SECTION XI. 

 ON THE TABLES OF MODIFJ CATIONS. 



THE preceding sections have explained the theory 

 of crystals in reference to 



The forms of their molecules. 



The manner in which those molecules are aggre- 

 gated in the production of crystals, and to 



The nature of decrements, or the manner in 

 which the secondary forms of crystals may 

 be conceived to be produced. 



The most important practical purpose of this theory 

 is, to enable the mineralogist to determine the species 

 to which any crystallized mineral belongs, from an 

 examination of any of its crystalline forms. 



Minerals which differ in species, may belong either 

 to different classes, or to the same class, of the pri- 

 mary forms. If several species belong to the same 

 class, they will be found, with the exception of such 

 as crystallize in cubes, regular tetrahedrons, regular 

 octahedrons, and rhombic dodecahedrons, to differ 

 from each other, sometimes in the angles at which their 

 primary planes incline to each other, and sometimes in 

 the comparative lengths of some of their primary edges. 



Thus the general class of square prisms may con- 

 sist of any number of particular prisms, belonging to 

 as many different species of minerals; and these indi- 



