TERMINOLOGY. . 141. 



of the forms as the preceding one (. 139.) } in every combi- 

 nation. 



. 141. SYMMETRY OF COMBINATIONS. 



The Symmetry of combinations is founded upon 

 the two laws in . 139. and . 140. 



The Symmetry of combinations consists in the same- 

 ness of disposition of the faces, edges, and angles of each of 

 those simple forms which they contain, in respect to the 

 homologous parts of the other ; or it consists in the same- 

 ness of situation of the different edges and angles produced 

 by the combination of these simple forms. Symmetry re- 

 fers only to combinations, Regularity only to simple forms 

 ( 45.). 



All combinations produced by nature are symmetrical, 

 and experience thus confirms the truth of the two above- 

 mentioned laws ; since the symmetry of combinations de- 

 pends upon the relative dimensions and the position of 

 simple forms. These Laws of Combination, and not the 

 Symmetry of the latter, are fundamental laws in Crystallo- 

 graphy, because the latter is a mere and necessary conse- 

 quence of the former. 



Sometimes there occur in nature apparent exceptions 

 to this Symmetry. Yet they are merely accidental, and 

 arise from an unequal and disproportionate enlargement of 

 certain faces of Crystallisation ; and this sometimes goes so 

 far, as to cause some of these faces entirely to disappear. 

 Sometimes, however, certain faces are enlarged, and others 

 diminished, according to constant laws, and then the sym- 

 metry is not destroyed altogether, though it assumes a pe- 

 culiar character, different from what it has been before. 

 The differences thus produced, are called the Character of 

 Combinations, and will be considered afterwards in greater 

 detail. 



In considering the combinations, we must abstract from 

 all casual deviations from symmetry, as in like manner has 



