ARSEKIATED COPPER. <2,55 



derable j It then takes the appearance of a rhomboidal tef rahe- 

 <lra! prifm of S4-° and 96*^, terminated at its extremities by a 

 dihedral fummit with ifofceles triangular planes, the fummit of 

 which is placed on the edges of 8i°, and the bafes meet with 

 each other in an angle of 112". This form has not hitherto 

 fliown any other modification except being replaced by a plane, 

 larger or fmaller, on the edges of 96°. Its planes are ufually 

 very fmooth and brilliant, and I have never been able to dif- 

 cover an appearance of divifion (clivage) in any of them. 



This third fpecies paifes by the greatly lengthened o6lahe- 

 dron to the determinate capillary variety, as well as to that 

 which is indeterminate, and in this cafe the colour appears 

 cither to fend more to green or to take a more defined yellow, 

 which fometimes has the brilliancy of gold. 



The firft fpecies does not exhibit any thing which refembles 

 (hefe various tranfilions; it is always the fame obtufe odahe* 

 dron, and only varies by a very flight prolongation of its cryftals 

 pacallel to the leaft inclined faces. To make the formation of 

 the acute odlahedron of the third fpecies, fecondary to the ob- 

 tufe one of the firft, you fuppofe a decrement at the bafe of 

 the latter of two rows above and below the edges of the union 

 of the leaft inclined faces, and another of foyr rows at that of 

 the union of the moft inclined faces, and by this, you get an 

 \ acute o6iahedron, whofe moft inclined faces meet at the fum- 

 mit, in an angle of 86^ 24-' and at the bafe in an angle of 

 93° 36' ; and the others meet at the fummit in an angle of 7 1 °, 

 and at the bafe in one of 109°. 



I acknowledge that this approximation to the meafures 

 which I have given is feducing, and, confidering the natural 

 (inallnefs of the cryftals of this fpecies, it would perhaps be 

 difficult for me to pronounce determinately whether the mea- 

 fures which I have taken are much more exa6t than thofe to 

 which you have attained by calculation; but this I can aifure 

 you, that no indication whatever, in either of thefe two o6ta- 

 hedra, leads to the fuppofition which has given you this re- 

 fult. 



From the details I have now given, it is eafy to deduce the Reafons for 

 reafons which impel me to adhere to the divifion which I have Jj'^^^^/"^^'' 

 thought it right to make in the arfeniated copper, and prevent fpecies. 

 me from adopting the approximation to which your ingenious 

 hypothefes have led you. Every thing ftill fcems to me to tend 

 3 to 



