142 Dr Turner's Analysis of a Mica from Cornwall. 



the prism follows, =120° 7. The faces of this prism are 

 very rough, but those parallel to its short diagonal, which 

 change it into an hexagonal prism, are generally smooth, and 

 possess a distinct vitreous lustre. The plane of the resultant 

 axes of double refraction is parallel to this face. This variety 

 occurs along with crystals of topaz, quartz, and felspar. It 

 is deserving of notice, as Mr Haidinger remarks, that most 

 of the lithion-micas are attended by topaz and other mi- 

 nerals which generally accompany tin- ore. The lepido- 

 lite from Moravia is associated with white topaz ; the dark 

 grey mica from Zinnwald includes Pycnite, which is a com- 

 pound variety of topaz ; the Cornish variety, from St Mi- 

 chael's Mount, is found along with topaz and tin-ore ; another 

 Cornish variety from Carclaze, of a greyish-white colour, con- 

 tains granular masses and small crystals of white topaz ; the 

 brown variety, which I have analysed, is mixed with quartz 

 and apatite. Topaz has also been discovered at Zinnwald, but 

 the mica of that place is commonly associated with tin-ore, 

 quartz, wolfram, and tungstate of lime. It occurs in fine crys- 

 tals, and was selected for analysis by Klaproth as being a cha- 

 racteristic variety. My analysis of it is in progress, and I 

 have already obtained unequivocal proof of the existence of li- 

 thia in it. The solution of the mixed sulphates of lithia and 

 potash, after the addition of muriate of platinum, was brought 

 to perfect dryness, and the soluble parts taken up by a little 

 cold water. A white salt was procured, after removing the 

 excess of platinum, which fused readily when heated, and af- 

 terwards dissolved easily in cold water. It was decomposed 

 by acetate of lead, and an acetate of lithia was formed in the 

 way already described. When a particle of this salt was 

 brought into contact with the flame of a spirit-lamp, it instantly 

 communicated a fine red colour to it. When decomposed by 

 heat a white carbonate was procured, which was characteriz- 

 ed by its singular fusibility, its sparing solubility in water, by 

 acting upon the surface of platinum when fused upon it, and 

 by causing a brown stain when placed on moistened turmeric 

 paper. 



