I3j5 Defcnptmi of the Arjeniates 



edses, as AB, (fig. 15.) produced by the replacing of the 

 folid r.ngles, unite together, and give birth lo a new equila- 

 teral triangle, placed in a direftion contrary to the primitive 

 one; and if, at the fame time, the cryftal has fuch a length 

 that the fccondary planes terminate at the oppofite bafe, and 

 are very acute ifofcele? triangles, the cryllal will jjroient the ap- 

 pearance of a kind of truncated hexaedral pyramid, the bafe and 

 apex of which will be equilateral triangles, (iig. 18. PI. IV.) 

 The fix triangular planes which compofc the pyramid of this 

 cryftal, are always acute ifofccles triangles; but three of them 

 have their acute angle much fmallcr than the three others. 

 The fides of the bafe of this kind of pyramid are oppofite to 

 the lead acute angles; and its truncated apex is oppofite to 

 the moft acute ones ; the triangular planes being placed al- 

 ternately in an oppofite direftion. I have feen feveral in- 

 ftances of this form ; but I never faw fuch intermediate va- 

 rieties as the fecondary plane, reprefented by the dotted lines 

 in fig. 15. would give, if it exifi;ed at the fame time in the 

 three angles. 



By a longer duration of the aft of cry^fiailization, under 

 the fame modification, the plane corrcfponding to the trun- 

 cated apex of the pyramid (fi^. 18.) becomes progreflively 

 fmaller; the mofl: acute ifofceles triangular planes, which 

 anfwer to the fecondary ones, encroach on the leaft acute, 

 all which are the primitive planes of the cryfi;al, and the py- 

 ramid becomes truly triedral at its upper extremity, whilft it 

 remains hexaedral at the bafe, on account of thofe parts of 

 the three planes of the primitive cryfial which are Ilill pre- 

 ferved. (Fig. 19.) 



By a flill more confiderable duration of the aft of cryfl:al- 

 'lization, the pyramid would become completely Iricdral, and 

 would not be truncated at its apex. I have never met with 

 ihis modification fo complete; but I have feen the variety 

 reprefented in fig. 19; which, however, as well as fig. 18. is 

 very uncommon. 



The triedral prifni is fubjeft to a fourth modification, 

 which takes place at the three edges of one of its two bafes 

 or terminal furfaces only, and replaces each of thofe edges by 

 a plane, much more inclined on the fide of the prifm on 

 which it is placed than on the terminal furfacc. (Fig. so.) 

 I have not been able to determine, in ihefc cryftals, the 

 angles formed by thefe new planes, either with the fides of 

 the prifm or with the terminal furfaces; but the varieties, 

 belonging to this modification demonftrate that thefe angles 

 are the fame as thofe which the fecondary planes of the folid 

 angles make, cither with the terminal furfiices, or with the 



edges 



