* yj V SILVER, GOLD, PLATINUM 523 



reddish, colloidal condition. The color is improved by toning 

 with a solution containing gold chloride, as part of the silver goes 

 into solution and gold (purplish) is deposited in its place. The 

 print is fixed with hypo, washed, and dried. Papers like velox 

 (invented by Baekeland) are essentially like plates (silver bromide 

 in gelatine), and are exposed, developed, and fixed in the same 

 way. 



GOLD Au 



Occurrence and Extraction. Gold is found in the free 

 condition in veins of quartz and in alluvial deposits, resulting 

 from the breaking up of such rock. It is found also in combination 

 with tellurium (p. 273). ~-&Cst*^*-+^dL<^ 



In vein mining (e.g. in the Transvaal) the rock is pulverized 

 with iron stamps working in an iron trough. The powder is 

 washed in the form of mud over plates of copper amalgamated 

 with mercury, in which 55 per cent of the gold dissolves. The 

 amalgam is afterwards scraped off, the mercury removed by dis- 

 tillation, and the gold residue refined. The tailings still contain 

 45 per cent of the gold, adhering to the particles of rock. They 

 are covered with sodium^ cyanide (p. 393) solution, and exposed to "* 

 the air, until the gold has been dissolved as sodium aurocyanide 

 NaAu(CN) 2 . From this solution the gold is deposited by elec- 

 trolysis, or displaced by zinc. 



The alluvial deposits are washed, on a small scale, in " cradles " 

 (shallow pans) and, on a large scale, by being carried by water 

 down a long trough (Placer Mining). The gold, having a much 

 higher specific gravity than the rock, sinks to the bottom, while 

 the rock particles are carried away. In the trough the gold settles 

 between elects nailed across the bottom. In hydraulic mining, a 

 modification of placer mining, very heavy streams of water are 

 thrown against the deposit. 



In 1920, the gold production of the world exceeded $330,000,000 

 in value. $165,000,000 of this came from the Transvaal, and 



