162 PROCEEDINGS OP SECTION B. 



gold, like the so-called base metals, becomes tarnished under 

 ordinary, evexyday conditions. 



Anyone can readily satisfy himself of the truth of my assump- 

 tions by a simple and easy test. 



When clean gold is plunged into clean mercury it instan- 

 taneously alloys with the mercury forming a white amalgam ; 

 but if the gold is not quite clean the amalgamation is retarded or 

 altogether prevented. 



The above is my principal test to discover the actual condition 

 of gold surfaces. 



With a little clean mercury and a few threads of gold any one 

 may easily prove for himself the fact that gold is subject to the 

 influences which affect the base metals. 



It was while personally investigating into the great loss of 

 gold at the Thames Gold-field, in Auckland, that I became aware 

 that a notable portion of it could not l)e properly referred to the 

 causes which were then generally supposed to account for it. 



The gold there is, as a rule, highly argentiferous, sometimes 

 assaying as high as thirty per cent, of silver, and I thought that 

 the alloy was possibly sulphurized, that is, as regards its surface. 

 I therefore cleaned some of the moi'e argentiferous of this gold 

 and afterwards exposed it to a jet of sulphuretted hydrogen for a 

 considerable time. I expected it to darken somewhat from the 

 formation of sulphide of silver, but I was unable to detect any 

 change in its colour. As a further test I plunged it into mercury 

 and found that it would not amalgamate in the least. 



I was greatly surprised that gold, containing so high a pei*- 

 centage of silver, did not sensibly darken, though highly 

 sulphurized as proved by the mercury test. 



I exposed a sample of pure gold to a current of sulphuretted 

 hydi'ogen and then plunged it into mercury, and found that it 

 also refused to amalgamate. I afterwards found that a few 

 seconds in the gas jet was suflicient to render any gold neutral to 

 mercury, and that gold was similarly aftected when brought into 

 contact, for a short time, with hot water in which sulphur was 

 present, or aqueous solutions of alkaline sul^^hides. 



Pure gold, after treatment with sulphuretted hydrogen gas, 

 gave a good reaction of sulphur, but could be readily brought 

 back to an amalgamable condition by a short contact with cyanide 

 of potassium, chromic acid, and nitric acid. 



The gold had most certainly absorbed sulphur, and the 

 important question to decide was — What is the nature of this 

 absorption ? Is it mechanical as in the case of gaseous absorption 

 by platina, or is it chemical 1 



If this absorption is mechanical, the sulphur must be in one of 

 the two following conditions : — 



1. As free sulphur. 



2. Combined with hydrogen, as sulphuretted hydrogen. 



