30 TERMINOLOGY. . 3. 33. 



enlarged, and assumes a figure different from what it 

 should be, and dependent upon this enlargement. Crys. 

 tallography takes no notice of these irregularities, in as 

 much as they are accidental, and because this science is in- 

 tended to promote the study of the forms in their peculiar 

 Regularity and Perfection, in order to enable us to develope 

 their relations to each other, and to facilitate the applica- 

 tion of both to the phenomena of Nature* 



. 32. HOMOLOGOUS OR EQUAL EDGES. 



Edges are said to be of equal magnitude, or of 

 equal value, if the faces meeting in them are equally 

 inclined to each other, or produce an equal angle of 

 incidence ; they are said to be of equal length, if 

 they are formed by equal sides of the faces ; and if 

 they are both of equal magnitude and equal length, 

 and at the same time similarly situated, they are 

 termed equal or homologous. 



The inclination at the edges is invariable in nature ; and 

 upon this constancy of the angles of incidence, is founded 

 the application of crystallography to nature. The length 

 of the edges is subject to variation, as well as the figure of 

 the faces. In the crystals themselves, all the edges of 

 equal quantity which are similarly situated, although per- 

 haps not of equal absolute length, are considered as homo- 

 logous. 



. 33. DENOMINATION OF THE SOLID ANGLES. 



Solid angles are denominated according to the 

 number of faces contiguous to them, or according 

 to the quality of the edges produced by the inter- 

 section of these faces. Solid angles formed by homo- 

 logous faces, are said to be Homologous themselves 



