1090 A TREATISE ON METAMORPHISM. 



in an 8 per cent solution of sodic carbonate containing an excess of carbonic 

 acid and containing- sodic silicate." Becker has shown that gold is easily 

 soluble in sodic sulphide and in sodic sulphy drate. b It has also been 

 held that gold is soluble in ferric sulphate. Recently Stokes has experi- 

 mentally investigated the solubility of g'old. He finds that gold is 

 somewhat readily soluble at a temperature of 200° C. in solutions of ferric 

 chloride and cupric chloride according to the following reactions: 



Au+3FeCl 3 ;^±AuCl3+3FeCl 2 

 Au+3CuCl 2 ^AuCl 3 +3CuCl 



It thus appears that when gold is dissolved by ferric chloride or cupric 

 chloride it is transformed to a chloride. The greater the amount of these 

 chlorides and the stronger the solutions the more gold may be dissolved. 

 With a given amount of ferric chloride or cupric chloride, with constant 

 temperature, the reactions cease when equilibrium has been reached. 

 Stokes's work has further shown that when this condition exists a rise in 

 temperature causes the reaction to continue. That is to say, with increasing 

 temperature a given amount of ferric or cupric chloride in a given solution 

 is capable of taking an increasing amount of gold into solution. 



Stokes finds that gold is not appreciably dissolved in ferric sulphate 

 unless chlorides are present at the same time, thus furnishing ferric chloride 

 and making the solution really that by the reaction already given. His 

 experiments have not been carried to sufficient refinement to prove that 

 ferric sulphate may not dissolve gold to an extent to be of consequence in 

 the segregation of the metals under natural conditions. 



Lenher, in advance of publication, has kindly given me the results of 

 experiments which show that gold is soluble in sulphuric acid, phosphoric 

 acid, and various other acids, if a compound be present which liberates 

 oxygen, as, for instance, manganese dioxide. The reaction takes place at 

 0° C, but is greatly accelerated by heat. This principle may be of con- 

 siderable importance in the solution of gold, since all of those compounds 

 occur rather plentifully under natural conditions. The frequent association 

 of gold ores with manganese minerals strengthens this suggestion. 



« Doelter, C, Einige Versuche fiber die Loslichkeit der Mineralien : Tschermaks Mineral. Mittheil., 

 vol., 11, 1890, p. 329. 



b Becker, G. F., Geology of the quicksilver deposits of the Pacific slope: Mon. U. S. Geol. Survey, 

 vol. 13, 1888, p. 433. 



