Theory of Cryflalllzaiion. 29 



and at a certain term the points RG becoming confounded 

 with the points YZ, the points SR with the points UY, &c. 

 there remained nothing of the pentagons in queftion but 

 the triangles YIZ, UXY, &c. {fig. 3). Beyond that term 

 the fections happening to pafs over the furface of thefe tri- 

 angles diminifhed gradually their extent, until, at length, 

 thefe triangles were loft ; and then the folid which arofe 

 from the hexaedral prifm became a rhomboid a e {fig. 4) en- 

 tirely like that commonly diftinguifhed under the name of 

 Icelandic fpar. 



A refult fo unexpected made me immediately conceive the 

 idea of fubjecting the other calcareous cryftals to the fame 

 proof; and all yielded to the mechanical divifion in fuch a 

 manner, that, when the external faces had difappeared, the 

 kind of nucleus which remained under the inftrument was 

 ftill a rhomboid of the fame form as the firft- The only 

 thing neceffary was to find the direction of the fections 

 which conducted to the central rhomboid. 



To extract, for example, -this rhomboid from that fpar 

 called commonly the lenticular, and which is a much more 

 obtufe rhomboid, having its great plane angle equal to 

 1 1 4 18' 56", it was neceffary to begin at the two vertices, 

 making the feelions pafs through the leffer diagonals of the 

 faces. If it were wifhed, on the contrary, to get at the nu- 

 cleus of the rhomboidal fpar with acute vertices *, it would 

 be neceffary to direct the feclions of the planes parallel to 

 the edges adjacent to the vertices, and in fuch a manner 

 that each of them fhould be equally inclined to the planes 

 it cuts. 



Thefe refults are the more worthy of attention, as it would 

 at firft feem that cryftallization, after having once adopted 

 the rhomboid, in regard to one determined fpecies of mine- 

 ral, ought always to re-produce it with the fame angles. But 



* This rhomboid, as well at the preceding, will be farther explained ir» 

 >he courfe of the prefent article. 



D4 tht 



