130 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



reduction of the "pulp" and imperfect determination of the contents of 

 the litharge. If proper care is taken in melting, most of the silver contained 

 in the rock can be collected in the lead button. It cannot be expected that 

 all the silver will be obtained, but what remains in the slag can be reduced 

 to an almost constant quantity by reproducing in all assays nearly the 

 same conditions, such as fineness of "pulp," length of time of melting, 

 quantity of flux, etc. Assays of the same rock have been repeated several 

 times with identically the same results. The quantity of silver produced 

 after a certain point does not seem to vary perceptibly with the time during 

 which the assay is kept melted. The second source of inaccuracy is the 

 more difficult to control, namely, the impossibility of obtaining a flux abso- 

 lutely free from silver, or of correctly determining the amount of silver in 

 the button which is derived from that source. 



Action of bitartrate of potash on litharge. — The quantity of litharge used in assay- 

 ing Eureka rocks was about 770 grains. To reduce this litharge 165 grains 

 of bitartrate of potash were added, and the resulting button of lead, when 

 reducing gases from the fire had been excluded as far as possible, usually 

 weighed in the neighborhood of 425 grains. Almost all the limestones in 

 Eureka district carry more or less free carbon." The quantity of sesquioxide 

 of iron present was so slight that only a very small portion of the carbon 

 was absorbed in reducing it to the protoxide (FeO.). Where there was 

 any quantity of that mineral present in the rock it was necessary to increase 

 the amount of bitartrate used in order to obtain a lead button of about 425 

 grains in weight. 



Bearing of the silver in the litharge on the results. The silver Contained in the 425 



grains of lead reduced from the 770 grains of litharge was 6 cents per ton 

 (0.0001591 per cent.) when the flux itself was assayed. This amount scarcely 

 ever varied, but frequent check assays were made. When peroxide of iron 

 or other substances requiring reduction were present the weight of the lead 

 button was less and the amount of silver it contained was also less. When 

 other reducing substances were present, such as organic matter in the lime- 

 stone, the weight of the lead was greater as well as the amount of the silver 

 resulting from cupellation. This increase or decrease in the amount of silver 



"This is particularly the case with the so-called "back limestone." 



