462 
Class I. 
Order I. 
Crystals. 
SIMPLE STONES. 
is directed against the edges of the crystal (stuck upon 
glass), it easily melts into a clear compact glass ; but 
when the flame is directed against the faces, they pre 
serve their lustre, and the edges alone slowly melt. 
According to the analysis of Dodun, it is composed 
46 silica 
36 alumina 
16 oxide of iron 
98 
When this stone is exposed to the atmosphere, it is 
apt to decay : Its surface becomes iridescent, and at 
last changes to ochre-yellow : Its specific gravity is 
2.3 or 2.212; and when breathed upon, it gives out an 
earthy smell. 
Sp. 6. Mica*. 
This stone forms an essential part of many moun 
tains, and has been long known under the names of gla 
cies maria and Muscovy glass. It consists of a great 
number of thin laminæ adhering to each other, some 
times of a very large size. Specimens have been found 
in Siberia nearly 24 yards square?. 
It is sometimes crystallized : Its primitive form is a 
rectangular prism, whose bases are rhombs, with an 
gles of 120° and 60°: Its integrant molecule has the 
same form. Sometimes it occurs in rectangular prisms, 
whose bases also are rectangles, and sometimes also in 
short six-sided prisms ; but it is much more frequently 
in plates or scales of no determinate figure or size f. 
* Kirwan, i. 210.—Gmelin, Nov. Com. Petropol. xii. 549. 
Hict. General de Voyages, t. xvii. 272. quoted by Hauy, Jour. de Mia. 
No. xxviii. 299. 
t Hauy, Jour. de Min. No. xxvii. 296.