424 Mr. D. Forbes on Darwinite, 



These strings or veins are described as entirely composed of 

 this mineral in a state of great purity, but very narrow, seldom 

 attaining the breadth of more than 2 inches across. The spe- 

 cimen which was examined appeared to represent the entire 

 thickness of such a vein, and is a mass of about 1£ inch across, 

 .composed of the pure mineral free from gangue or extraneous 

 metallic compounds, but having its sides coated with red oxide 

 of copper, and in some parts spots of a green arsenite or arseniate 

 of copper. 



From its colour, lustre, and great specific gravity, it had, on 

 discovery, been supposed to be native silver, and workings were 

 accordingly commenced on the veins, — which, however, were at 

 once abandoned when the assays made at Copiapo showed the 

 very small amount of silver entering into its composition; and 

 as when thrown into a forge it evolved arsenical fumes in abun- 

 dance, it was regarded as " Arseniko," or native arsenic, and, 

 strange enough, does not even appear to have been assayed for 

 copper. 



The mineral is massive, without trace of cleavage, is rather 

 brittle ; but although easily broken, its surface may be distinctly 

 impressed by the hammer before yielding : fracture even ; hard- 

 ness 35. 



Lustre metallic ; colour of freshly-fractured surface dark silver- 

 grey, tarnishing on exposure to a dirty bronze-yellow; streak 

 metallic, dark silver-grey ; opake. 



The specific gravity of three separate fragments was found to 

 be respectively 8'69, 8*67, and 8-57 ; consequent mean 8*64. 



Heated in a closed tube, the mineral does not alter, or at 

 most a faint trace of arsenious acid sublimes on to the side of 

 the tube, arising from the action of the air contained in the same ; 

 in an open tube, a distinct white sublimate of arsenious acid is 

 obtained. 



Before the blowpipe on charcoal in reducing flame, it fuses 

 readily to a silver-white globule, which in the act of cooling 

 evolves arsenical fumes and becomes slightly red on surface ; in 

 the oxidizing flame on charcoal, it evolves abundant arsenical 

 fumes, rotates, and ultimately leaves a globule of metallic copper, 

 malleable, but still retaining some arsenic : on cupelling this but- 

 ton of copper with lead, a minute globule of silver is obtained ; 

 with fluxes, it gives the reactions of copper only. 



A qualitative examination by the wet way confirmed these 

 results, and showed the presence of copper, arsenic, and silver 

 without other admixture. Sulphur and iron were especially 

 tested for, but not found present. In one specimen a trace of 

 lead was detected, which appeared evidently to be an accidental 

 impurity. 



