494 SCIENCE PROGRESS. 



of the globules of mercury with black scum, which effectually 

 prevents the amalgam from adhering either to gold or to 

 amalgamated plates. This "sickening" is doubtless also 

 caused by the formation of compounds of the mercury 

 itself. Ores containing sulphides of arsenic or antimony 

 (which are reduced by mercury) are particularly apt to 

 cause " sickening," but manganese dioxide, partly decom- 

 posed copper pyrites, zinc blende, and galena are also 

 harmful. 



It has been well known ever since the time of Agricola 

 that the gold contained in these minerals, although not easy 

 to extract by mercury, may be readily obtained by concen- 

 trating the ore (all finely divided free gold being, of course, 

 lost in the process) ; the concentrates are then smelted, the 

 gold accumulated in a reduced metal, such as lead or 

 copper, and subsequently separated by cupellation or 

 other means. The impossibility of applying this method 

 of procedure to individual mines far distant from coal beds, 

 and in places where a mixture of different kinds of ore 

 cannot be obtained, prevents the universal application of 

 the method, although in the neighbourhood of such smelt- 

 ing centres as Freiberg and Denver nothing better is 

 required. 



The chlorination process, now nearly fifty years old, is of 

 more general value for treating concentrates. Chlorine is a 

 somewhat slow solvent for gold, any particle occurring 

 native which is visible to the unassisted vision requiring 

 many hours for its complete dissolution ; but it is well 

 adapted to dissolve the fine flakes existing in pyrites. 

 Unfortunately chlorine has a strongly preferential action 

 on sulphides, and to avoid the enormous waste of gas 

 which a small percentage of these substances cause it is 

 necessary to precede chlorination by careful and complete 

 roasting. After this there is little difficulty in the process. 

 Oxides of the metals, except the alkaline earths, are very 

 slowly attacked by chlorine ; and when the alkaline earths 

 are present salt is added in the roasting furnace. Chlorine 

 is applied to the slightly clamped ore in the form of gas, 

 or, in more modern practice, as a strong aqueous solution. 



