2g MINER ALOGICaL SYSTEMS. 



folld angles, and twelve edges. If the cube be truncated in 

 a parallel to one of its faces, a redangled parallelipipedon will 

 be produced, and the equality of the faces will be defiroyed. 



If the eight folld angles of the cube be llruck ofl', eight 

 new faces will replace the eight folid angles, and in place of 

 fix fides we (hall have fourteen. If the twelve edges be taken 

 off, twelve new faces will fucceed the flrait lines, and the 

 folid will have eighteen fides. Such are De L'Ifle*s fimple 

 troiications. They may be then combined with each other, and 

 made more or lefs deep; hence an immenfe variety of new 

 figures. But thefe new forms again may be truncated in the 

 directions either of their faces, folid angles, or edges, and 

 (hefe new troncations more or lefs deep, called by De L'Ifle 

 Jur-troncatures, may alfo be combined with each other. Here 

 the forms mud multiply to infinity, and their boundlefs num- 

 bers will foon bury the primitive cube in oblivion. 



It muft not be fuppofed that nature has furniQied us with this 

 infinite feries of forms; indeed Mr. De L*J(le in his tables has 

 only mentioned ihofe he had obferved, with fome few addi- 

 tional fuppontious figures, of which feveral have been fince 

 difcovered to exifi. 

 Account of This ingenious naturalift has given us, as I have already 



Del'ine'sfeven f^i^ f^^ren cryfialloojraphical tables. In the I ft he defcribes 

 tables of cryftals. , , , "^ ,*. ' ,.r • • •, 



the tetrahedron and its modincations; m the 2d the cube; m 



the 3d the rectangular octahedron ; in the -ith the rhomboidal 



parallelipipedon; in the 5th the rhomboidal octahedron; in the 



6th the dodecahedron, w^ith triangular faces, and to each are 



fubjoined their refpedlve modifications. The obj-ecl of the 



7th table is to point out certain modifications of the o6tahe- 



dron and parallelipipedon, whether reftangularor rhomboidal. 



Plates accompany each table, where the figures are drawn, 



and in the obfervations and notes on them are to be found the 



meafuresof the principal angles. 



They contain Thefe cry ftallographical tables exhibit only general repre- 



general folids } fentations of folids, which Mr. de L'Ifle in the courfe of his 

 which are Alb- , ,. , ,.„^ -, , ,- , , , i i • 



fequently applied work applies to the diHerent cryltals which had already been 



tothecryftdis difcovered, and fallen within his obfervation. His work con- 



fafdfjiotm, and ^^^^ of three parts. In the firft he treats of faline cryftals; in 



^stctah, the fecond of ftoney (pierreux) cryftals, and in the third pf 



metallic cryftals. Thofe of the firft clafs are artificial, thofe 



of the two latter claflesare natural cry fta,ls, and are fubdividqd 



intp genera, fpecies, and variciirs. 



When 



