68 bastin: colloidal gold and silver 



after standing 1^ hours. If either gelatinous sihca or gelatine 

 is present in the solutions similar gold suspensions result, but 

 from these the gold settles only very slowly. 



Summary for gold. From the foregoing results it is evident 

 that the readiness with which colloidal gold solutions may be 

 obtained under the conditions of these experiments bears a close 

 relation to the nature of the gold deposited. The dull brown or 

 black condition of gold is favorable and the lustrous yellow con- 

 dition unfavorable to their formation. In the experiments 

 conducted at room temperatures the colloidal gold appeared to 

 originate on the surface of the metal or sulphide, and to slowly 

 diffuse out into the liquid. These phenomena suggest that the 

 dull brown or black gold may be in a finely divided condition 

 approaching that of a coagulated colloid. 



Brown gold occurs occasionally in nature usually if not always 

 in the oxidized zone. Lindgren^ states that the gold derived from 

 the oxidation of tellurides is commonly dull brown in color. 

 While it is very probable that the brown color of some natural 

 gold is due to impurity or to coatings, the possibility that some 

 of it is in a condition approaching the colloidal should not be 

 overlooked. 



In the experiments with chalcocite and with ferrous sulphate 

 colloidal silica appeared to function similarly to gelatine in de- 

 laying the complete precipitation of gold, some being retained 

 in colloidal suspension. The possibility of gold being taken into 

 colloidal solution in the oxidized zone of ore deposits in the 

 presence of such emulsoids as colloidal silica appears to be worthy 

 of further investigation. 



silver 



In the preliminary paper by Palmer and Bastin already re- 

 ferred to, attention was called to the fact that certain metallic 

 minerals are capable of reducing silver sulphate in dilute aqueous 

 solutions. With some minerals the only silver mineral formed 

 was metallic silver, in other cases native silver and a compound 

 of silver fonned, and in still other cases only a silver compound. 



^ Lindgren, W., Mineral Deposits, 434. 1913. 



