the Albt tldiiy's Theories of Crysialhgraphy . 16*1 



rallel to one of its faces, a rectangled paralleiopipcclon wfllf 

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

 If the eight solid angles of the cube be struck oif, eight new 1 

 faces will replace the eight solid 'angles ; and in place of six 

 Sides we shall have fourteen. If the twelve edges be taken 

 off, twelve new faces will succeed the straight lines, and 

 the solid will have eighteen sides. Such are Be V Isle's sim- 

 ple truncations. They may be then combined with each 

 other, and made more or less deep : hence an immense va- 

 riety of new figures, put these new forms again may be 

 truncated in the directions either of their faces, solid an- 

 gles, or edg",s ; and these new truncations, more or less* 

 deep, eallcd by De l'lsle sur-troncatures, may also be, com- 

 bined with each other. Here the foims must multiply to 

 infinity, and their boundless numbers will soon bury the 

 primitive cube in oblivion. 



It must not be supposed that nature has furnished us with 

 this infinite series of forms ; indeed Mr. de l'lsle in his ta- 

 bles has only mentioned those he had observed, with some 

 few additional supposititious figures, of which several have 

 been since discovered to exist. 



This ingenious naturalist has given us, as I have already 

 said, seven crystallographical tables. In the first, he de- 

 scribes the tetraedron and its modifications 5 in the second", 

 the cube, in the third, the rectangular octaedron ; in the 

 fourth, the rhomboidal parallelopipedon j in the fifth, the 

 rhomboidal octaedron ; in the sixth, the dodecaedron with, 

 triangular faces ; and to each are subjoined their respec- 

 tive modifications. The object of the seventh table is to 

 point out certain modifications of the octaedron and pa»- 

 rallelopipedon, whether rectangular or rhomboidal. Plates 

 accompany each table, where the figures are drawn, and 

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

 measures of the principal angles. 



These crystallographical tables exhibit only general repre- 

 sentation's of solids, which Mr. de l'lsle in the course of 

 his work applies to the different crystals which had already 

 been discovered, and fallen within his observation. His work 

 consists. of three parts. In the fitstrhe treats of saline crys- 

 tals ; in the second, of stony VpierWi'ix) crystals ; and in the 

 • third, of metallic crystals, Those of the first class are arti- 

 ficial ; those of the two latter classes are natural, and sub- 

 divided into genera, species, and varieties. 



When treating of a species or of a variety, he refers firs 



reader to the table where the figure of that species or variety 



'is to be found, and nVthen enumerates every thing relating 



4 to 



