o/ Copper and of Iron. 7 



Thefe planes are commonly fmooth and bright; fome- 

 times, however, they are ftriated in a direftion parallel to 

 their edges. 



The tour planes feldom terminate in one and the fame 

 point : more commonly the apex is formed into a ridge, the 

 oftaedron being lengthened^ parallel to its lefs inclined planes; 

 the bafe is then a fquare, or at leall approaches very nearly 

 to that form. (Fig. 2.) 



Thefe two varieties are the only ones I have obferved in the 

 form of the cryftals of this fpecies, althoug^h I have had the 

 opportunity of examining a great number of fpecimcns. 



This arfeniate is very light ; its average fpccific gravity, 

 taken on fix pieces perteAly pure, was 2,881. Its hardnefs 

 is likewife very inconfiderable; it eafily fcratches calcareous 

 fpar, but makes no impreffion on fluor fpar. 



It is feldom perfedtly tranfpareut, but has generally a 

 cloudy afpeft. 



The ufual colour of this fpecies (for this character is as ef- 

 fential in metallic fubftances as it is immaterial in ftones) is 

 a beautiful deep fky-blue; fometimes, but very rarely, it 

 inclines more or lefs to Pruffian blue. It is frequently of a 

 very fine grafs green J the cryitals have then a much more 

 beautiful tranfparency. I have fcen fome which were of a 

 fine apple green'; others white, having a flight blue caft. 

 In one piece, the cryftals of which were of a green colour, 

 and lefs tranfparent than they commonly are, I difcovered, 

 on breaking them, that the colour of their central part, for 

 about half their thicknefs, was blue. From the obfcrvations 

 made by Mr. Chenevix in his analyfis of thefe arfeniates, 

 it appears that the variation in their colour principally de 

 pcnds on the quantity of water which enters as a conftituent 

 part into their formation. 



This fpecies is found mixed with all the other kinds of 

 arfenical copper ore; but that which moft commonly accom- 

 panies it, is the prifmalic triedral fpecies. 



I have never dilcovered in tiiis fpecies any thing which 

 could induce me to fuppofe it fufccptible of decompofition, 

 or even of change. 



Second Species. — Arfeniate of Copper in hexaedral Lamina; 

 <with inclined Sides. 



This fpecies is commonly found in very fine hexaedral 

 laminae. The fix narrow fides of ihefc laminae liave an in- 

 clined pofition, allernalcly in a contrary direction, on the 

 two broad planes, in fuch a manner that each of the planes 

 is encompaflcd by three fides, which are inclined upon 

 A4 it. 



