8 Defcript'ion ef the Arfeniates 



it. (Fig. 3.) As far as the fmall fize, and more particularly the 

 thinnefs of thefe cryftals, has enabled me to judge, two of 

 thefe three inclined fides form an angle of about 135' with , 

 the broad planes on which they incline j and the third, one 

 of 115°. 



The two broad planes are fmooth, and have a very bril- 

 liant luftre. The fix narrow fides are rendered very dull by 

 the great number of ftria? with which they arc covered ; moft 

 of which are very prominent, and all are parallel to the 

 edges of the broad planes. In confctinence of this, thefe 

 cryflals may be divided, parallel to the planes, almoft as 

 eafily as cryllals of mica. 



This ftrufture prevents the cryftal from being confidercd 

 as a modification of the oftacdron : that which would be 

 produced by an increafe of the inclined fides, would only be 

 a fecondary cryfl;al ; and none of the fpecimens I have leen 

 give me reafon to fuppofe the exificncc of fuch a variety. 



The colour of this fpecies is a fine deep emerald green ; 

 fometimes, though rarely, it is found of a lighter colour. 

 The luftre of its broad planes, which are the only parts of 

 the cryftal that forcibly ftrike the eve, give it pretty nmch 

 the appearance of thole coloured metal plates which are 

 known by the name oi foil. 



This fpecies is ftill Icfs heavy than the preceding, its fpe- 

 cific gravitv being only 2,548. 



It is alfo lefs hardj it eafily fcratches gypfum, but not 

 calcareous fpar. 



When its cryftals are very thin, they are very tranfparent j 

 but their tranlparen'cy is diminifilied when they have any 

 dtgree of thickncfs. 



When expofed to fire, this fpecies decrepitates very ftrongly. 



This arfeniate, the matrix of wiiich is generally quartzous, 

 is occafionally found mixed with fome other arfeniates of 

 copper, and particularly with the acite oclaedron in the ca- 

 pillary or fibrous ftate. (Spec, 3. Var. i and %.) But the 

 ore moft commonly found with it is the red copper ore, 

 which is frequently verv abundant. 



I have never obferved in this fpecies any appearance of 

 deconipofition. 



Third Species. — Arjcn'iate of Copper in the Form of an acufs 

 Oilaedron. 



TTie rao^ fimple form in which this third fpecies is found, 

 19 likewife an o6laedron ; bijl this oftaedron, inftead of being 

 obtiil'e, like that of the firft fpecies, is flighily acute. It has, 

 like that, in each of its pyramids, two oppofile planes more 



inciiri...'d 



