578 MINERALOGY 



Metals which are precipitated from acid solution by hydrogen 

 sulphide as sulphides, and the sulphides of which are insoluble in 

 ammonium sulphide. 



Common metals: silver, mercury, lead, bismuth, copper, and 

 cadmium. 



Rare metals: palladium, rhodium, osmium, ruthenium. 



Silver, Ag. Atomic weight, 107.87. Fusing point, 955 C. 



a. Reduction. Compounds containing silver when fused on 

 coal with 4 parts soda in R. F. yield metallic silver. If the silver 

 button does not appear bright, it may be heated on coal in the 

 O. F. beside borax ; the base metals will oxidize first ; their oxides 

 are dissolved by the borax, leaving the silver button bright. 



When the amount of silver in a mineral is small, considerable 

 of the powdered mineral should be mixed with 2 volumes of test 

 lead and one volume of powdered borax ; this charge is placed in a 

 deep cavity in the coal and heated with the R. F., gently at first, 

 until the lead is fused ; the reduction is continued for a couple of 

 minutes, shaking the coal now and then while under the flame to 

 collect the lead in one globule. All silver and other reduced 

 metals the charge may have contained will be collected in the 

 lead globule. The O. F. is now used to refine the lead, by oxidiz- 

 ing arsenic, antimony, or any easily oxidized elements the lead 

 may contain. When the lead begins to boil freely, the assay is 

 allowed to cool, the lead is picked out and cleaned of slag by 

 hammering on the anvil. A cupel is now prepared by cutting a 

 cavity 6 mm. deep and 12 mm. in diameter near the end of a firm 

 piece of coal ; this is packed full of dry bone ash and pressed down 

 with the large end of the agate pestle, giving a twist to the pestle 

 at the last which will leave the concave surface smooth and hard. 

 The loose bone ash is blown off and the cupel is ignited in the 

 O. F. to drive off all moisture. The clean lead button is placed 

 on the cupel and fused with the R. F. ; after the lead is fused and 

 the cupel is hot, a well-pointed 0. F. is directed on the lead. 

 The flame should be no hotter than is necessary to keep the lead 

 fused and the bone ash directly underneath it well heated or it 

 will not absorb the oxide of lead as it is formed. It is better to 

 roll the lead by tilting the cupel to fresh portions of the surface 

 from time to time, but the lead should never be allowed to freeze 

 or solidify, once the oxidation has commenced. The experiment 



