70 bastin: colloidal gold and silver 



covered with peacock tarnish, due presumably to the formation 

 of some double sulphide of silver and copper. 



When a colloidal silica sol prepared by dialysis was substituted 

 for the gelatine solution under the conditions of the experiments 

 described above colloidal solutions were obtained upon boiling, 

 with chalcopyrite and metallic tin. With metallic copper and 

 with chalcocite, and enargite, only fine suspensions of silver were 

 obtained with the comparatively dilute silica solution used, but 

 with a solution of sodium silicate^ much richer in silica true 

 colloidal solutions of silver were readily obtained with all these 

 substances. 



Hydrogen sulphide gives with a dilute Ag2S04 solution a black 

 precipitate of Ag2S, the solution becoming very pale yellowish, 

 probably from the presence of minute amounts of Ag2S in colloi- 

 dal solution. In the presence of gelatine, however, no Ag2S 

 is precipitated, but the solution becomes deep brownish-yellow, 

 all the AgoS presumably going into colloidal solution. Entirely 

 analogous results were obtained with cupric sulphate solutions, 

 the gelatine-bearing solution yielding no precipitate, but a yellow- 

 ish-brown colloidal solution (presumably of sulphides of copper). 

 When a colloidal silica sol was substituted for the gelatine sol 

 in the above experiments, some sulphide was precipitated, but 

 some also went into colloidal suspension, strongly coloring the 

 solutions. 



Summary for silver. The above experiments show that, in 

 the presence of an emulsoid such as gelatine, part of the metallic 

 silver or silver compounds that would otherwise be precipitated 

 from silver sulphate by certain metallic minerals is likely to be 

 retained in colloidal solution. Silica appears to function similarly 

 to gelatine in favoring the entrance of the silver or its com- 

 pounds into colloidal solution. Because of the fact that basic 

 rocks, particularly those rich in olivine, yield colloidal silica in 

 weathering more readily than acid rocks, it is in ores associated 

 wHh such rocks that we might expect to find the most evidence 

 of colloidal downward transport of these metals. The writer 



" Sodium silicate in aqueous solution undergoes partial dissociation, yielding 

 sodium hydroxide and silica. 



