40 TERMINOLOGY. . 48. 49. 



With these degrees of regularity, the rest of the quality 

 of simple forms is in the closest agreement, as will be seen 

 hereafter. The first degree of regularity contains two of 

 those forms, and the second one of those forms, which are 

 geometrically regular. 



. 48. CLASSIFICATION OF SIMPLE FOEMS. 



Simple forms are divided, according to the num- 

 ber of their principal axes, into such as have only 

 One Principal Axis, and such as have Several. 



The forms with one axis are of the third and fourth, 

 those with several axes are of the first and second degree 

 of regularity. 



. 49. NOMENCLATIVE OF SIMPLE FOBMS. 



The forms of one axis receive their names ac- 

 cording to the figure of their faces, or according to 

 some general property ; those of several axes, ac- 

 cording to the number of their faces ; and when 

 a more accurate determination is necessary, accord- 

 ing to certain peculiarities of these forms them- 

 selves. 



Systematic nomenclature remedies the want of confer- 

 mity and precision, which has hitherto prevailed in the 

 method of denominating crystalline forms ; and at the same 

 time produces a distinct idea of the forms themselves, since 

 it is in fact their abridged description. This shews the use- 

 fulness of the systematic nomenclature, and justifies its in- 

 troduction. 



Forms of a single axis, whose faces are rhombs, are term- 

 ed Rhombohedrons ; others, whose faces are triangles, are 

 called Pyramids. 



A form of several axes, which is contained under four faces, 

 is a Tetrahedron, or the Tetrahedron, because there exists 



