104 On Cnjstallographif. 



of the rhomboidal form which serve? here to disguise itself, 

 and conceals fixed characters under vaiiable ontsides. 



Let us choose for instance, among these ditlerent rhom- 

 boids, that in which the angle at the stin)mit is 73° 3l' -20", 

 and which is represented tig. 8, circnmscnbtd to its nucleus. 

 Rome de I'lsle called it muriatic calcaieous spar, and I 

 denoniinale it inverse carbonated lime. In order to divide 

 this rh(nnboid mechanically, the secting planes must be di- 

 rected parallel to the six extreme edges > viz. st, sn, sn, 

 on one hand, and st', su', sn', on the other, in such a 

 way that these planes are equally inclined upon the faces 

 which thcv cut into. The first sections will exhibit six pen- 

 tairons rrrr'r'n^ (fig. 9), parallel to the faces of the nu- 

 cleus ; and it is easy to conceive, that by continuing the di- 

 vision always in the same direction, until the residues ot the 

 faces of the rhomboid A A' (fig. 8) have disappeared, we 

 shall have a new rhomboid, wliich will be the primitive 

 form. 



We may remark, that the faces of this last rhomboid are 

 inclined iri the same quantity upon the common axis, with 

 the edo;es st,s ?/, s n, &c., to which these faces are parallel. 

 Now tlie edges in question form with the axis larger angles 

 than the oblique diagonals drawn from s to ?/, from s to t' , 

 from J to ?/, or, what comes to the same thing, than (hp 

 faces . 5 ^ 7i' ?/, snt'u, stv'n; whence we conclude that 

 in the rhomboid, extracted by mechanical division, the 

 an^le of the summit should be sensibly more open than that 

 •which corresponds with it in the divided rhomboid. From 

 what has been said above, this last angle is smaller than the 

 other by 2ti'' O' 53". 



If we try to divide a crvstal of another species, you will 

 have a different nucleus. For instance, a cube of fluated 

 lime will give a regular octahedron, which you will succeed 

 in extracting by dividing the cube upon its eight solid angles, 

 which will in the first place discover eight equilateral tri- 

 angles, and by pursuing the division, always parallel to the 

 first sections, until nothing more remains of the faces of the 

 cube, the nucleus of the crystals of sulphated barytes will 

 be a straiglit prism with rhombous bases ; that of the crystals 

 of phosphated lime a regular hexahedral prism ; that of sul- 

 phuretted lead a cube, &c.; and each of these forms will be 

 constant relative to the entire species, in such a manner 

 that its angles will not undergo any appreciable variation. 



With regard to crystals which refuse to be mechanically 

 (divided, the theory seconded by certain indications, which 



we 



