446 PRINCIPLES OF CHEMISTRY 



in nuggets or grains of greater- or less magnitude. It always contains 

 silver (from very small quantities up to 30 p.c., when it is called 

 * electrum ') and certain other metals, among which lead and rhodium 

 are sometimes found. 



The separation of the silver from gold is generally carried on with 

 great precision,, as the presence of the silver in the gold does not 

 increase its value for exchange, and it can be substituted by other 

 less valuable metals, so that the extraction of the silver, as a precious 

 metal, from its alloy with gold, is a profitable operation. This 

 separation is conducted by different methods. Sometimes the argenti- 

 ferous gold is melted in crucibles, together with a mixture of common 

 salt and powdered bricks. The greater portion of the silver is' thus 

 converted into the chloride, which fuses and is absorbed by the slags, 

 from which it may be extracted by the usual methods. The silver is 

 also extracted from gold by treating it with boiling sulphuric acid, 

 which does not act on the gold but dissolves the silver. But if the 

 alloy does not contain a large proportion of silver it cannot be extracted 

 by this method or at B,ll events the separation will be imperfect, and 

 therefore a fresh amount of silver is added (by fusion) to the gold, in 

 such quantity that the alloy contains twice as much silver as gold. 

 The silver which is added is preferably such as contains gold, which is 

 very frequently the case. The alloy thus formed is poured in a thin 

 stream into water, by which means it is obtained in a granulated 

 form j it is then boiled with strong sulphuric acid, three parts of 

 acid' being used to one part of alloy. The sulphuric acid extracts 

 ,all the silver without acting on the gold. It is best, however, to' 

 pour off the first portion of the acid, which has dissolved the silver, 

 and then treat the residue of still imperfectly pure gold with a fresh 

 quantity of sulphuric acid. The gold is thus obtained in the form 

 of powder, which is washed with water until it is quite free from 

 silver. The silver is precipitated from the solution by means of 

 copper, so that cupric sulphate and metallic silver are obtained. This 

 process is carried out in many countries, as in Russia, at the Govern- 

 ment mints. 



Gold is generally used alloyed with copper ; since pure gold, 

 like pure silver, is very soft, and therefore soon worn away. In 

 assaying or determining the amount of pure gold in such an alloy 

 it is usual to add silver to the gold in order to make up an alloy 

 containing three parts of silver to one of gold (this is known as 

 quartation because the alloy contains of gold), and the resultant 

 alloy is treated with nitric acidi If the silver be not in excess over 

 the gold, it is not all dissolved by the nitric acid, and this is the reason 



