LAWS OF DECREMENT. 287 



It has been already remarked, thai one half the 

 number of planes by which any crystal is bounded, 

 are generally shewn in front of the engraved figure 

 of that crystal. And as we know that the opposite 

 angles, edges, and planes, which are supposed to form 

 the back of the engraved figure, are respectively 

 similar to those which appear on its front, if the 

 decrements on these be described, the decrements on 

 the hidden or back planes may be conceived to be 

 described also. And again, as the law of symmetry 

 requires that all similar angles and edges shall be 

 similarly modified, if among the modified angles and 

 edges, which are supposed to be in front of the 

 figure, there be two or more, similar to each other, it 

 is obviously sufficient to investigate the decrement 

 upon one of these, in order to determine the character 

 of the modifying planes upon the others. 



Decrements, as we have already seen, take place 

 on the ^edges or angles of crystals, and are of two 

 principal kinds ; one of which produces planes inter- 

 secting the primary planes, in lines, of which one 

 at least, is parallel to an edge or diagonal of one of 

 those planes ; and the planes produced by the other 

 intersecting the primary planes in lines, not any of 

 which are parallel to an edge or diagonal of any of 

 those planes. 



The effect of both these classes of decrements upon the 

 primary form, is similar to that which would take 

 place, if we conceive the enlarged crystal to have been 

 completed, and the whole of the omitted molecules to 

 have been then removed from it in one, mass. 



This will be readily perceived, if we refer for an 

 example to modification a of the rhomboid. Let 

 us conceive a rhomboid of a given dimension to 

 hare been formed; and during its further increase in 



