tf Copper and of Irort^ 9 



Inclined than the other two. The more inclinec] planes meet 

 at the apex in an angle of 84°; and at the bale, in one of 96': 

 the other two meet at the apex in an angle of 68°; and at 

 the bafe, in one of 113''. (Fig. 4.) 



In this oftaedron it fometimes happens, that the planes 

 which compofe its pyramids tend to one and the fame point, 

 in order to form the apex; but it is much more comniou'to 

 find it extended in a line parallel to the lefs inclined planes 

 of the pyramid. (Fis. 5.) The cr\-n;al is (till more frequently^ 

 found ill the form Si' a. long tetraedral rhomhoidal pril'ni of 

 84- and 96', terminated by a diedral apex, with ifofceles tri- 

 angular planes, which are'placed on the angles of 84°, and 

 meet in an ancle of of 1 12 . (Fig. 6.) 



Moft commonly, both in the perfeR and the lengthened 

 ocfaedron, the angles of 96° are replaced by a plane, which 

 is equally inclined ~c)n the adjacent fides, (fig- 7.) and is fre- 

 quently vcrv broad : (fig. 8.) then the tetraedral prifni of 84" 

 and 96^ is changed into a 'fiat hexaedral prifm, having two 

 angles of 84=, ami the other four of 138 . I never faw the 

 angles of 84° replaced. 



The average fpecific gravity of this arfeniate of copper, 

 taken on five pure pieces, was 4,280. 



it is furticiently hard to fcratch fluor fpar, but is not hard 

 enough to fcratch glafs. 



Its^ufual colour is a brown, or bottle green, fo dark that 

 the cryltals appear of a blackilh colour, when they are not 

 oppofed to the light : fometimes, but very feldom, in the re- 

 gular cryftals, which happen to be rather thicker, this colour 

 IS a clearer green ; in other fpccimens the crvftals have a yel- 

 lowifh caft, and the furface then often reflcds the light of a 

 golden tint. 



The tranfparency of this fpecies is generally pretty great- 

 It is not always cryfiailized in a determinate form, but is 

 an abfolute Proteus, both with refpe6t to the diflerent foruis 

 in which it appears, and the various colours it exhibits. I 

 have obferved the five following varieties of it : 



Variety i. Capillary, of a determinate Form. 



In this variety, the cryftals are extremely {lender, yet pre- 

 fcrvc their form, which is that of a very lengthened od^ae- 

 dron. The fmall {lender cryftals often form theml'elves into 

 a confuted group ; fomclinics, however, they form iinall ma- 

 iiiilla;, by the divergence of a nuiViber of tnein from a com- 

 mon centre. Tljcir colour is either a fine grals green, or a 

 vcllowidi green, tn- a golden yciiow; and they liave gene- 

 rJly a Iciutiful traufpareiicy. 



(i Varlely 



