ERUPTIVE ROCKS. 595 



REMARKS ON TABLE IV. 



For the estimatiou of such extremely small quantities of silver and gold as it waa 

 supposed some of the eruptive rocks from the Leadville region might contain, and 

 even for their detection alone, a most extreme degree of care and precaution was 

 imperative. It being necessary to operate upon large quantities of material, it was 

 decided to make the determinations by crucible assay, this process combining the 

 greatest accuracy with the least expenditure of time. It was found, however, after a 

 number of tests, that none of the lead or litharge obtaiuable was sufficiently free from 

 silver for the present purpose. The silver contained in the lead or litharge used for 

 an assay was generally so largely in excess of that in the powdered rock mixed with 

 it that the prills of silver obtained from the regular assay upon rock known to contain 

 silver and from a check assay upon the lead or litharge alone frequently differed in 

 weight only within the allowable limits of error. Recourse was then had to lead 

 acetate, of which several lots were examined. These were all found much freer from 

 silver than either of the substaices previously tested, and one lot of commercial acetate 

 from Mallinckrodt & Co., of Saint Louis, Mo., was used for all the assays tabulated 

 above. 



Preparatory to using, it was dehydrated by fusing in a large iron vessel till 

 sadden swelling up and solidification of the whole mass took place, and then finely 

 pulverized. This material, containing about 73 per cent, of lead by assay, was found 

 bj- repeated tests, conducted, as given below, upon the same amounts as used for the 

 rock assays, to carry 0.004 ounce silver per ton of 2,000 pounds, or 0.0000137 per cent., 

 including a trace of gold far too small for estimation. The latter was left on solution 

 of the silver in nitric acid as a minute black speck, indistinguishable without the aid 

 of a lens. By collecting into one button the silver from 500 to 600 grams of dehy- 

 drated lead acetate, parting with great care, bringing the gold upon a sheet of white 

 writing paper and flattening it out with a knife blade, the yellow reflection of gold 

 could readily be observed by examination with a lens, and sometimes with the naked 

 eye. 



The process of assay was as follows : Four Hessian crucibles, of suitable size, 

 were each charged with one assay ton (29,166 milligrams) of the sample to be assayed, 

 two and one-half assay tons of the dehydrated lead acetate, and a proportionate 

 amount of a flux consisting of soda, borax, and a little argol. After mixing well, a 

 layer of salt was placed on top, and, if much pyrite was present, an iron nail inserted. 

 The four charged crucibles were then placed covered in a wind furnace fired by coke, 

 and left, with proper regulation and final strong increase of temperature, till fusion 

 was complete. The contents were then poured into molds, the lead reguli, weighing 

 each about 55 grams, reduced by scorification in a muffle to a smaller size, the reduced 

 reguli united two and two and rescorifled, and the two resulting therefrom again 

 united and reduced by scorification to a single button of suitable size for cupellation. 

 Toward the end of cupellation, which was always conducted with the greatest care 

 and, as nearly as possible, under the same conditions of temperature for each assay, 

 the button was poured from its cupel into another one immediately behind the first, 

 in order that the cupellation might be finished upon a smooth bottom. If this jirecau- 

 tiou was neglected, the silver button was occasionally not to be found in the roughened 

 surface of the cupel. After a little experience, no loss need be apprehended in pour- 



