I6(? Parallel of Rome de V Isle's and 



Lctttr from the Able Buee to Mr. * * *, on M. Rome 

 - de LTsle's and the Abbe Hauy's Theories of Crystallo- 

 graphy. 



. In consequence of your request, I send you the parallel of 

 the two theories of crystallography which seem to divide mi- 

 neralogists in this country; those of Mr. Rome de Plsle and 

 of the Abbe Haiiy. You arc perfectly acquainted with the 

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

 Jived for si\-and-thirly years in habits of intimacy with the 

 Abbe, I dwell with pleasure on his works, and will do my 

 utmost to satisfy vour curiosity. 



To Mr. de I'lsle is due the merit of having called the 

 attention of naturalists to that neglected branch of mine- 

 jalogy, crystallography ; of having discovered that that 

 branch, though neglected, was perhaps the most interest- 

 ing part of mineralogy, and tjie only part which could 

 raise it to the dignity of a correct science ; in short, of hav- 

 ing discovered order, bv numerous observations, as inge- 

 nious as new, where a Cronskdt, a Bergman, a Buffon, or 

 a Kirwan, could perceive nothing but contusion ; and thus 

 seemed to rescue nature from the charge of caprice, almost 

 imputed to it, because great mineralogists had neglected to 

 .study its unerring laws. 



It was exclusive! v reserved to the Abbe Haiiy to point out, 

 to explain, and apply those laws. He. demonstrated where 

 De Plsle affirmed. He discovered those hidden facts, which 

 he has since shown to be the mathematical consequences of 

 facts observed by De 1'Isle. If the latter furnished a part 

 of the materials, the Abbe has augmented and employed 

 them. 



The discoveries of these two writers force me to subdi- 

 vide crystallography into two distinct parts ; descriptive, 

 and philosophical : and under these two heads I will rapidly 

 describe the labours of each author. 



- Descriptive. The most important part of Mr. de ITsle's 

 work consists in his crystallographical tables. In each of 

 these tables (seven in number) he describes one of the 

 principal forms a-sumed by 'crystals ; and then delineates 

 the different modifications- of vvnich that form is suscepti- 

 ble, by -mean* of different truncations (troncatwes), as he 

 calls them. 



For elucidation, take a cube the primitive form of the 

 second table. A cube, it is known, has 6 faces, 8 solid 

 angles, and 12 edges. If the cube be truncated in a pa- 

 . E rallel 



