ig2 ON AR.SENIATED COPPER, 



Comparifon of In a direflion parallel to the faces of the two pyramids, of which 



the acute and •, • ^j^g afforegate *. Proceedin? on this datum, I was curious 

 obtufe o«ahe- , .°*'. ^ . rr-i 5 . o. -.i .l r r .u 



^a^ to know if it was not pombie to connect with the form ot the 



obtufe o6lahedron in queftioHj that of the acute odahedron, 

 which M. de Bournon has taken for the type of his third 

 fpecies. Let P, P' (Fig. 2) be ftill an obtufe odahedron, in 

 which the incidence of jP onp is reckoned to be 50^* 4^ and 

 that of P' on //, 65** 8', conformably to the meafures indicated 

 above: if we imagine another o6tahedron (Fig. 3) the fign of 



24. 

 which is ^ , we (hall find that the incidence of I on I' is 



I r 

 109", and that of r on r' is 93* 36', Now the correfpondent 

 incidences determined by M. de Bournon, are one 1 12'=*, and 

 the other 96"; which in one cafe makes a difference of 3°, 

 and in the other of 2° 24.^ 

 The differjnces If the meafures had been taken on cryftals fo well defined, 

 in the angles ac- ^j^^^ jj-,g differences could be confidered as real, we muft have 

 concluded that they formed two diftindl fpecies, becaufe even 

 thefe differences could only have been done away by fuppofing 

 the laws of decrement to be much too complicated to be ad- 

 miffible. 



But if the cryftals were not capable of very accurate meafure- 

 inent, we can the better conceive that the differences were 

 fimply apparent, and that it may be poffible that the error did 

 not wholly arife in one obfervation, fince it was neceffary ta 

 make two, in which fmall deviations might have been pro- 

 duced on oppofite fides; and then mechanical divifion alone, 

 by giving different refults with refped to the two oftahedra, 

 would have fliown that a conformity between the angles ob- 

 ferved and thofe meafured, would be purely accidental. 

 Comparifm of ^ afterwards compared the lamelliform variety with alternate 

 the latiiel iiorm bevils, which is the fecond fpecies of M. de Bournon, with the 

 ob!u?oa'ahe'.^' fame odahedron with obtufe fummits. Now, if we fuppofe 

 ^ron, two interfering planes, parallel to the face P', and which meet 



the center, they will detach an odahedral fegment which can- 

 not be fuppofed to have much thicknels, and whofe two large 

 faces will be hexagons, and the fix lateral faces, trapeziums, 



♦ Journ. de Phjftque, Pluviofe, An. X. p. 131. 



inclined 



