192 ALUMINA. 



oxyd of iron 4'5, fluoric acid 0*7, water 1*8. 'Before the 

 blowpipe infusible, but becomes opaque white. 



Varieties. — A variety in which the scales are arranged in 

 a plumose form is called plumose mica ; another, in which 

 the plates have a transverse cleavage, has been termed pris 

 matic mica. 



Dif. Mica differs from talc in affording thinner folia and 

 being elastic ; also in not having the greasy feel of tha. 

 mineral. The same characters, excepting the last, distin- 

 guish it from gypsum ; besides, it does not crumble so readily 

 on heating. 



Obs. Mica is one of the constituents of granite, gneiss 

 and mica slate, and gives to the latter its laminate structure. 

 It also occurs in granular limestone. /Plates two and three 

 feet in diameter, and perfectly transparent, are obtained at 

 Alstead, Acworth and Grafton, New Hampshire. Other 

 good localities are Paris, Me. ; Chesterfield, Barre, Brim- 

 field, and South Royalston, Mass. ; near Greenwood furnace, 

 Warwick and Edenville, Orange county, and in Jefferson 

 and St. Lawrence counties, N. Y. ; Newton and Franklin, 

 N. J. ; near Germantown, Pa., and Jones's Falls, Maryland. 

 Oblique prisms from near Greenwood are sometimes six or 

 seven inches in diameter. 



A green variety occurs at Unity, Maine, near Baltimore, 

 Md., and at Chestnut Hill, Pa. Prismatic mica is found at 

 Russel, Mass. 



Uses. Mica, on account of the toughness, transparency 

 and the thinness of its folia, has been used in Siberia for glass 

 in windows : whence it has been called Muscovy glass. It 

 was formerly employed in the Russian navy, because not 

 liable to fracture from concussion. It is in common use for 

 lanterns, and also for the doors of stoves. It affords a con- 

 venient material for preserving minute objects for the micros- 

 cope, and is sometimes used for holding minerals before the 

 blowpipe flame. 



The best localities of the mineral in this country for the 

 arts, are those of New Hampshire. 



Lepidolite, or Lithia mica. Occurs in crystals or laminae, of a pur- 

 plish color, and often in masses consisting of aggregated scales. A 

 gpecimen from the Ural consisted, according to Rosales, of silica 477 t 



How does mica differ from talc and gypsum ? Of what rocks is 

 t a constituent ? What are its uses 1 What is the peculiarity of 

 fejjidolite ? 



