226 ON THE APPLICATION OF THE 



us to refer our crystal to any particular class of 

 primary forms ; and by comparing the characters we 

 thus observe with those described in the tables, we 

 shall probably discover the class of primary forms to 

 which our crystal belongs. 



Let us now suppose our crystal to be contained within 

 any series of vertical planes, and to be terminated, not 

 by a horizontal plane, but by a single oblique plane, 

 the crystal may then belong to the class of oblique 

 rhombic prisms, doubly oblique prisms, or rhomboids. 



If there be four oblique planes, inclining to each 

 other at equal angles, the crystal may belong to the 

 class of square prisms, or of octahedrons with square 

 bases. 



If there be four oblique planes, each of which in- 

 clines on two adjacent planes at unequal angles, the 

 crystal will probably belong to the class of right 

 rectangular prisms, right rhombic prisms, or octa- 

 hedrons with rectangular or rhombic bases. 



If the series of vertical planes consist of 6, 9, 12, or 

 some other multiple of 3, and if there be a single hori- 

 zontal plane, the crystal may belong to one of the 

 classes of right prisms, rhomboids, or hexagonal 

 prisms. 



If there should be three oblique planes, the primary 

 form is a rhomboid. 



But if the termination consists of six oblique 

 and equal planes, the crystal may belong to the class 

 of rhomboids or hexagonal prisms. 



Crystals not falling within any of the preceding 

 descriptions, may yet be found to resemble some of 

 the secondary forms given in the tables. 



Those which belong to the class of doubly oblique 

 prisms, are sometimes very difficult to be understood; 

 and the relation between the primary and secondary 

 forms of this class, can be learned only by a coin- 



