CRYSTALLINE FOEMS. 91 



primary forms, of layers of crystalline matter, gradually de- 

 creasing in breadth, such layers being composed of molecules, 

 so infinitely minute as to be invisible ; 

 so that a pyramid (fig. 26.) formed of 

 such particles, would present on its 

 surface, no inequality which would be 

 perceptible to the eye. 



With regard to the forms of these 

 ultimate molecules, it is known that 

 secondary crystals may be cleaved into 

 layers parallel to the planes of a primary 

 nucleus of a different form, as the cube 

 into the octohedron, as in the case of fluor, &c., while these 

 layers may be divided by cleavage in other directions, so as 

 to afford small bodies of determinate shape, which, divide 

 and sub-divide them as we may, still preserve, as long as 

 they are visible, the same form. These forms, therefore, 

 whether cubes, octohedrons, &c., are considered as those of 

 the component molecules of which these crystals are com- 

 posed. 



The arrangement of the molecules is found to be different 

 in different forms, particularly in the case of the rhombic 

 and pentagonal dodecahedrons. By placing a low three- 

 sided pyramid on each plane of the regular octohedron, a 

 rhombic dodecahedron may be formed, as (fig. 27). 



FIG. 28. FIG. 29. 



The rhombic dodecahedron may also be formed from the 

 cube, by placing a low quadrangular pyramid on each face of 

 the cube, as (fig. 28). 



The pentagonal dodecahedron may likewise be formed on 

 the cube, by placing, on each of its sides, an equal and 

 similar pyramid, having two triangular and two quadrangular 

 planes (fig. 29). 



