64 bastin: colloidal gold and silver 



material with different heat treatments, correction cannot be 

 made to standard temperature from data obtained from other 

 materials. 



III. Unless the temperature coefficient is known for the par- 

 ticular material under test, temperature control offers the only 

 means of avoiding the error due to temperature changes, at least 

 where errors as great as 1 per cent are to be avoided. 



GEOCHEMISTRY. — Experiments with colloidal gold and silver. 

 Edson S. Bastin/ Geological Survey. 



GOLD 



In a series of preliminary experiments on the precipitative 

 action of metallic sulphides, arsenides, etc. on gold held in dilute' 

 solutions, the results of which were published in 1913, ^ it was 

 noted that chalcocite (CU2S) precipitated gold -from a dilute 

 aqueous solution of auric chloride (AuCls) as a dark brown dull 

 coating sometimes exhibiting moss-like protuberances, whereas 

 most other common metallic minerals precipitated it as a thin 

 lustrous yellow or orange coating. The brown gold coating 

 chalcocite when touched with the blowpipe flame becomes 

 orange-yellow and lustrous. 



As the brgwn gold showed no indication of being anything but 

 pure gold and as brown gold of somewhat similar appearance is 

 known to occur in nature the suggestion presented itself that the 

 brown color was largely due to a very fine state of division of 

 the gold, its condition possibly approaching that of a coagulated 

 colloid. To test the accuracy of this supposition advantage 

 was taken of certain well known properties of colloidal solutions. 



Colloidal solutions may be conveniently divided into two classes 

 according as the ''dissolved" substance is solid or liquid. If the 

 "dissolved" substance is solid, as in the case of a colloidal solution 

 of gold in water, the solution is termed a suspensoid. If the 

 ''dissolved" phase is liquid, as in a solution of gelatine in water, 

 the solution, is termed an emulsoid. Even very small amounts 



1 Published with the permission of the Director of the U. S. Geological Survey. 



2 Palmer, Chase, and Bastin, Edson S., Metallic minerals as precipitants of 

 silver a,nd gold. Economic Geology, 8: 140-170. 1913. 



