1823.] Mr, Brooked Introduction to Crystallography. 149 



" It is for the sake of rendering- our notions of a primary form 

 more precise, that we give this Hmiting, and in some degree arbi- 

 trary, definition of the term. Our purpose throughout this trea- 

 tise is, to find the shortest and most direct road from the 

 secondary crystal to the mineral species to which it belongs. 



" But as we must travel first from the secondary to the primary 

 form, it is essential that our ideas of that figure which we agree 

 to call the primary form, should be as precise as possible.''(P- ^9.) 



In Sect. VIII. Secondary Forms are briefly considered, in a 

 general manner ; in Sect. IX. Hemitrope and Intersected Crys- 

 tals are described ; and Sect. X. defines Epigene and Pseudo- 

 morphous Crystals. 



In Sect. XI. are described the nature and use of the tables of 

 modifications of the primary form which succeed it. 



" In these tables," the author says, " not merely the observed 

 modifications of crystals y hut all the numerous modifications of 

 which each class of primary form is susceptible, while influenced by 

 the law of symmetry, are reduced into classes, and arranged in an 

 orderly series ; and I have added some of the observed mstances 

 of departure from this law, in the production of peculiar and 

 anomalous secondary forms." (P. 98.) 



*' The figures of the primary and secondary forms given ia the 

 following tables, are not to be regarded as representations of crys- 

 talline forms of any particular minerals, but as exhibiting a type, 

 or general character, of each of the classes of primary forms, and 

 of the modifications belonging to each of those classes." (P. 101.) 



These tables exhibit 150 classes of the modifications of the 

 following 15 primary forms : the cube, regular tetrahedron, regular 

 octahedron, rhombic dodecahedron, octahedron with a square 

 base,octahedron with a rectangular base,octahedron with a rhombic 

 base, right square prism, right rectangular prism, rhombic prism, 

 oblique-angled prism, oblique rhombic prism, doubly obfique 

 prism, hexagonal prism, rhomboid. They are followed by a 

 . table exhibiting the relations to the primary forms, of those 

 secondary forms of crystals which are similar to some of the 

 classes of the primary, and also of those which, when complete, 

 are different from all the primary forms. 



Sect. XII. treats of the Application of the Tables of Modifica- 

 tions. In Sect. XIII. (numbered XIV. by mistake), the Use of 

 Symbols for describing the secondary forms of crystals is 

 explained. To this section succeeds a series of tables, which 

 terminate this part of the work, showing the Relation of the 

 Laws of Decrement to the difi'erent Classes of Modifications. 



A large portion of the Appendix consists of an outline of the 

 method of applying the theory of decrements to determine the 

 relations between the secondary and primary forms of crystals. 

 Abbe Haiiy has used plane trigonometry in his calculation of the 



generally used to designate this original or parent form, and which seems to imply some- 

 thing more intrinsic, and absolute, than is required by the science into which it is intro- 

 duced," 



