plane- 

 symbols adoi)ted here the axial relation is calculated on : 

 a:h-.c = 0,8935 -. i : 1,9198 



B^orms observed: .s=:jl()l|, strongly predominant and yielding 

 sharp reflexes; 'r=|101| much narrower, but gives a good reflection ; 

 ^ =: |001|, narrower than r ; g = {Ollj, also narrow, but reflects 

 w^ell. The habit is flattened along |101| with considerable elongation 

 along the /^-axis. 



The following angular values were determined : 



Measured -. Calculated -. 

 c.q = (001) : (Oil) =r * 62°21' — 



</:6^=(011):(10l) = *80^57' — 



c : r = (001) f (101) = * 60° 4' — 



7': 6^=: (101): (101)=: 49^45' 49°45' 



s:c = (lOf) : (OOT) = 70°11' 70°TJ ' 



A distinct plane of cleavage was not found. 



In the orthodiagonal zone the direction of extinction is orientated 

 everywhere perpejidicular to the direction of the ^>-axis. On |101| no 

 perceptible dicliroïsm is obserxable, 



The optical properties of the substance in convergent polarised 

 light are very remarkable. 



For the rod, yellow and most of the green rays of the spectrum 

 the axial plane = |010| ; cxtraordinai'ily strong, inclined dispersion ; 

 the axial angle for the red is much larger than that for the green. 

 The character of the double refraction is positive. 



On the other hand, the axial angle for the blue and violet rays 

 is situated perpendicular on |010| with a horizontal dispersion. The 

 axial angle for all rays is but small. 



The curious colourphenomena in white light exhibited b}' this 

 substance, which thus possesses at the same time inclined and hori- 

 zontal dispersion, lend themselves particularly well to the demon- 

 stration of anomalous dispersion in biaxial crystals. 



The sp.gr. of the crystals is 1.419 at 15" ; the equivalent volume 

 is therefore 126.85. 



Topic parameters : / : i|' : w — 3,7578 -. 4,2058 : 8,0744. 



