216 TERMINOLOGY. . 160. 



. 160. DEVIATIONS FROM REGULARITY, DEPEND- 

 ING UPON THE CONTACT WITH OTHER INDI- 

 VIDUALS. 



Crystals may either touch on all sides, those mi- 

 nerals to which they are adjoining, or they may ad- 

 here to them only by some of their parts. 



Crystals, surrounded and inclosed by the solid mass in 

 which they are found, or in which they have been formed, 

 are in contact with this mass on all sides. This mass 

 may either be homogeneous (. 23.) to that of the crystals, 

 or may not be homogeneous with it. In the first case, 

 the regularity of the form is almost without any exception 

 so much disfigured, that not even a trace of it will re- 

 main. One of the individuals prevents the other individual, 

 by their contact, from assuming that regular form which is 

 peculiar to it ; and, in fact, we have very often occasion to 

 observe, that the individual really assumes this regular 

 form, whenever a part of it emerges from the contact with 

 other individuals. 



Examples of this are frequently found in the compound 

 varieties of rhombohedral Lime-haloide, and of other spe- 

 cies. The individuals in these compositions are real crys- 

 tals, which only have been prevented by their mutual contact, 

 from assuming their peculiar regular forms. Very often we 

 find cavities or empty spaces in the interior of sucli com- 

 pound varieties ; the individuals lining these cavities pre- 

 sent regular forms, wherever they do not touch the rest of 

 the compound mass. 



In these compositions the crystals sometimes lose only 

 the regularity of their form, while they still continue to 

 present its general aspect. Thus, for instance, the particles 

 of such species whose forms belong to the tessular system, 

 often Jhave their three dimensions nearly equal, while many 

 of those of other systems have two dimensions greater or 

 less than the third. The System of Crystallisation, however, 

 cannot be inferred from this observation. If one or two of 



