372 Review of Dana’s Mineralogy. 
Sillimanite, Kyanite and Andalusite, minerals chemically identi- 
cal, but mineralogically considered, distinct. 
Chapter IV, is on the deter- 
mination of primary forms; 
we find under it the following 
figure and description of an 
ingenious and useful improve- 
ment in the reflecting goniom- 
eter of Wollaston, for adjust- 
ing the crystals; it is drawn 
from a German instrument. 
The contrivance acd is al- 
so an important addition. It 
contains a slit at d for sight- 
ing the crystals, by using 
which, one of the lines may 
be dispensed with. It slides 
up and down’ in the part ad, 
and also moves back and forth, 
parallel with the plane of the graduated circle, on the pivot by 
which it is attached to the stand of the goniometer. 
The chapter on practical erystallogeny is au interesting one, 
-and largely illustrated by facts drawn from American sources. 
_ Crystallized minerals, especially when the individuals are large, 
are so rarely homogeneous in structure, that the attention of chem- 
ists (as before suggested, p. 367,) ought to be directed specially 
to a consideration of the circumstances under which the crystal 
was produced, before deciding definitely as to the essential na- 
ture of minute quantities of accidental ingredients, particularly if 
the mineral owes its origin to crystallization from solution. It 
seems probable from the observations of Beudant, that symmetr- 
cal crystals are seldom produced in clear or homogeneous So! 
tions. Quartz, if pellucid and pure, is almost never regulat 
or normal in the relation of its several secondary planes, while 
the highly ferruginous quartz, from Expailly, is always in Teg 
lar bipyramidal prisms, although the quantity of foreign matter 
mechanically disseminated through the crystals, is such as 10 
make them quite opaque. We see then the risk incurred in as 
suming that regularly crystallized opaque minerals are of course 
free from accidental impurities. 
io 
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