MINERALOGICAL SYSTEMS, Q^ 



A Letter from the Abbe BuSe to Mr. — , on Mr. Rome deUIJleh 

 and the Abb6 Hauy*$ Theories of Cryfiallography, 



Sir, 



IN confequence of your requefl-, I fend you the parallel of ^^''^V^^ between 

 .1 ^. • /• ^ n 11 t I • 1 r 1- . 1 the theories of 



the two Theories of Cryltallography, which leem to divide Rome de I'llle 



mineralogifts in this country, thofe of Mr. Rom6 de L'Ifle and and Hauy. 



of Ihe Abb^ Hauy. You are perfe6lly acquainted with the 



former theory, but nearly a ftranger to the latter. Having 



lived for fix and thirty years in habits of intimacy with the 



Abbd, I dwell with pleafure on his w^orks, and will do ray 



utraoft to fatisfy your curiofity. 



To Mr. de L^Ifle is due the merit of having called the at- De rifle firft 

 tention of naturalifts to that negleded branch of n^'neralogy, "^J'^J^^^^"'^^^ 

 cryflallography; of having difcovered that that branch, though phy. 

 negleded, was perhaps the moft interefting part of mineralo- 

 gy, and the only part which could raife it to the dignity of a - 

 correal fcience; in fhort, of having difcovered order, by nu- 

 merous obfervations as ingenious as new, there, where a 

 Cronftet, a Bergman, a Buffbn, or a Kirwan could perceive 

 nothing but confufion; and thus feemed to refcue nature from 

 the charge of caprice, almoft imputed to it becaufe great mi- 

 neralogifts had negle6led to ftudy its unerring laws. 



It was exclufively referved to the Abbe Hauy to point out, Hauy firft ex- 

 to explain, and apply thofe laws. He demonftrated where P['^^'^^^'^^^^^ 

 De L'Ifle affirmed. He difcovered thofe hidden fads, which 

 he has fince (hewn to be the mathematical confequences of 

 fads obferved by De L'Ifle. If the latter furniQied a part of 

 the materials, the Abbd has augmented and employed them. 

 The difcoveries of thefe two writers force me to fubdivide 

 cryflallography into two diftin6t parts; defcriptive and phi- 

 lofophical; and under thefe two heads I will rapidly defcribe 

 the labours of each author. 



Defcriptive, — The moft important part of Mr. de LTfle*s Defcriptive cry^ 

 work confifts in his cryftallographical tables. In each of thefe D^^nfl^.^Luu 

 tables (feven in number) he defcribes one of the principal 

 forms affumed by cryftals, and then delineates the different 

 modifications of which that form is fufceptible, by means of dif- 

 ferent troncations (troncature) as he calls them. 



For elucidation, let us take a cube, the primitive form of Modifications of 

 the fecond table. A- cube, it is known, has fi)i faces, eight cryftals defcribed 



° by troncations. 

 lOha Inftancethe 



cube. 



