g24 On Crystallogroplnji 



corresponds with one of the taccs A E O T of the nuc}eii^,i 

 in such a manner that the djajona! v\hich passes hy the 

 points e, i, is parallel to that which goes fron) fcl to I, and 

 has niereiv a more elevated position. 



Observation shows that ihis result is realized hy cry^taU 

 lization in a varietv of oiigisfe iron (specular iron ore), which 

 bears the name of binary oligiste iron. 



The decrements which take place on the angl*^, whether 

 superior or inferior, are su^cepiihle of several variations, on 

 >vhich it is proper to give some details. I shall atieuipt to 

 represent these variations by the aid of a -graphic method, 

 which will facilitate its comprehension. 



Let G^ (fig. 29) be any given rhomboid which has its 

 summits in S and s, let S g' s G' (fig. 30) be a quadrilater 

 taken on the oblique diagonals S^", G' s, of two opposite 

 faces, and on the edges S G", sg", comprised between these 

 diagonals. This quadrilater, which I call the principal sec- 

 tion of the rhomboid, is here subdivided into a multitude 

 of similar small quadrilaters, which represent the principal 

 sections of so nuuiy molecules : lastly, letSGg'G' (fig. 

 31) be the same face as figure 29, subdivided into facets 

 of uKilecules. If we suppose that the angle g' undergoes a 

 decrement by a simple course, the small rhomboid which 

 answers to n o g" z, on tlie first lamina of superposition, will 

 be subtracted ; whence it follows, that the edge of this la- 

 mina will have the direction o z, and the distance between 

 the an^le q', which is the parting line of decrement, and 

 the same edge, will be measured by an oblique demi-diago- 

 nal g' r of the molecule. 



If the decrement takes place by two courses, in which 

 case the edge of the first lamina of superposition will cor- 

 respond with c d, the distance in question will be measured 

 bv an entire oblique diagonal g"n of the molecule. Thence 

 \ve shall conclude, that in general, in decrements on the 

 angles, the distance between one lamina and the succeedf 

 ing, which is the same with that between the point of de- 

 parture and the edge of the first lamina, is equivalent to 

 as many demi-diagonals of this molecule as there are course* 

 subtracted ; whereas, in the decrements on the edges, the 

 distance bctveen two consecutive laminae contains a num- 

 ber of entire breadths of the molecule equal to the number 

 of courses subtracted. 



This being done, let us conceive a decrement hy two 

 courses on the angle g". In this case the quadrilater neap^ 

 (fie. 30) being a st-gment made on the first lamina of su- 

 perposition. 



