CLEAVAGE. 59 



readily in general, will sometimes present a con- 

 choidal fracture.* 



As a crystalline solid cannot be contained within planes 

 lying in less than three directions, it is obvious that it 

 cannot be produced by a single or double cleavage. 



The solids obtained by cleavage may therefore, 

 according to what has preceded, consist either of 

 primary forms produced by triple, fourfold, or sixfold 

 primary cleavages, or of other forms resulting from 

 the supernumerary cleavages, either alone or com- 

 bined with the primary. 



But another class of solids may also result from 

 cleavage when that takes place parallel to some only 

 of the primary planes of those forms which possess 

 fourfold or sixfold cleavages. 



From a primary triple cleavage it. is clear that only 

 a single solid can be produced, that solid being a 

 parallelepiped . But from either a fourfold or sixfold 

 primary cleavage, more than one solid may result, 

 according as the cleavage takes place parallel to all, 

 or only to some, of the primary planes. 



* Some practice is necessary in order to cleave minerals neatly, and 

 some experience in the choice of the instruments to be used for this 

 purpose. 



In many instances, the mineral being placed on a small anvil of iron 

 or lead, a blow with a hammer will be sufficient for dividing it in" the 

 direction of its natural joints ; and sometimes a knife or small chissel 

 may be applied in the direction of those joints, and pressed with the 

 hand, or struck with a hammer; or the crystal may be held in the 

 hand and split with a small knife; or it may be split, by means of a pair 

 of small cutting pincers whose edges are parallel. 



A small short chissel, fixed with its edge outward in a block of wood, 

 is a convenient instrument for resting a mineral upon which we are 

 desirous of cleaving. 





