ON ARSENIATED COPPER. 1^3 



inclined to the great faces *. But thefe trapeziums will not be 

 fituated alternately in oppofite direftions. The three which 

 will form obtufe angles with the large faces, will be contiguous 

 to each other, and the fame will be the cafe with thofe which 

 form acute angles with the fame faces. For example, thofe 

 of the trapeziums which form obtufe angles with the large face 

 analogous to P', will correfpond to the two faces of the adja- 

 cent o6taheclra at B, B\ and to the face fituated behind At 

 parallel to P. The inclination of this latter face on P is, ac- 

 I cording to M. de Bournon, 115^; and the two others, as I 

 J have indicated them above according to my calculations, are 

 each about 139i°. 



Now, of the three lateral trapeziums in the lamelliform ar- 

 fenical copper, one has the fame inclination of 115^ on this 

 bafe, according to M. de Bournon, and the two others have 

 135°; an eftimate which he only gives as an approximation, 

 and which only differs by 4§° from that correfponding to it in 

 the obtufe octahedron, (Fig. 2). 



The great difference confifts in this, that the three lateral 



trapeziums which look towards the fame bafe, in the octahedral 



fegment I have defcribed, are contiguous to each other, as I 



I have faid ; while thofe of the lamelliform arfeniated copper 



\ alternate with the three others which look towards the oppofite 



i bafef. 



j But there is a method of removing this difficulty. Let us 

 conceive that the two fedtions made in the o6tahedron (Fig. 2), 

 inflead of being parallel to the face P^ are fo to the face P. 

 In this cafe the lateral trapeziums, fituated on the two fides of 

 the edges B, B' , will always have an inclination of 139|*' io 

 the fuperior bafe. Now, if the fegment parallel to p makes an 

 angle of 115^ with the bafe analogous to P, the three ieg" 

 ments will preferve, with refped to thofe turned towards the 

 oppofite bafe, the alternation indicated by M. de Bournon, 



• Several fubftances, among others the fpinelle, offer example* 

 of fimilar fegments. 



f The figure given by M. de Bournon, of which that is a copy 

 in Plate XI. Fig. 5, feems to have been traced according to the 

 condition that the three trapeziums turned towards the fame bafe 

 (hould be contiguous. This was doubtlefs an overfight of the 

 draughtfman. 



Vol. VIIL— July, 1804 O But 



