596 



GEOLOGY AND MINING INDUSTRY OF LEADVJLLE. 



ing from one cupel to the other. The silver was then weighed upon an Oertling 

 assay balance, indicating a difference in weight of 0.02 milligram with great exactness 

 and of 0.01 milligram with tolerable accuracy. After deducting from the weight of 

 silver found that due to the lead acetate, which, where ten assay tons had been used, 

 would be 0.04 milligram, division of the remainder, if any, by the number of assay 

 tons of rock taken gave directly the contents in ounces and decimal fractions of an 

 ounce troy per ton of 2,000 pounds avoirdupois, since 29,166.6 ounces troy make one 

 ton of 2,000 pounds avoirdupois and an assay ton contaius 29,166.6 milligrams. The 

 silver was then dissolved in nitric acid, but the presence of a trace of gold, derived 

 from the lead acetate, rendered the detection of gold from the rock impossible, unless 

 its amount considerably exceeded that of the lead salt. An example will best show 

 the degree of accuracy attainable. Suppose rock and lead acetate to have been taken 

 in the usual amounts: Four assay tons (116.66 grams) of the former to ten assay tons 

 (291.66 grams) of the latter, and the final silver button to weigh 0.06 milligram. 

 From this is to be deducted 0.04 milligram, and the remainder divided by 4, the 

 number of assay tons of rock tested, gives 0.005 ounce per ton as the accurate result. 

 Had the weight been 0.05 milligram, the correctness of the result, 0.0025 ounce, might 

 be more open to doubt, as the balance cannot be counted upon to indicate differences 

 of only 0.01 milligram with certainty. Hence, for the above quantities of sample and 

 lead acetate, 0.005 ounce per ton is about the limit of accuracy. 



There will be noticed in the table occasional instances, notably in No. 41, where 

 lower figures are given. In these cases the amount of rock assayed had been increased 

 without at the same time increasing the lead acetate. In the case of No. 41 it was 

 impossible to decide from 4 assay tons whether silver was present or not, though the 

 weight seemed to slightly exceed 0.04 milligram. By doubling the amount of sample 

 and using still only ten assay tons of lead acetate, the weight of the silver sensibly 

 increased, thus showiug beyond reasonable doubt that the rock was argentiferous. It 

 did not appear advisable, however, as a rule, to reduce the proportion between the 

 weights of sample and lead acetate much below 4 : 10 for fear the reduced lead might 

 not be sufficient to extract and collect the silver entirely. 



LIMESTONES. 



TABLE V. Complete analyses. Dolomitic limestones. 



00.000,022. 5 per cent. 



I. Type of the Silurian or White Limestone. Co'J. No. 164. From quarry in California gnkh. 



n. Type of the Lower Carboniferous or Blue Limestone. Coll. No. 170. Silver Wave claim, Iron Hill. 



