2()6 Theory of Cryjlellifation. 



ment of the decrement on the fuperior angles (hall have beeil 

 retarded. This difference of epochs becomes fenfible by in- 

 fpefting the dodecaedron, j%. 2, which is one of the refults 

 of the mechanical divifion of the prifm. It is there feen that 

 the pentagonal lamina of the fummits, fuch as AOIRSj 

 decreafe only by their edge R S, which correfponds to the 

 inferior angle b df {fig. 4), while, by their upper parts, they 

 continue to envelop the cryftal without experiencing any 

 decrement towards that fide ; fo that it is only on the la- 

 minae molt diftant from the axis, as that correfponding to 

 ps u /, that the two decrements take place at the fame time. 



The refult which we have explained is general ; that is to 

 fay, that, whatever may be the angles of the primitive rhom- 

 boid, the fecondary folid will always be a regular hexaedral 

 priVm. 



Amphkrigonous Iron Ore, 



(Fig. 51 reprefents this cryftal in a homonta! projection, 

 and^%. 52 in perfpe&ive.) 



Mine defer a 24 faces. Daubenton Tab. Miner, edit. 1792, 

 p. 30,. n° 2. De 1'Ifle Cryfial/ographie, torn. iii. p. 103 et 

 fuiv. var. c - , 6,. 7. 



Geomet. charatl. Refpe&ive inclination of the triangles 

 gen, gcd, &c. from the fame fummit 146 26' 33"; of the 

 lateral triangles bg u, bgq, to the adjacent pentagons, fuch 

 as gutmn y i54°4S' 39"- 



This form is that under which the iron ore of the ifland 

 of Elba moft commonly appears. It refults from a decre- 

 ment by two ranges on the angles c, n (fig. 46.), to the fum- 

 mits of a cubic nucleus which produces the ifofceles trian- 

 gles gen, gcd, nc d (fig. 51 and 52), and of a fecond de- 

 crement by three ranges on the lateral angles ebp, erp y 

 ers t &c. which produce the triangles tnnr, rnk t u g b 3 

 qgby &c. Thefe two decrements ftop at a certain term* 

 fo that there remain faces parallel to thofe of the nucleus, 

 Viz. the pentagons gutmn, bdnk I, &c. (fig. 5 \ .) 



The 



