PRIMAKY FOJtMS. 81 



frequently happens, two or more solids can be ex- 

 tracted from a crystallized mineral by cleavage, we 

 must refer to its secondary forms, in order to deter- 

 mine which of those solids ought to be adopted as its 

 primary form. And if the secondary forms can be 

 derived from one of such solids by the operation of 

 single decrements, while it would require two or more 

 decrements to operate simultaneously on the other, 

 in order to produce the same secondary forms, that 

 solid will be adopted as the primary, from which the 

 secondary forms belonging to the species might be 

 derived by the single decrement. 



A case however may occur in which two different 

 solids may be produced by cleavage, from either of 

 which, the secondary forms of the particular species 

 of mineral in which it occurs, may be derived by laws 

 of decrement equally simple. Whenever this hap- 

 pens we shall be at liberty to adopt either as the 

 primary, and we should probably adopt that which 

 predominates most among the secondary forms. 



In the section on cleavage we have seen that the 

 same set of cleavages will produce either a regular 

 tetrahedron, or a regular octahedron, or a particular 

 rhomboid, one only of which is to be regarded as the 

 primary form of the species in which such cleavages 

 occur. 



In this case the secondary forms of the mineral we 

 are supposed to be examining, can alone enable us 

 to determine which to adopt. 



The secondary forms of a rhomboid differ so much 

 from those of the octahedron and tetrahedron, as to 

 admit no doubt in the instance of fluor, that the pri- 

 mary form of this substance is not a rhomboid. 



According to the law of symmetry, the modifications 

 of a regular octahedron should equally affect all its 



