148 Analyses of Books, [Aug, 



relations between tlie primary and secondary forms of crystals. 

 And under this view of the subject, we ought to divest our 

 notions of molecules and decrementSy of that absolute reahty, 

 which the manner in which it is necessary to speak of them m 

 order to render our illustrations intelligible, seems generally to 

 imply." (P. 43-^2.) 



Sect. III. relates to the Structure of Crystals ; and Sect. IV, 

 to Cleavage : in tlie latter, the author thus explains the relation 

 of the tetrahedron to the octahedron, in reference to the theory 

 of cubic molecules. 



" The Abbe Hauy's theory, it will be recollected, supposes 

 that if the tetrahearon obtained by cleavage from the octahe- 

 dron, were to be successively reduced to an octahedron, and four 

 still smaller tetrahedrons, we should at length arrive at a tetra- 

 hedron consisting of four single tetrahedral molecules ewc/05/wg 

 ciily an octahedral space, instead of an octahedral solid. 



" But according to the structure assigned to the octahedron 

 by the theory of cubic molecules, that hgure is an entire solid ; 

 and the smallest tetrahedron that can be imagined to exist, will 

 contain an octahedral solid, and would be reduced to an octahe- 

 dron by the removal of four cubic molecules from its four solid 

 angles, and not of four tetrahedrons. 



" Thus the necessity of adopting the tetrahedron as the mole- 

 cule of the octahedron is removed, and in consequence a more 

 simple theory of the structure of the octahedron, may be substi- 

 tuted for that which has been established upon the adoption of 

 tetrahedral molecules. 



" By a similar mode of reasoning, the compatibility of the 

 cubic molecule with the solids obtained by cleavage from the 

 rhombic dodecahedron, might be shown ; and by adopting the 

 cubic molecule, a more simple theory of decrement, in relation 

 to the rhombic dodecahedron, may be substituted for that which 

 has been established upon the assumption of the irregular tetra- 

 hedron as the integrant molecule, and the obtuse rhomboid as 

 the subtractive molecule." (P. 66, QQ.) 



Sect. V. is allotted to the explanation of Decrements ; Sect. 

 VI. to Symmetry ; and Sect. VII. to Primary Forms. 



" The derivative or parent form,'* observes the author, " from 

 which the secondary forms of any crystallized mineral may be 

 conceived to be derived by the operation of certain laws of 

 decrement, has been denominated the primary form of such 

 mineral. 



" It may be added that the piimary form of a mineral should 

 not be inconsistent with its known cleavages, and it should gene- 

 rally be such also as would produce the secondary forms of those 

 species to which it belongs by the fewest and simplest laws of 

 decrement.* 



• " Tlie term primary^ so defined, is merely relative, being used in contradistinction 

 to i€C»ndury. It appears therefore preferable to the term priinHive^ which has been 



