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Art. LXV.—On the Rapid Action of Strong Cyanide Solutions on Gold superficially.
[Read before the Wellington Philosophical Society, 16th February, 1898.]
A Very singular phenomenon may be observed in regard to the action of the cyanide solution upon gold such as that which Professor Faraday used for his experiment. This gold is deposited from very weak solutions of its chloride by phosphorus or hydrogen in such finely divided particles that they were indistinguishable—in fact, invisible—under the highest-powered microscope of that time; they are able to remain in suspension in water for an indefinite period, and it was only by reflected light Faraday could observe them. Now, if we neutralise, or slightly alkalize, this red-looking auriferous fluid (we cannot designate it as a solution), then divide into two parts, and mix one part with, say, three volumes of the dilute solution, and mix the other part with a strong cyanide solution—say, the concentrated solution—it will be found that it is the gold in the strong cyanide that first disappears from view. Nor is it a neck-and-neck affair by any means, for in the strong solution three seconds may suffice to complete the operation, while in the case of the orthodox dilute solution the time required will be about five minutes—that is, about one hundred times longer. This is, of course, just what one would have expected a few years ago, before our working cyaniders had taught us the extraordinary potency of the dilute solution over the strong solution as a gold-dissolver.
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Both the blue and the red gold of Faraday give the results stated.
These results clearly prove that, even in the supposed inert solutions of the cyanide, the strong solutions rapidly act upon gold to a limited extent. It is not a matter of superior causticity, for it was found that strongly alkalizing the weak cyanide did not hasten the dissolution of the gold.
The explanation of this enigma is, I think, this: In the strong solution cyaniding goes on at once, but only so far as to produce a colourless cyanide of gold that is insoluble, or but very slowly soluble, in the liquid. I have before shown that gold taken out of a strong cyanide solution refuses to amalgamate, or, at least, but very slowly amalgamates, showing, as I said, that an insoluble saline film of some gold-cyanide has enveloped it. In further support of this idea I would note here the fact (which may or may not have been observed before) that gold-leaf upon a strong cyanide solution, in disappearing, leaves floating thereon a pale white ghostly-looking figure of its departed self, exact to the shape. In the case before us, however, there is no transmutation of a vast number of metals—nor even of one: the white floating mass left by the gold is its cyanide, and it is this substance that, I believe, always forms upon gold in strong cyanide solution, thus preventing or retarding its dissolution.
In the strong solution, then, we have not only the limited supply of oxygen against the cyanider, as Maclaurin has shown, but we have also the rapid enfilming of the gold by a salt that is but very slowly removed therefrom.