. 159. OF THE IMPERFECTIONS OF CRYSTALS. &09 



\ 



designated by - (P). (J)> while Fig. 78- is expressed 

 2 2i 



O C* 



by (P) -- _ . (r), since the trigonal-dodecahedron r in 



the latter combination is the inverse of the trigonal-dode- 

 cahedron I in the former, the tetrahedron in the normal 

 position being common to both. These combinations oc- 

 cur in tetrahedral Copper-glance. 



IV. OF THE IMPERFECTIONS OF CRYSTALS IN RESPECT TO 

 THEIR FORM. 



. 158. TWO KINDS OF THIS IMPERFECTION. 



The imperfections of crystals in respect to their 

 form, originate either in the very formation of the 

 crystals themselves, or they are the consequence of 

 the contact of these with other minerals. 



The imperfections of the crystalline forms are deviations 

 from that regularity which has been supposed to take place 

 in the preceding considerations of forms. This regularity 

 requires the faces of crystals to be planes of a certain 

 figure and extent, and the edges in which they intersect 

 each other to be straight lines. It is very seldom met with 

 in nature, perhaps never, if we examine the natural pro- 

 ductions with the utmost accuracy ; the deviations which it 

 presents are founded in some cases upon the formation of 

 the crystals themselves, if we find ourselves entitled to sup- 

 pose that nothing external has had any influence upon the 

 quality of the form ; in other cases they depend upon the 

 contact into which one crystal has come with another ; for 

 this is the means by which one of them could influence 

 the shape of the other. The most interesting of these ob- 

 jects is the consideration of the first kind of these imper- 

 fections. The other will be considered more at large when 

 VOL. i. o 



