of the red oxyd of Copper and arseniated Iron, 29 



less, to that analysis, it has been called by Brongniart, Cuivre 

 oxidule. 



The colour of this mineral varies from carmine red to metallic 

 grey, occasionally inclining to black. 



Its lustre is considerable — very considerable in the more trans- 

 lucent crystals. 



It is not very brittle. 



It easily cuts calcareous spar, but will not scratch fluor spar. 



It gives, when rubbed on paper, a slight red streak. 



It emits no smell when rubbed. 



When powdered it is of a brick-red colour. 



It emits in that state no light when thrown on a hot iron. 



Its specific gravity is 5fi. 



It is soluble with effervescence in nitric acid, to which it imparts 

 a greenish tinge. 



The fracture of the crystals, particularly of the more translucent 

 ones, is very smooth and inclines to conchoidal ; but is frequently 

 uneven, and inclining to shattery, in those of a darker hue. I have 

 met with some octohedrons that admitted a fracture in the direction 

 of their faces, but have not found any of a cubical form in which a 

 division parallel to the circumscribing planes was practicable. 



The form of the primitive crystal, according to Haiiy, is the regu- 

 lar octohedron (fig. 1.) and of the integrant molecule, the regular 

 tetrahedron. 



The crystals of this substance are mostly well defined, but do not 

 in general exceed a line in length. The largest in my collection is 

 nearly half an inch ; I have seen others somewhat larger. They are 

 however frequently so small, as to appear to the naked eye a mere 

 point, but by the assistance of the lens, the perfection of their 

 geometrical forms may easily be discovered : a perfection rarely 



