74 TERMINOLOGY. 



the forms of cleavage crystals arej the octahedral form of cleaV* 

 age in Fluor and the Diamond, the dodecahedral in Blende, the 

 rhomboidal in Carbonate of Lime, and Bitter Spar, and the hexago- 

 nal in Apatite ; in each of which species, the same forms occur 

 among their natural crystals. In these cases, and others which 

 are similar, it is to be understood that the faces under which the 

 form of cleavage is contained, are parallel to the faces of the crystal- 

 line form, and therefore the solids are perfectly similar in the rela- 

 tions of edges and angles. 



In the correspondence above referred to, abstraction is made of 

 those slight alterations which crystals suffer in consequence of the 

 replacement of their edges and angles. 



2. But there are a few instances where the solid yielded by cleav- 

 age, does not resemble the crystal from which it is obtained, or any 

 others of the same species. Of this, the form of cleavage in Corun- 

 dum which is a Rhomboid, is an example. The form possessed by 

 the crystals of this substance, is either that of an hexagonal Prism, 

 or of an hexagonal Prism surmounted by a six sided pyramid. A 

 second example of this disagreement occurs in the cleavage forms 

 of the Leucite, which (as has before been noticed) are, a Cube, and 

 a rhombic Dodecahedron, while its only crystalline form is that of 

 the trapezohedron. 



According to the present proposition, the hexagonal Prism is de- 

 rivable from the Rhomboid, and the trapezohedron from the rhom- 

 bic Dodecahedron, agreeably to those transitions of one form into 

 another from symmetrical replacements upon their edges or angles, 

 or both. That this is the case, may be perceived by referring to 

 . 50. If the hexagonal Prism be shown to sustain this relation, it 



peculiarity, in the right quadrangular prism, particularly denoted by the 

 right rectangular Prism. 



Where the form of cleavage presents solids, contained under dissimi- 

 lar faces, as the right rectangular Prism, it is natural to infer that those 

 cleavages which are parallel to the larger faces, will take place with 

 greater difficulty than those parallel to the smaller faces, since the re- 

 sistance produced by a large surface must necessarily be greater than 

 that by a small ; accordingly, those faces which correspond to cleavages, 

 effected with the greatest facility, and which of course present the most 

 distinct planes, are regarded as the lesser, and those on the other hand 

 which are effected with more difficultly, are conceived to belong to the 

 larger faces. 



