384 Count de Bournon on Bardigliorte, 



of the Individuals that compose the species are afterwards found to 

 be destitute. 



Thus for instance M. Cordier, in a paper in which he has de- 

 scribed with great perspicuity and accuracy a substance of a violet- 

 blue colour, which is met with either at Cape de Gat in 

 Spain, among volcanic products, or near Nijar, also in Spain, in a 

 granitic rock, gives this substance the name of Dichrdite^ a word of 

 Greek etymology implying double colour^ because its crystals present 

 a very deep blue when viewed in a direction parallel to their axis, 

 while they appear of a brownish-yellow, when viewed in a direction 

 perpendicular to this axis. But mica exhibits precisely the same 

 phenomenon of refraction. I have a variety from Somma in very 

 fine short hexahedral prisms, which, if viewed perpendicularly 

 to their axis, are of a green, more or less deep in proportion to 

 their thickness ; but when viewed parallel to that axis, through their 

 sides, are of a very deep reddish-orange yellow. A specimen in my 

 collection, likewise from Somma, and perhaps unique for the beauty 

 of the very bright, slightly greenish-yellow topazes it includes, con- 

 tains small crystals of mica, in incomplete acute hexahedral pyramids, 

 very transparent, and having the lustre of the hardest stones ; these 

 have a slightly yellowish-red colour, refracted through the sides of 

 the prisms, in consequence of which they are pretty constantly mis- 

 taken for very fine garnets. I have also some specimens of that 

 quartz, which comes from Macedonia, and is known by the name of 

 Leuco-sapphire, polished en cabochon^ which, seen in one direction, 

 are of a light bluish-grey, or nearly colourless, while in a direction 

 perpendicular to the former, they have the fine blue of the deepest 

 coloured sapphire. Dr. Wollaston, to whom mineralogy is daily under 

 important obligations, has observed tourmalines likewise possessing 

 the same property. If a person, who has never seen the Dichroite^ 



