302 Theory of Cr\fallifation. 



ifiS' $7) re tu\t from a decrement by two ranges parallel to 

 the ridges A O, N R {Jig. 58); 4th, that the plane cfgh 

 and its oppofite (fg. 50), which correfpond to my, dc 

 {Jig. 57), refult from a decrement by fciT ranges parallel to 

 the ridges GDBH (fg. 58); 5'h, that the plane ctYg 

 and its oppofite [fig- 56), which corrcfpcnd to fy, ca 

 {fg. 57), refult from a decrement by two ranges parallel to 

 the fame ridges G D, B H (fg. 58), which decrement takes 

 place on the other fide of thefe ridges. It may be feen by 

 what has been already faid, that decrements different in their 

 meafuie give rife to planes fimilarly fituated, fuch as on km 

 and cfgh (fg- $6), which is a confequence of the particular 

 figure of the molecule. 



With regard to the faces of the fummit, the heptagon 

 p Gicdez (fg. 56) is fituated parallel to the bafe BR GO 

 (fg. 58). The enneagon B sr"P oji bez (fg- 56) is pro- 

 duced in confequence of a decrement by one range on the 

 angle OBR (fg. 58), or parallel to the diagonal OR; 

 which decrement does not attain to its full extent, and leaves 

 fubfifting the neighbouring heptagon parallel to the bafe 

 13 R G O. It may be readily conceived, after what has been 

 faid on the pofition of the diagonal OR, why the line ez 

 (fg. 56), which feparates the two large faces of the fummit, 

 is fituated horizontally, fuppofing that the planes have a 

 vertical pofition. 



The trapeziums dafc, ApGC, refult from a decrement 

 by one range on the ridges GO, BR, (fg. 58). The 

 facet deb a (fg 5 b) arifes from a decrement by two ranges, 

 parallel to the ridge BO (fg. 58;. With regard to the 

 other facet ABsft, which has the fame pci'iion as the pre- 

 ceding, in regard to the oppofite part of the cryftal, it refuks 

 from an intermediary law by a range of double molecule on 

 the angle OBR (_/%. 58). The rhombufes bclh, hlfu 

 (fg. 5-1), reprefent the horizontal fe£tions of two of thefe 

 cloubl^ moleculte, taken in the fame range, and whofe rela- 

 tion to the reft of the afTortment will become fenfible by 



comparing 



