SECTION IV. 

 CLEAVAGE. 



The spilling a crystal in the manner already des- 

 cribed, is, in the language of Mineralogy, termed 

 cleaving it. 



The direction in which the crystal can be split is 

 called the direction of the cleavage , or the natural 

 joint of the crystal. 



The direction of the natural joints may depend, 

 according to the preceding theory, upon the com- 

 parative degrees of molecular attraction existing in 

 the different directions of the molecules. This may 

 be so proportioned in different directions, as to occa- 

 sion other cleavages than those which are parallel to 

 the planes which we may assume as the primary 

 planes, as in the instances already cited of the corun- 

 dum, and tungstat of lime. 



When this occurs the crystal is said to possess two 

 or more sets of cleavages. Those which are parallel 

 to the planes of the primary form, are called the 

 primary set, and those which are not parallel to those 

 planes are termed supernumerary sets. 



The oxide of tin, described by Mr. Phillips in the 

 Geological Transactions, has three sets of cleavages ; 

 one parallel to the planes of an obtuse octahedron 

 with a square base, which is considered the primary 

 set, and two others which are sitperminierftr?/, and 



