8 PHYSIOGRAPHY. . 256. 



by their crystallographic signs, and partly also illustrated 

 by figures. It may be supposed here, that the use of the 

 signs will be entirely familiar to those who have read the 

 preceding part of the present work. Upon this supposition 

 the few lines referring to the combinations will be more 

 useful to the reader, than the descriptions of crystalline 

 forms commonly to be met with in mineralogical books, 

 which, though sometimes extended to several pages, yet 

 seldom suffice for representing with any degree of accuracy, 

 combinations of three or four simple forms, far less such as 

 are still more complicated. These descriptions never can 

 arrive at any thing like mathematical precision, whereas 

 the crystallographic designation employed in this work ad- 

 mits of the most accurate calculations, and will answer 

 every crystallographic question, which regards the desig- 

 nated compound form. 



The phenomenon of cleavage being in the nearest rela- 

 tion to the crystalline forms, the next place in the col- 

 lective description has been assigned to it. The forms of 

 cleavage are likewise represented by means of their crystal- 

 lographic signs ; and as to the faces of cleavage, due atten- 

 tion has been paid to their degree of perfection, some of 

 them being more easily observed, and on that account dis- 

 tinguished from others, which require a more minute ex- 

 amination, sometimes even the assistance of a very intense 

 light (. 162.). Thus in rhombohedral Lime-haloide the 

 faces of cleavage obtained in the direction of the faces of 

 the rhombohedron R, commonly present high degrees of 

 perfection, while thos2 in other directions, if ever they 

 occur, generally appear less distinctly. 



Fracture, as far as it is contained in the collective de- 

 scription itself, refers only to simple minerals. Although 

 in itself rather insignificant, yet for the sake of complete- 

 ness we cannot pass it by unnoticed. Several varieties of 

 fracture, if mentioned in one and the same place, denote 

 the limits, between which the varieties range, which occur 

 in the species. Also in respect to fracture, it will be indi- 

 cated, whether it be easily or difficultly obtained, lihom- 



