a Nezo Mineral Specksjbund in Gle-nco. 219 



Between the faces F and A, I observed other two very imper- 

 fect ones, which were inclined 156° and 147° respectively to 

 A. 



From the irregularity of this prism, of which only two of 

 the opposite sides are parallel, Mr Haidinger, who had like- 

 wise measured the angles, was led to believe that the crystals 

 are compound, and by computing the angles of the com- 

 pound prism, on the supposition that one of the individual 

 crystals was turned round 180°, he obtained a very satisfac- 

 tory confirmation of his opinion. 



The hardness of Withamite is about 6.5, scratching glass 

 with facility. Its specific gravity, as determined by Dr Tur- 

 ner, is about 3.1537, and the specific gravity of the rock about 

 2.669. 



In examining the optical characters of this substance, I 

 have experienced considerable difficulties from the minuteness 

 of the crystals. By plunging them, however, in oil of cassia, 

 the oil of the highest refractive power, I was enabled to ascer- 

 tain that their ordinary refraction greatly exceeds that of oil 

 of cassia, — that their double refraction, which is considerable, 

 is negative in relation to the axis of the prism, — and that the 

 two images may be easily separated by looking through any of 

 the two acute angles of 03° or 76°. 



The most interesting optical property of Withamite is its 

 dichroism, or double colour, which it exhibits both in common 

 and polarised light. When common light is transmitted 

 through the two parallel faces of the prism, the tint is of a 

 crimson or amethyst colour, with a mixture of straw yellow. 

 Upon turning the crystal round, the yellow tint disappears, 

 and the colour becomes a deep crimson red. On continuing 

 to turn the prism, the colour changes to a straw yellow, and 

 at the end of half a revolution the crystal resumes its com- 

 pound tint. In the groupes of crystals which have penetrated 

 the quartz, some of them occupy, accidentally, the position 

 which gives the yellow colour, — others that which gives the 

 red colour, and some that which gives the compound tint ; so 

 that, without a knowledge of their dichroitic property, the 

 groupe might have been considered as composed of three dif- 

 ferent sets of crystals. 



