On Crystallography, 217 



fn<?nt of the structure will assist us in forming a clear ide* 

 Ct the^c variations. 



Let a bdf ((ig. 19 A) be the same rhomb with fig:. 18, 

 what WL- have lo say with respect to tliis rhombus may be 

 easilv a|)|)lieii 10 the five others. Suppose it to be divided, 

 -as represented in fig. I9, into 81 partial rhombs, which 

 mav l)e considered as the external facets of so many mole- 

 cules: licnce we have 729 molecules for the entire nucleus. 



'i'he first lamina of superposition which must be ap|-lied 

 on the rhond) a h d f. will be that which we see in fig. 16 B, 

 in which \J lZ,d' represents the grand external face, and 

 C \J I A', X Z / //, ihe two small superior faces. We shall 

 ariapge this laniina with respect to the face a h dj (fig. 1 9 A) 

 so that the point // may be blended with the point k, the 

 point A' with the point A, and the point B' wiili the jpoint 

 B. VVc at first see bv this arrangement that the two su- 

 perior ranires of the face abdj (fig. 19 A), both of them 

 cominued l)el>.\en the lines nb, Ah, on one hand, and 

 aj, B A, on the other, remain exposed ; which puts in ejc- 

 ecutii.n the law of deLrement bv one range of molecules. 



The lannna in question is a pentagon resulting from t-he 

 retrvurhment of the three small rhomboids destined to com- 

 plete It towards the base. This retrenchment is necessary, 

 that the lamina may, by its figure, beacconnnoda'ed to the 

 fcfiect ot the decrement, as we shall presently explain. 



The two rauiies of rhomboids situated one bv one on 

 both sides ol the lines D d\ Kd', (hg. B) are added, in or- 

 der that the nucleus may be enveloped, and continue t« 

 increase towards the edgCN b d, f'd. (fig. I9 A) which cor- 

 re-<])ond with these lines. These two ranses being suffi- 

 cient lor fillinir up the vacuum, we perceive that it is not 

 iieees-^arv to add similar ones towards the adjacent edses of 

 the laminae of superposition applied on the adjoining faces, 

 Tlie o|ieration will indicate of ilbelf what is lo be done re- 

 lative to this kind t)f additions. 



Fiff. 19 C represents tlie second lamina of superposition, 

 which (fught to be applied on the preceding one in such a 

 way that the points i', D', E', d, may be confounded with 

 tiio<e which arc marked by the saine letters (fig. 19 B). As 

 llie i-rvsial ouiiht to assume a new increment towards the 

 cdaes which correspond with Fd', Gd', we conceive that 

 jiiitiad of one range added on both sides of the lines Dt/*, 

 lid', (t\-s. 19 ' ) it is necessary to add two (fig. I9 C) on 

 (he !\' o sidcb of the lines Vd', G d' . 



\Vc shall place successively in the same way the two la- 

 mi n(p 



