Mechanical Assay of Quartz. 143 



carbon and the oxidizing nitre, we have the means of 

 liberating any desired quantity of metallic lead in our bath 

 of fused quartz, &c. However much the minerals accom- 

 panying the quartz might otherwise interfere with such a 

 result, we can reduce metallic lead by the addition of char- 

 coal and we can counteract the undue reduction of lead by 

 addition of nitre, so as to obtain a convenient quantity of 

 reduced lead and no more. 



We Ultimately mix our roasted quartz with oxide of 

 lead in excess and charcoal, we add carbonate of soda, we 

 may increase the fusibility of our bath by addition of borax 

 or similar flux ; and exposing this mixture, in an earthen 

 crucible, to a red heat, the mixture melts, metallic lead is 

 produced throughout the bath, the globules of lead pick 

 up and alloy with the gold and gravitate to the bottom, 

 forming a button of lead in which all the gold of the quartz 

 is contained. The lead globules immersed in the molten 

 bath have a ijerfectly hvi^i metallic surface, and being under 

 these conditions very greedy of gold, and being generated 

 in every part of the mass during incipient fusion, no 

 particle of gold can escape contact with, and alloy 

 with the lead. We obtain a button of auriferous lead, 

 and a superposed layer of glassy substance or slag, quite 

 free from gold. We break the cold crucible, separate the 

 lead button, and submit it to cupellation, — that is to say, we 

 expose this lead to a full red heat in an oven of earthenware, 

 on a little porous cup or cupel made of the ash of burnt 

 bones ; the lead and any base metals which it may 

 contain are again oxidized, the fusible oxide of lead 

 with other oxides of the base metals are absorbed by 

 the cupel, and any gold or silver derived from our sample of 

 quartz is left as a brilliant globule on the cupel. This 

 globule of precious metals ^is often veiy minute, and if the 

 ascertaining the contents in gold only is our object, we may 

 add a little pure silver to the lead on the cupel and this 

 will ensure a residue of convenient size : we obtain a 

 tangible bead of silver, in which all the gold collected from 

 the quartz by the reduced lead is concentrated. This bead, 

 dissolved in aquafortis, leaves the gold as brown powder or 

 flocks ; these we can wash with two or three affusions of 

 distilled water, carefully dry, and heat to redness, whereby 

 the full metallic aspect of gold is produced, and then the 

 weight of this residue of gold multiplied by 5,000 gives the 

 contents of the stone expressed in terms of the ton avoirdupois. 



