bastin: colloidal gold and silver 65 



of electrolytes added to suspensoids will cause the suspended 

 solid particles to coagulate, i.e. to assemble into larger groups 

 which settle out of the solution. Resolution after coagulation 

 is impossible without introducing entirely new conditions. If 

 on the other hand small amounts of an emulsoid (such as a 

 gelatine solution) are mixed with a suspensoid, coagulation may 

 be much delayed in spite of the presence of electrolytes in the 

 solution. 



It seemed possible therefore that if the brown gold developed 

 on chalcocite was colloidal in character, its deposition might be 

 delayed by mixing an emulsoid with the gold chloride solution 

 before inserting the piece of chalcocite and that by this means 

 some of the gold might be obtained in colloidal suspension. This 

 was readily accomplished. 



In the experiments first to be reported the solutions were heated 

 to boiling, since it has been determined that in reactions of 

 metallic minerals on gold chloride solutions, heating increases the 

 rate without as a rule greatly affecting the character of the 

 reactions. Much time was thereby saved. In the first experi- 

 ment a solution of gelatine in water and a ^ solution of auric 

 chloride were employed. Two solutions of equal strength as 

 regards gold were prepared by adding to one volume of AuCls 

 solution (1) in one case one volume of gelatine solution, and (2) 

 in the other case one volume of water. The solutions therefore 

 differed only in the presence in one of an emulsoid solution of 

 gelatine. Into each were introduced a few small pieces of 

 chalcocite a,nd both were then heated to boiling and kept there 

 until the reactions were complete. The solution without gelatine 

 rapidly changed from clear yellow to clear pale green-blue in 

 color as the auric chloride was replaced by cupric chloride. At 

 the same time a coating of brown gold formed on the chalcocite. 

 Upon further heating, after the yellow color had wholly disap- 

 peared, the solution again changed color becoming deeper blue 

 by transmitted and pale reddish by reflected light. After cool- 

 ing and standing for about an hour this pseudo-fluorescent color- 

 ation disappeared, the solution again showing the pale green- 

 blue of cupric chloride.. With the gelatine-bearing solution the 



