266. Account of some new Vesuvian Minerals. 
olours. Topaz-yellow, brown, and colourless. It is 
found limpid and transparent. 
Dimensions. The largest crystals hitherto found do not 
ordinarily exceed six millimetres in diameter. One crystak 
of the fourth variety has 12 millimetres length, 8 breadth, 
and 6 thickness. : 
Position. Granitoid aggregates of pyroxene and mica 
usually contain Biotina mixed with its fragments. These ag- 
é S are not firm, and are found in the matters ejected 
in different eruptions: The crystals are distinguished from 
those accompanying them, by their superior splendour. 
Distinguishing Characters: Yt is easily distinguished 
rom those crystals which have the same primitive form, as: 
t is di 
‘ongly scratches quartz, while this scarcely scratches glass- 
Cymophane has the straight rectangular prism for its primi- 
‘ve form, while Biotina has the rhomb. 
