CLEAVAGE, 



Fig. 94. 



If the cleavage take place parallel only to the 

 planes c d and i k, and be continued until only the 

 four cleavage planes remain, an irregular tetrahedron, 

 fig. 94, will be produced, whose planes meet at an 

 angle of 90 at the edges n o, p q, and at an angle 

 of 60 at the other edges. 



Thus an obtuse octahedron, d on m, fig. 92, mea- 

 suring 60 % an obtuse rhomboid of 120", and an" 

 irregular tetrahedron , obtained by partial cleavages 

 from the rhombic dodecahedron, may be regarded as 

 imperfect dodecahedrons, to which figure the?/ may be 

 reduced, by detaching from each solid the portions of 

 cubic molecules which respectively cover the obscured 

 dodecahedral planes. 



But it is very obvious that these imperfect forms 

 may be obtained as well by cleaving at once through 

 the interior of the crystal, in directions parallel to 8 to 

 6 or to 4 only of the primary planes, as by beginning 

 to cleave from the outside, and arriving by degrees at 

 the new figure in the manner already described. 



Hence it appears that a dissection of the octahedral 

 and dodecahedral crystals by cleavages, parallel to some 

 only, of the primary planes, will yield only the imper- 

 fect solids above described, not any of which will 

 represent the molecules of which the crystals are com- 

 posed.* 



* Blende will afford the student an opportunity of producing, by 

 cleavage, the solids represented by fig-. 91 to 94. 



