217 



Obviously, therefore, some foreign substance occupied these non-amalgam- 

 able surfaces, excluding the mercury from contact with the auriferous alloy, 

 which substance was dissipated or decomposed by heat, and capable of removal 

 by the several re-agents specified ; tlie question then arose. What could this 

 substance be 1 and I began to suspect that sulphur, in some form or other, was 

 this substance, since its deportment under these conditions would certainly be 

 very similar, 



I therefore well cleaned the surfaces of several samples of Thames and 

 Otago gold, and one of pure gold, and placed them for a few seconds in 

 sulphuretted hydrogen gas, well washed them afterwards, and dipped them into 

 clean mercury, when, in the place of being instantly whitened over their whole 

 surfaces by the mercury, they absolutely refused to amalgamate with it upon 

 any part, and even seemed to exercise a repulsive force upon it.* 



The same effects, or rather non-effects, in relation to the mercury, followed 

 when alkaline sulphides were substituted for the surphuretted hydrogen, also 

 when the samj^les of gold were kept a short time in boiling water, in contact 

 with sulphur — a platinum crucible being used as a precautionaiy measure 

 against the introduction of alkaline matters. As in the case of the first series 

 of specimens, these were rendered amalgamable by treatment with cyanide of 

 potassium, nitric acid, chi'omic acid, or chloride of lime, also by the application 

 of heat to the less cupreous ones, t The efiects of heat on those samples 

 containing 6 per cent, of copper (and perhaps much less) being to produce such 

 a film of the oxide upon their surfaces that the mercury is still excluded 

 from them, though from a different cause from that above mentioned. These 

 samples also readily amalgamated with sodium amalgam. 



Gold thus treated with sulphuretted hydrogen, sulphur, or alkaline 

 sulphides, and thoroughly washed, then put in pure cyanide of potassium, gave 

 a good reaction of siilphur to the nitro-prusside test ; and I have also obtained, 

 in the same manner, very distinct reactions of sulphur upon several of the 

 native gold samples in the Museum. 



These resixlts cleaily show that gold, even when in its purest state, is by 

 no means so negative to sulphur and its compounds as is supposed, but 

 that, on the other hand, it absorbs sulphur with great avidity ; and they 

 further show, that when this sulphur is thus absorbed by gold, even when 

 only in very minute quantity, the metal refuses to amalgamate, although there 

 is no visible change induced in its appearance. 



Taken in connection with the presence of sulphur (or a compound of it) 



* This was illustrated at the close of the meeting. 



+ The chlorides of gold and mercury, also nitrate of silver, have been found lately to 

 have the same effect. See paper ' ' On the reduction of various metals from ther solutions 

 by metallic sulphides, etc." 



2f 



