CETSTALLIFE FOKMS. 



89 



terminating in an edge, like that of a chisel, it is said to be 

 bevilled. 



When a solid angle is replaced by three or more converg- 

 ing planes, it is said to be acuminated, that 

 is, it is replaced by planes meeting in a point. 



The forms of crystals are divided into pri- 

 mary and secondary, or derived forms. This 

 distinction is founded on the relation of certain 

 geometrical solids to each other ; on the tran- 

 sition of one form to another exhibited in 

 many minerals ; on the replacement or trun- 

 cation of their edges and solid angles; and on the facility with 

 which most crystallised minerals split in certain directions; so 

 that, however various the forms of their crystals, they may a]l 

 be reduced by cleavage, into some simpler form, presenting 

 smooth and shining faces, like the natural planes of a crystal. 



To exemplify this process, the student has only to take 

 some substance which may be cut with ease, as a piece of 

 soap or cork, and practise these cleavages as follows : First 

 cut the object into a cube, thus (fig. 19). 



FIG. 18. 



FIG. 19. 



FIG. 20. 



Then replace the solid angles ; in plainer terms, cut off 

 the corners, thus (fig. 20). 



Continue the process (taking care to preserve the layers 



FIG. 2 



FIG. 22. 



thus removed) till the planes of the cube disappear, and the 

 octohedron will be the result (fig. 21). 



