96 ON THE TABLES OF MODIFICATIONS. 



vidual or particular prisms may be conceived to differ 

 from each other in the relation of their respective 

 heights to the length of the edge of their square base. 

 The class of rectangular prisms may be supposed to 

 contain many particular prisms, which vary from each 

 other in the relative dimensions of their planes. 



The individuals of the class of rhombic prisms may 

 vary from each other in their relative heights, and in 

 the angle at which the lateral planes incline to each 

 other. Those of the class of rhomboids will differ in 

 the angles at which their planes incline respectively 

 to each other ; constituting a series of particular 

 rhomboids, of which the most acute, and the most 

 obtuse, will be the two extremes ; and similar dif- 

 ferences may be imagined to exist among the indi- 

 viduals belonging to such of the other classes of pri- 

 mary forms as admit of analagous variations. 



But crystals rarely present themselves under their 

 respective primary forms ; they are usually modified 

 by new planes, producing secondary crystals, from 

 which the primary forms are to be inferred. 



And although, as we have already seen, we may, 

 to a certain extent, be guided by cleavage in our 

 attempts to discover the primary forms of minerals, 

 those forms cannot in general be determined without 

 a reference to the secondary crystals. 



Hence the relations between the various secondary 

 forms of crystals, and their respective primary forms, 

 constitute a highly important feature in the science, of 

 which we are treating. 



The secondary forms of crystals have been ex- 

 plained to consist of modifications of the primary, 

 occasioned by decrements on some of their edges or 

 angles. 



The character of the modifying planes, and their 

 geometrical relations to the primary form, as con- 



