444 PRINCIPLES OF CHEMISTRY 



disintegration of the hard rocky matter containing gold. 30 These dis- 

 integrated rocks, washed by rain and other water, have formed gold- 

 bearing deposits, which are known as alluvial gold deposits. Gold- 

 bearing soil is sometimes met with on the surface and sometimes under 



bringing it into contact with substances capable of depositing gold upon their surface. 

 The first efforts might be made upon the extraction of salt from sea water, and as the 

 total amount of sea water maybe taken as about 2,000,000,000,000,000,000 tons, it follows 

 that it contains about 10,000 million tons of gold. The yearly production of gold, is about 

 200 tons for, the whole world, of which about one quarter is extracted in Eussia. It ia 

 supposed that gold is dissolved in sea water owing to the presence of iodides, which, under 

 the action of animal organisms, yield free iodine. It is thought (as Professor Konova- 

 loff mentions in his work upon 'The Industries of the United States,' 1894) that 

 iodine facilitates the solution of the gold, and the organic matter its precipitation. 

 These facts and considerations to a certain extent explain the distribution of gold in 

 veins or rock fissures, chiefly filled with quartz, because there is sufficient reason for 

 supposing that these rocks once formed the ocean bottom. R. Dentrie, and subse- 

 quently Wilkinson, showed that organic matter for instance, cork and pyrites are able 

 to precipitate gold from its solutions in that -metallic form and state in which it occurs 

 in quartz veins, where (especially in the deeper parts of vein deposits) gold is frequently 

 found on the surface of pyrites, chiefly arsenical pyrites. Kazantseff (in Ekaterinburg, 

 1891) eve^n supposes, from the distribution of the gold in these pyrites, that it occurred 

 in solution as a compound of sulphide of gold and sulphide of arsenic when it penetrated 

 'into the veins. It is from such considerations that the origin of vein and pyritic gold 

 is, at the present time, attributed to the reaction of solutions of this metal, the remains 

 of which are seen in the gold still present in sea water. 



30 However, in recent times, especially since about 1870, when chlorine (either -as a 

 solution of the gas or as bleaching powder) and bromine began to be applied to the extrac- 

 tion, of finely-divided gold from poor ores (previously roasted in order to drive off arsenio 

 and sulphur, and oxidise the iron), the extraction of gold from quartz and pyrites, 

 by the wet method, increases from year to year, and begins to equal the amount 

 extracted from alluvial deposits. Since the nineties th,e cyanide process (Chapter 

 XIII., Note 18 bis) has taken an important place among the wet methods for 

 extracting gold from its ores. It consists in pouring a dilute solution of cyanide of potas- 

 sium (about 500 parts of water and 1 to 4 parts of cyanide of potassium per 1.000 parts 

 of ore, the amount of cyanide depending upon the richness of the ore) and a mixture 

 of it with NaCN, (see Chapter XIII., Note 12) over the crushed ore (which need not be 

 roasted, whilst roasting is indispensable in the chlorination process, as otherwise the 

 chlorine is used up in oxidising the sulphur, arsenic, &c.) The gold is dissolved 

 very rapidly even from pyrites, where it generally occurs on the surface in such 

 fine and adherent particles that it either cannot be mechanically washed away, or, 

 more frequently is carried away by the stream of water, and cannot be caught by 

 mechanical means or by the mercury used for catching the gold In the sluices. 

 Chlorination had already given the possibility of- extracting the finest particles of gold ; 

 but the cyanide process enables such pyrites to be treated as could be scarcely worked 

 by other means. The treatment of the crushed ore by the KCN is carried on in simple 

 wooden vats (coated with paraffin or tar) with the greatest possible rapidity (in order that 

 the KCN solution should not have time to change) by a method of systematic lixiviation, 

 and is completed in 10 to 12 hours. The resultant solution of gold, containing AuK(CN) 8> 

 is decomposed either with freshly-made zinc filings (but when the gold settles on the 

 Zn, the cyanide solution reacts upon the Zn with the evolution of EL, and formation of 

 ZnH^Oj) or by sodium amalgam prepared at the moment of reaction by the action of an 

 electric current upon a solution of NaHO poured into a vessel partially immersed in 

 mercury (the NaCN is renewed continually by this means). The silver in the ore passes 

 into solution, together with the gold, as in amalgamation. 



