TERMINOLOGY. . 164. 



however, particularly in respect to lustre, will be consider- 

 ed more accurately on another occasion. 



Faces of cleavage, which are of the same quality, and ap- 

 pear in one and the same individual, or in one and the 

 same species, are said to be homologous ; faces of cleavage of 

 different qualities, if they appear under the same circum- 

 stances, are considered as being faces of cleavage not homo- 

 logous with each other. 



Sometimes faces of cleavage appear to be curved. Irre- 

 gularities of this kind require to be considered, like those 

 mentioned above in respect to the faces of crystallisation. 

 But. very often curved faces of cleavage result from the 

 composition of several individuals in a position little differ- 

 ent from the parallel one. Fig. 80. shews a remarkable in- 

 stance in rhombohedral Lime-haloide, where the axes of 

 the individuals diverge very little from a common centre, 

 by which the compound product of cleavage assumes the 

 appearance of a rhombohedron, of which three faces are 

 convex, and the opposite ones concave. 



. 164. DIRECTION OF CLEAVAGE. 



The direction in which the individuals of a spe- 

 cies allow themselves to be cleaved, is the Direction 

 of Cleavage. 



The direction of the faces of cleavage is constant ; its 

 situation in respect to the fundamental form, or any other 

 derived crystalline form of the species, is determined, and 

 not subject to any alteration. 



The directions of cleavage in every individual are not 

 found in the same number. The individuals of most of 

 those species which constitute the order Mica, can be 

 cleaved only in one direction, and therefore possess only 

 one direction of cleavage. Many species of the order Spar 

 contain two ; rhombohedral Lime-haloide and hexahedral 

 Lead-glance contain three ; octahedral Fluor-haloide con- 

 tains four ; dodecahedral Garnet-blende six ; and in many 



