240 HISTORY OF MINERALOGY. 



represented by the singly-oblique or doubly-oblique 

 prism; they are termed by Weiss ttvo-and-one mem- 

 bered, and one-and-one membered; by other wri- 

 ters, Monoklinometric, and Triklinometric Systems. 

 There are also other peculiarities of Symmetry, 

 such, for instance, as that of the plagihedral faces 

 of quartz, and other minerals. 



The introduction of an arrangement of crystal- 

 line forms into systems, according to their degree 

 of symmetry, was a step which was rather founded 

 on a distinct and comprehensive perception of ma- 

 thematical relations, than on an acquaintance with 

 experimental facts, beyond what earlier minera- 

 logists had possessed. This arrangement was, how- 

 ever, remarkably confirmed by some of the properties 

 of minerals which attracted notice about the time 

 now spoken of, as we shall see in the next chapter. 



