FORMS OF CLEAVAGE AND PRIMARY FORMS. 75 



is plain, that the hexagonal Prism terminated at both extremities, 

 with six sided pyramids, (whose faces correspond to those of the 

 prism) or even the Dodecahedron with isosceles faces may be said 

 to come into the same connection ; since both these forms flow from 

 the hexagonal Prism, simply through the truncation of its terminal 

 edges. 



<. 65. RELATION BETWEEN FORMS OF CLEAVAGE AND 

 PRIMARY FORMS. 



The forms of cleavage and primary forms, (in those 

 systems of crystallization where the former are observa- 

 ble,) are identical. 



Those forms which are obtainable by cleavage, or which may be 

 inferred to exist from certain appearances, are similar to the solids 

 which are considered as the primary forms; and among the crystals 

 of the different species, the one may be substituted for the other. 



But it will no doubt be recollected, that in two or three instances, 

 the crystals of the same mineral yield two different cleavage forms. 

 In these cases, we have two primary forms, either of which may 

 be selected, the choice being regulated only by a single circum- 

 stance ; viz. the predominance of one of the forms among the sec- 

 ondary crystals of the mineral. Thus, in Phosphate of Lead, the 



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dodecahedron with isosceles faces; but as the crystals are decided- 

 ly prismatic in their shape, the former is regarded as the primary 

 form. 



. 66. PRIMARY FORM, IN THE ABSENCE OF CLEAVAGE, 

 HOW ESTABLISHED. 



When no cleavages are observable among the crystals of 

 a mineral, the primary form is chosen from analogy. 



Thus, the crystals of Native Gold, (which are Octahedrons un- 

 modified, and Octahedrons with their edges and angles truncated ; 

 Cubes and Dodecahedrons,) possess no indications of cleavage ; but 

 as they assume those forms which other minerals affect whose cleav- 

 age forms are Octahedrons, the Octahedron is presumed to be the 

 primary form in this instance, 



