74 DECREMENTS. 



For the purpose, however, of affording a clearer 

 illustration of the theory of decrements, it has been 

 found convenient to imagine that the primary form 

 of any modified crystal had attained such a magni- 

 tude, before the law of decrement had begun to act 

 upon it, as to require for the completion of the mo- 

 dified crystal, the addition of only those defective 

 plates of molecules by which the modifying planes 

 were produced. A primary form of this magnitude, 

 is evidently the greatest that could be inscribed in 

 the given secondary form. And a primary form so 

 related to the secondary, is in theory termed the nu- 

 cleus of the secondary form. 



This nucleus may frequently be extracted from the 

 secondary crystals by cleavage. 



If we take a crystal of carbonate of lime of the 

 variety called dog-tooth spar (the metastatique of 

 the Abbe Haiiy,) and begin to cleave it at its sum- 

 mit, we shall first remove those molecules which 

 were last added in the production of the crystal ; and 

 by continuing to detach successive portions, thus pro- 

 ceeding in an order the inverse of that by which the 

 modified crystal has been formed, we may remove 

 the whole of the laminae, which enclose or cover the 

 theoretical primary nucleus. 



As far as we have proceeded with the theory of 

 decrements, we have supposed the diminished plates 

 of molecules to be laid constantly upon the primary 

 form, in order to produce the modifications which 

 are found to exist in nature. But from a comparison 

 of the angles at which some secondary planes incline 

 on the primary, and on each other, it is probable 

 that the decrements sometimes take place on second- 

 ary crystals. Thus, for example, we may conceive 

 decrements to take place on any secondary rhomboid 



