140' On the Arjcniales of Copper and of Iron. 



eafilv caufc them to efcape the notice of common obfcrva- 

 tion, particularly when they are not in pretty large groups. 



The matrix of thcfe two arfeniatcs is exa6lly the fame as 

 that of the arfeniatcs of copper; confining, like that, of 

 quartz, mixed with j^ellow, gray, and vitreous ores of cop- 

 per, with oxides of iron, and with mifpickel. The mines 

 of Hue] Gorland and Muttrell, although not fituated in the 

 diftricl of the tin mines, have yet produced fome fpecimens 

 of tin, the cr.vltals of which are covered with thofe of the 

 arfeniatc here fpokcn of. Two fpecitncns of this kind are in 

 the colledion of Sir John St. Aubyn.' 



Species I. Simple Arfcnlate rf Iron. 



This fpecies cryftallizes in perfeil cubes, (fig. 28.); fomc- 

 times, though rarely, they are a little flattened ; their fides are 

 fmooth and brilliant. 



The only modification I have obfervcd in this form is, that 

 four of the eight folid angles of the cube are replaced by an 

 equal number of equilateral triangular planes, fituated in fuch 

 a manner, that every one of the fides of the cube becomes 

 an elongated hexagon, having two angles of 90" each, and 

 four of i^^)''. (Fig- '29.) Cryfrals modified in this way are 

 very fcarce. I have never feen but one fuch fpecimcn, which 

 is in the collection of Sir John St. Aubyn. The cryfials of 

 it are pretty large, and very well defined. 



The f]iecific gravity of this fpecies is 3,000. Its hardnefs 

 is jull fufficicnt to fcratch calcareous fpar. Its cryftals, which 

 are tolerably tranfparent, are of a dark green colour, with a 

 brownifh tinge; fometimes they are rather yellowifli; and 

 there exift fome fpecimens of a biown yellow colour like 

 refin. I have never feen this fpecies in any other ftate than 

 that of perfect cryftallization. 



Sometimes, indeed, a decompofition takes place, which 

 caufes the cryftals to pafs into the ftate of a pulverulent oxide, 

 of a fine rcddifli yellow colour. In this cafe, as the bulk of 

 the cryfials is confiderably diminiflied, there is perceived, 

 npon breaking them, a confiderable number of fmall cavities 

 in their 'fubitance. 1 hefe cavities are analogous to thofe 

 which appear in the cryllals of the fpathofe ores of iron 

 vvhen they have pafled into a fimilar ftate of decompofition. 



Species II. Cupreous Arfeniatc of Iron. 



The cryftals of this fpecies are of uncommon brilliancy, 

 and are perfectly tranfparent. Their form is a rhomboidal 

 tetraedral prifm, having two of its edges very obtufe, and 

 t)ie other two very acute: but, owing to the minute fize of 



thefe 



