580 MINERALOGY 



fusion is cut away from the coal and ground in the mortar under 

 water. If pressure is used in the grinding the lead buttons will 

 be flattened out in thin scales, when if a gentle stream of water is 

 allowed to run in the mortar, the coal and slag, being much lighter 

 than the lead buttons, will be carried away. When very little 

 lead is present, the end of the pestle must be examined for metallic 

 streaks; also the coal, for the yellow oxide coat (see below). 

 The button of lead may be further tested by dissolving it in nitric 

 acid and adding a drop of H 2 S0 4 , when if it is lead a white precipi- 

 tate of lead sulphate will form. 



6. Yellow lead coat. Minerals and compounds containing 

 lead are roasted in the O. F. for a time ; then reduced with the R. F. 

 lead is reduced to metal, and if the heat is continued, volatilizes, 

 forming a yellow oxide of lead coat. The yellow coat may, when 

 the mineral contains much sulphur, be masked by a white sul- 

 phate coat ; if the white coat is heated in the O. F. until it glows, 

 the white coat will be reduced and on cooling will appear yellow. 



c. Bismuth yields a yellow oxide coat which at times is dis- 

 tinguished from the lead coat with difficulty. In this case the 

 powdered mineral is mixed with an equal volume of von Kobell's 

 flux and treated with a moderately hot R. F. on coal, when if 

 lead is present, a lemon-yellow iodide of lead coat will form, at a 

 distance from the assay. Under similar conditions bismuth will 

 yield a brick-red coat. 



Illustration. For the above tests use galena, PbS. 



Bismuth, Bi. Atomic weight, 208. Fusing point, 269 C. 



a. Coat. Bismuth compounds, when heated in a moderately 

 hot R. F. with soda and borax as in the case of lead, yield metallic 

 globules, which, however, are not malleable. If the flame is con- 

 tinued, the metal volatilizes, forming a yellow oxide coat. 



6. Von Kobell's test. The finely powdered mineral is mixed 

 with 2 parts of von Kobell's flux and treated with a moderately 

 hot R. F. on coal ; if bismuth is present, a brick-red iodide of bis- 

 muth coat will form at a considerable distance from the assay, 

 inside of which there may also form a yellow oxide coat. This 

 brick-red coat is a very characteristic test for bismuth and serves 

 to distinguish it from lead. 



Illustration. Use bismuth oxide. 



Copper, Cu. Atomic weight, 63.57. Fusing point, 1084 C. 



a. Flame test. A very delicate test for copper is to roast the 

 mineral in O. F. on coal ; the charge is allowed to cool and is then 



