

, MOLECULES, 



Fig. 78. 



47 



Fig. 78 in the rhombic dodecahedron. 



These arrangements of cubic molecules cannot be 

 objected to on account of any supposed imperfection 

 of surface which would be occasioned by the faces of 

 all the primary forms, except the cube, being con- 

 stituted of the edges, or solid angles, of the molecules* 

 For as we observe that the octahedral and dodeca- 

 hedral planes of some of the secondary crystals of 

 galena, which are obviously composed of the solid 

 angles, or edges, of the cubic molecules, are capable 

 of reflecting objects with great distinctness, it is 

 evident that the size of the molecules of galena is 

 less than the smallest perceptible inequality of the 

 splendent surface of those planes, and hence we in- 

 fer generally that there will be no observable difference 

 in brilliancy between the surfaces of the planes obtained 

 by cleavage parallel to the sides of molecules, and of 

 those which would expose their edges or solid angles. 



This theory may be reconciled with the cleavages 

 which are found to take place parallel to the primary 

 planes of the tetrahedron, the octahedron, and the rhom- 

 bic dodecahedron, as well as to those of the cube, if we 

 suppose the cubic molecules capable of being held to- 



