PRIMARY FORMS. 83 



other, and another ef, g h, such that the angle e o g, 

 measures about 100 and the angle g of, about 80. 



Now these cleavages are respectively parallel to 

 the sides of either a rectangular, or a rhombic prism, 

 and either of these may therefore be the primary form 

 of petalite. But there is no cleavage which indicates 

 with certainty whether the prism, whichever of the 

 two we may adopt, be right or oblique. 



We cannot therefore determine the primary form 

 of petalite, without a reference to crystals possessing 

 their natural terminal planes, or such modifications 

 of those planes as will shew whether the primary 

 crystal be right or oblique. 



If we discover from the crystal of petalite that the 

 primary form is a right prism, we should still be at 

 liberty to take either the right rectangular or right 

 rhombic prism. 



But as the angles at which the planes of the rhom- 

 bic prism incline to each other, are those by which 

 the particular species of mineral would in this case 

 be distinguished, we should at once adopt the rhom- 

 bic prism as the primary form. 



It has been observed in the section on cleavage, 

 that the character of the planes developed by cleavage 

 sometimes afforded indications of the primary form. 



The striae on the natural surfaces of the secondary 

 planes of crystals have been also considered to afford 

 indications of primary form. As those which are 

 parallel to the shorter diagonal of the dodecahedral 

 planes of the aplome, have been regarded as indica- 

 tions of the, primary form being a cube, as it appears 

 to be from its cleavage. 



