bastin: colloidal gold and silver 69 



Some differences were noted in the crystal forms of the silver 

 developed on different minerals, but all was silver-white, lustrous 

 and metallic in appearance. 



Following the experiments with gold, similar experiments 

 were conducted with silver using a solution composed of equal 

 parts of /o Ag2S04 and a dilute aqueous sol of gelatine. 



In the first experiment small pieces of chalcocite were boiled: 

 (1) with gelatine-bearing Ag2S04 solution, and (2) with the same 

 strength Ag2S04 solution without gelatine. The gelatine-bear- 

 ing solution yielded a brown colloidal solution of petroleum- 

 like appearance evidently containing silver and possibly silver 

 sulphide;^ only a slight precipitate of metallic silver formed on 

 the chalcocite. The solution without gelatine yielded no colloi- 

 dal solution of silver but became pale blue from copper sulphate 

 formed, metallic silver being deposited in abundance on the chal- 

 cocite. 



Colloidal solutions similar in general appearance to those ob- 

 tained with chalcocite were obtained from gelatine-bearing 

 Ag2S04 solutions with enargite, smaltite, maucherite (Ni4As3) 

 and niccolite. Some of the solutions carried presumably not only 

 silver but compounds of silver in colloidal suspension, but the 

 writer has had no opportunity to determine their composition. 

 In each case some metallic silver was precipitated on the mineral, 

 though in notably lesser abundance than when gelatine was not 

 present. 



From chalcopyrite, a mineral which does not precipitate metal- 

 lic silver from neutral Ag2S04 solution, '^ there was obtained upon 

 boiling with gelatine sol and dilute Ag2S04 a yellowish-brown 

 colloidal solution, probably of a sulphide of silver and copper. 

 This solution was quite different in appearance from the pe- 

 troleum-like solutions obtained with minerals that precipitate 

 metallic silver. During the reaction the chalcocite becomes 



5 Palmer has demonstrated the reaction CU2S+2 Ag2S04 = 2 Cu SO4+ Ag2S+ 2 Ag, 

 for a neutral electrolytic solution. 



'■' In the preliminary paper already referred to some precipitation of metallic 

 silver by chalcopyrite was reported but it was subsequently learned that tarnishes 

 on the fragment used produced this effect and that the perfectly clean mineral 

 precipitated no metallic silver from a dilute Ag2 SO4 solution. 



