210 TERMINOLOGY. . 159. 



treating of compound minerals ; at present it is only ne- 

 cessary to examine in what shape an individual will ap- 

 pear, which is prevented from assuming its regular form 

 by some external obstacle. 



159. DEVIATIONS FROM REGULARITY, DEPEND- 

 ING UPON THE FORMATION OF THE INDIVIDUALS 

 THEMSELVES. 



Those deviations from the regularity of crystal- 

 line forms, which arise from the formation of the 

 individuals themselves, refer both to simple forms 

 and to combinations. They appear either in the size 

 and figure, or in the physical quality of their faces. 



The deviations from regularity take place in two differ- 

 ent ways : 



1, By the disproportionate and irregular enlargement or 

 decrease of some of the faces, or 



2, By their curvature, or in general by the property of 

 not being mathematical planes. 



With respect to the first, it must here be observed, that the 

 regular enlargement of certain faces, considered above in the 

 combinations called hemi-rhombohedral, hemi-pyramidal, 

 &c., does not enter within the limits of our present exami- 

 nation ; on the contrary, simple forms and combinations of 

 that kind may, as well as any others, be subject to those 

 deformities of which we are now treating. 



We find many examples of such irregularities in simple 

 forms. The faces of the hexahedron, for instance, very 

 often are not squares, but oblong or rectangular figures ; 

 sometimes only two of them are squares. Of the faces of 

 the octahedron, four become irregular tetragons, and two 

 equiangular hexagons. The whole form of the dodecahe- 

 dron is sometimes elongated or shortened in the direction 

 of one of its axes : if this be a rhombohedral one, the 

 form will assume the aspect of a combination of the rhom- 



