bastin: colloidal gold and silver 67 



To determine whether colloidal gold solutions could be obtained 

 with the aid of substances known to precipitate lustrous yellow 

 or orange gold from AuCls solutions, fragments of pyrite, chal- 

 copyrite, enargite, and galena were boiled with the same gelatine- 

 bearing solution of AuCls used in the previous experiments. In 

 each case lustrous yellow gold was deposited on the mineral, 

 the solution at the same time losing its yellow color; but gold 

 was not obtained in colloidal solution even on prolonged boiling. 

 Metallic bismuth, which unlike zinc, copper, and cadmiimi pre- 

 cipitates lustrous yellow gold from a gelatine-bearing AuCls 

 solution, also failed to yield a colloidal gold solution. 



The experiments that have been described were conducted 

 at temperatures at or near 100° C. to speed the reactions, but 

 beautifully colored solutions of colloidal gold were also obtained 

 at ordinary room temperatures by the action of metallic copper 

 and of chalcocite on gelatine-bearing AuCls solutions. The 

 finely-divided gold first appears as a cloud over the surface of 

 the copper or chalcocite; this cloud, when the dish is jarred, de- 

 taches itself in streamer-like forms and finally becomes wholly 

 dispersed in the solution. The color of the solution gradually 

 changes from yellow to purplish red as these gold clouds disperse 

 through it. The facility with which the colloidal solutions can 

 be obtained at room temperatures is markedly dependent upon 

 the strength of the solutions and the most favorable conditions 

 have not been accurately determined by the writer. 



A colloidal silica solution was prepared by dialysis of an aque- 

 ous sol of sodimn silicate that had been neutralized by sul- 

 phuric acid. This sol was used instead of gelatine sol in experi- 

 ments similar to those previously described and yielded colloidal 

 gold solutions with chalcocite and copper, and purple of Cassius 

 with tin. 



It is well known that ferrous sulphate readily precipitates gold 

 from solutions of auric chloride. When dilute solutions of these 

 two compounds are mixed there results a solution that is blue 

 by tr,ansmitted light and brick-red by reflected light, due to the 

 presence of gold in very fine suspension. From the particular 

 solution prepared by the writer all of the suspended gold settled 



