38 



MOLECULES. 



If we reduce a crystal of carbonate of lime to frag- 

 ments, the planes of those fragments will be found to 

 incline to each other at angles which are respectively 

 equal to those of the primary rhomboid. We there- 

 fore infer that the molecule of carbonate of lime is a 

 minute rhomboid similar to the primary form. 



Sulphate ofbarytes may be split into right rhombic 

 prisms, whose angles are respectively equal to those 

 of the primary crystal. It is therefore supposed that 

 the primary crystal and the molecules of this substance 

 are similar prisms. 



Having thus found that crystals belonging to seve- 

 ral of the classes of parallelepipeds may be split into 

 fragments resembling their respective primary forms; 

 and having assumed that these fragments represent 

 the molecules of each of those forms respectively, it 

 has been concluded that the primary forms and the 

 molecules of all the classes of parallelepipeds are 

 respectively similar to each other. 



This similarity does not however exist between the 

 other classes of primary forms and their respective 

 molecules. 



Fig. 60. 



Fig. 61. 



If a regular hexagonal prism of phosphate of lime 

 be split in directions parallel to all its sides, it may 

 be divided into trihedral prisms whose bases are equi- 

 lateral triangles ; these may be regarded as the mole- 

 cules of this class of primary forms. 



Fig. 60 shews the hexagonal prism composed of 



