vi Introduction* 



most of their forms by separate names, he has pre- 

 sented those forms to the mind rather as indepen- 

 dent individuals, than as parts of such groups as 

 should render their relations to each other, and hence 

 their mineralogical relations, apparent. 



I have been induced, therefore, to attempt such an 

 arrangement of the various forms of crystals, as will 

 indicate their constant relations to, or differences 

 from, each other, for the purpose of more readily re- 

 ferring from the crystal to the mineral ; and this ar- 

 rangement is contained in the Tables of Modifications 

 which will be found in the following pages.* 



The best illustration of the manner in which some 

 of the forms of crystals may be conceived to be allied 

 to others, is afforded by the Abbe Haiiy's theory of 

 decrement. That theory appears, however, much 

 encumbered by his adoption of two kinds of molecules, 

 and by the forms which he has assigned to particular 

 molecules of one of those kinds ; in consequence of 

 this, I have ventured to propose a new theory, in re- 

 ference to several of the classes of primary forms, 

 which may in some respects be regarded as more 

 simple, and which forms the subject of the section 

 on molecules. t 



* Since these tables were constructed I have learned from Mr. Konig 

 of the British Museum, that he had for some time entertained an inten- 

 tion of framing a set of tables nearly on the same principle: and he has 

 shewn me a considerable number of drawings of the figures of crystals 

 which were made partly with a view to this object, yet serving at the 

 same time as records of many of the crystalline forms of minerals con- 

 tained in that rich collection, upon which his attention is so constantly 

 and so assiduously bestowed. 



+ The theory of spherical molecules which has been entertained by 

 some distinguished philosophers, has not been alluded to in this treatise, 

 as the laws of decrement appeared more readily explicable on the sup- 

 position of the molecules of crystals being solidb contained within plane 

 surfaces. 



