2p4> Theory of Cryfialiifation. 



Various Examples of compound fecondary Forms. 

 Prifmatic Calcareous Spar (Fig. i. Plate II.) 



Spath calcaire en prifme hexuedre. Daubenton Tab. Miner. 

 edit. 1792, p. 15, n° 6. De l'lfle Cryfiallographie, torn. i. 

 p. 514. var. 10. 



The bafes of this prifm are produced in confequence of a 

 decrement by a fingle range on the angles of the fummits 

 bafigaf, bag, dex, dec, cex (fg. 4.), of the primitive 

 form. The fix planes refult from a decrement by two 

 rangeson the angles bdf, fxg, beg, dfx, dbc, cgx, op- 

 pofite to the preceding. Let abdf (fig. 50.) be the fame 

 face of the nucleus as fig. 4. The decreafing edges fituated 

 towards the angle of the fummita will fucceffivelycorrefpond 

 with the lines hi, k I, &c. and thofe which look towards the infe- 

 rior angle dw\\\ have the pofitions pointed out hymn, op, &c. 

 But in confequence of the firft decrement taking place by 

 one range, we prove that the face which refults from it is 

 perpendicular to the axis ; and calculation fhews, in the like 

 manner, that the fecond decrement, which takes place by 

 two ranges, produces planes parallel to the axis, and thus 

 the feconchry folid is a regular hexaedral prifm. 



To difplay farther the ftruclure of this prifm let us re- 

 mark that, in the production of any one a b c nib (fig. 1.) 

 of the two bafes, we may confine ourfelves to confider the 

 effect of one only of the three decrements which take place 

 around the folid angle a (fig. 4.), for example, of that which 

 takes place on the angle b af, fuppofing that the laminse ap- 

 plied on the two other faces, fagx, bagc, do not decreafe 

 but to affift the refult of the principal decrement, which 

 takes place in regard to the angle b af. But here thefe auxi- 

 liary decrements are altogether fimilar to that the effect of 

 which they are fuppofed to prolong. 



The cafe will be totally different if we apply the fame ob- 

 fervation to the decrements which are effected by two ranges 

 on the inferior angles bdf y dfx,fxg y &c and which pro- 

 duce 



