ORDER 



11HOMBOHEDRAL TALC-MICA. 



and this is the case in the micas from St Gothard and from 

 Altenberg in Saxony. Several crystallised varieties from 

 Vesuvius, of a pale green colour, and others from various 

 localities, presenting green, brown, and black colours, pos- 

 sess one axis of double refraction. The forms of the mi- 

 nerals called Mica thus appear to belong to three different 

 systems of crystallisation ; they are sufficient to shew how 

 much there is yet to be done in examining their varieties. 

 2. Not less at variance, and partly in opposition with 

 each other, we find the results of chemical analyses of the 

 different kinds of mica, as comparatively instituted and 

 published by KLAPHOTH, ROSE, and PESCHIER. 



From a comparison of these analyses, it appears that there 

 exist differences among the varieties of mica, which, how- 

 ever, it is impossible to reduce to fixed points, so long as 

 we are in want of an accurate natural-historical deter, 

 inination of the species. This determination must flow 

 from principles of Natural History, and can the less 

 depend upon chemical relations, as the very object of a 

 great part of the present researches of chemistry consists 

 in examining the nature of the composition in those bodies 

 which form species in Natural History. Before the blow- 

 pipe, several varieties first lose their transparency, and then 

 melt into a scoria, white or coloured, or even black. Others 

 are infusible, and they shew in general as much difference 

 in this respect as in their composition. 



