72 TERMINOLOGY. 



. 63. BUT ONE FORM OF CLEAVAGE (GENERALLY) 

 IN THE MEMBERS OF A SPECIES. 



The various crystalline forms belonging to any one spe- 

 cies afford in general, the same form of cleavage. 



By this we are not to understand, that but one solid in the majority 

 of cases is actually obtainable by cleavage in whatever way it may be 

 performed ; the proposition is only true, when a simultaneous cleav- 

 age is effected in every direction, in a crystal that affords similar 

 cleavage planes. For it is obvious, that if in the case of Fluor, whose 

 cleavage solid is a regular Octahedron, we omit to cleave parallel 

 with certain of its planes, and cleave only parallel with the others, 

 we may obtain a Tetrahedron or an acute Rhomboid. Likewise 

 in Blende, where the cleavage form (obtained by all the similar 

 cleavages) is a regular Dodecahedron, we may obtain by partial 

 cleavages an obtuse Rhomboid, an Octahedron, an acute Rhom- 

 boid, and an irregular Tetrahedron.* 



It has been said, however, that the crystals of some minerals pos- 

 sess many cleavages, and in some instances two sets of cleavages, 

 may each result in forms of cleavage, which will disagree, as in the 

 case of Phosphate of Lead, whose crystals besides cleaving parallel 

 to the sides of a regular six sided Prism, also afford a Dodecahedron 

 with isosceles triangular faces. These instances are however, very 

 rare. In general, the additional cleavages (which rarely take place 

 in more than one direction) are of such a nature as to lead to no reg- 

 ular solid, and are therefore denominated supernumerary cleavages. 



. 64. RELATION BETWEEN FORMS OF CLEAVAGE AND 

 CRYSTALS. 



Forms of cleavage, either represent members of the se- 

 ries of crystallization of those species from the individuals 

 of which they have been extracted, or those individuals 



* See Brooke's Crystallography, p. 40, et seq. where these results 

 are illustrated by diagrams. 



