ON THE USE OF SYMBOLS. 235 



along that plane in the directions of whose edges the 

 greatest number of molecules have been abstracted. 



It may not be useless to remark, that when two or 

 more planes replace the solid angle of a crystal, if 

 an edge at which the secondary planes intersect each 

 other be parallel to the edge at which one of them 

 intersects the primary plane, they will generally both 

 result from simple or mixed decrements. 



Intermediary decrements however sometimes pro- 

 duce a series of planes whose intersecting edges are 

 parallel to each other, and when this happens, the 

 symbols of those planes will have two of their corre- 

 sponding indices in the same ratio to each other. 



The edges of such secondary planes as replace the 

 edges of crystals, and which result from simple or 

 mixed decrements, are always parallel. 



From what has been already stated it will appear, 

 that if we are about to describe a secondary crystal, 

 belonging to any species of mineral whose primary 

 form is known, and upon several of whose edges, or 

 angles, similar decrements have produced similar 

 planes, it will be sufficient, generally , to describe one 

 only of the new planes, produced upon one of those 

 edges or angles. 



And if two or more laws of decrement have con- 

 curred in the production of any secondary crystal, we 

 should be required, generally ', to describe only one 

 of the planes produced by each particular law. For 

 the change of figure which any primary form has 

 undergone would be, generally, thus indicated. And 

 in drawing the crystal we might construct planes, 

 similar to those which are described, upon all its 

 similar edges or angles. 





