372 



Review of Dana's Mineralogy. 



Fig. 9. 



a 



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 c 

 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 ab, 

 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 crystallogeny is an 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 symmetri- 

 cal crystals are seldom produced in clear or homogeneous solu- 

 tions. Quartz, if pellucid and pure, is almost never regular 

 or normal in the relation of its several secondary planes, while 

 the highly ferruginous quartz, from Expailly, is always in regu- 

 lar bipyramidal prisms, although the quantity of foreign matter 

 mechanically disseminated through the crystals, is such as to 

 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. 



