GOLD EXTRACTION PROCESSES. 493 



Admirable as is the amalgamation process in many 

 respects, it has always been recognised that the extraction 

 of gold by its use is generally far from complete. Besides 

 the comparatively large particles of free gold which are 

 readily saved by amalgamation, all ores contain more or 

 less gold in an excessively fine state of division (the aurum 

 larvatum or "disguised" gold of the last century) as well 

 as gold contained in sulphides {aurum miner alisatum), 

 and these particles cannot be extracted by the copper 

 plates. 



In an investigation on the dimensions of gold particles in 

 ores J. A. Edman 1 observed a single chip of quartz tV inch 

 in diameter which, when magnified 50 diameters, showed 

 over 300 particles of gold, varying in size from T oVo to 

 titoo inch not only on the surface of the stone but scattered 

 through the transparent mass. Higher powers showed still 

 greater numbers of smaller particles. The gold contained 

 in pyrites, if, as seems likely, it is generally free, must be 

 often still finer. It has been likened to the mortar in a 

 brick wall, and is almost as difficult to catch as the motes in 

 a sunbeam. Prolonged grinding with mercury no doubt 

 increases the chances of such gold being amalgamated, 

 and hence the success which has frequently attended the 

 use of the Mexican arrastra, where the grinding surfaces 

 are of stone, and of its successor the iron amalgamating 

 pan. 



Nevertheless, the yield of gold, mainly owing to the 

 "flouring" and "sickening" of the mercury, is not always 

 good even in these slow-working and therefore costly ma- 

 chines. Mercury, when triturated with ore for a long time, 

 tends to break up into very fine particles which, although 

 apparently clean and bright under the microscope, refuse to 

 run together, and are carried away by the stream of water and 

 lost, together with the gold already taken up by them. Such 

 mercury appears greyish-white, and is said to be " floured". 

 Moreover, when base metals are present in the ore they 

 become amalgamated, and then, oxidising, coat the surfaces 



1 Mining and Scientific Press. San Francisco, 12th August, 1892. 



