254? ARSENIATED COPPKR. 



I have fubmitted fame new cryftals of this fpeci^s (o rnea- 

 furement, and have found them agree perfectly in therneafures 

 of 1 15** and ISi'^ with thofe which I had examined before. 

 Tiie angle fuppofed by yon of 130*' 30', which I have tried ore" 

 a number of cryftals always appeared to me to be much too 

 A new variety of fm a! I. Thefe cryftals have offered me a new variety, in which 

 the lecond jj^^ ^^^^^ of the hexahedral laminae are lefs inclined on the ter- 



minal faces, with which they form an angle of about 105®. 

 The cryflal which afforded me this new variety is four lines in 

 diameter: it is only in perfe6l prefervation in one of its halves/ 

 but it admits of a judgment from it, that all its fides muft have 

 the fame inclination. Thefe new faces are perfedly fmoothy 

 and do not (how any flriae. On another cryftal, inftead of 

 thefe inclined fides, two planes are obferved, one of w^hich 

 belongs to that which made an angle of 105" with the termi- 

 nal face to which it inclines, and the other belongs either t& 

 that of 115° or that of 135". 



There was not any thing that I could difcover conneded' 

 with any of the planes of the obtufe odlahedron of the firft 

 fpecies. 



Comparifon of the Third Species with the Firjl. 

 Comparifon of The colour of the firfi is either a deep flvy-blue or a grafs- 

 the third fpecles ggj,^ That moft ufual in the third is a yellowini green, more 

 With the firft. ^ , r , , r .• ,. •,, 



or lets deep, but very frequently it can only be perceived by 



placing the cryftal between the eye and the light, the intenfity 

 of the colour making the cryfials appear black in every other 

 pofition. 



Tl:e fpecific gravity of the firfl is 2881 ; that of the third 

 4280. 



The hardnefs of the firfl: is not more than fufficient tofcratch 

 carbonated lime ; that of the third is fuch as to cut fluated lime. 



The firft fpecies has an obtufe rectangular oflahedron for a 

 unique and primitive cryfial, whofe dimenfions have been 

 given above: the figure of the third is an acute rectangular 

 odahedron, in which each pyramid has two faces more in-' 

 clined than the other two; the two moft inclined faces meet ;* 

 at the fummit, in an angle of 84'', and at the bafe in one of 

 96', and the two others meet at the fummit in an angle of 

 68", and at the bafe in one of 1 02®. This oftahedron is mofi 

 ufually cuneiform, and its prolongation is fometimes very confi- 



derable; 



