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Volume 28, 1895
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Art. LXXIV.—Notes on Mr. J. S. Maclaurin's Paper on the Action of Potassium-Cyanide Solutions upon Gold.

[Read before the Wellington Philosophical Society, 26th February, 1896.]

This paper is a reply to a part of a communication of mine that appeared in the Annual Report of the Mines Department for 1895, and, in justice both to Mr. Maclaurin and to myself, I make these few notes thereon.

In regard to the publications of his that he cites showing the great insolubility of oxygen in strong or concentrated solutions of potassium cyanide, all I wish to say is that at the time I wrote my contribution for the Mines Department my acquaintance with the facts that he has given and his theory respecting these was entirely gathered from short notices of his paper as scattered in other works, and I regret my know-ledge of his labours in this matter being then so slight as it was. However, Mr. Maclaurin will find that in a part of my annual Laboratory Report that was in page form on the 8th February,* and is now just about ready for issue, I had acknowledged both the accuracy and the great value of his discovery that potassic - cyanide solutions are absorptive of oxygen to a degree that is approximately the inverse of their strengths; and as I (in common with most chemists) have always held that free oxygen is required for the cyanide pro

[Footnote] * This fact is certified to by a letter I have from the Government Printer.

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cess, I allowed in that communication of mine that Mr. Maclaurin had solved the question in its practical bearings as to why gold is so slightly affected by strong cyanide solutions as experience has proved it to be; and, further, I proved that even in those cases where an electric current is produced during the solution of gold in potassic cyanide, free oxygen is also necessary.

One of the points upon which Mr. Maclaurin does not agree with me is where I state that a product is generated on the gold in the cyanide solution that hinders the dissolution of this metal to a more or less extent. In answer to this I would refer him to page 45 of the Laboratory Report cited, where, as an additional result supporting this, I prove that by the amalgamation test the cyanodizing or oxidizing of the gold is always much ahead of its dissolution. As to the author's statement that it is inexplicable why I should assume that the gold compound that first forms is insoluble in strong potassic cyanide, while the analogous silver compound is more rapidly dissolved therein than in the weak solution, I would answer that I have not tied myself to any particular gold compound—nor yet to any one—the formulæ of which is known, and my result certainly appears to show that at least there is a gold compound that forms in potassic cyanide which is very slowly soluble therein.

As to the inability of Mr. Maclaurin to confirm my statement regarding the results of electrically pairing platina with gold, except I allow that the platina must project out of the cyanide solution, I have to say that if the platina is wholly submerged therein, it all depends upon the area of the two metals relatively to each other as to whether the result I describe is easily observed or not. If the area of the platina considerably exceeds that of the gold—as was the case in my experiment—so much free oxygen present in the solution is available for oxidation that the dissolution of the gold is sufficiently accelerated to allow of its being easily observed. If the platina is allowed to project above the cyanide, it does, as Mr. Maclaurin observes, take the place of, gold, and the dissolution of that metal is still further accelerated. There is another difference in our mode of conducting the experiment the knowledge of which helps to reconcile our differences on this point: I worked with shallow depths of liquid; the author had his metals deeply submerged; and, as I rely somewhat on the absorption of aerial oxygen going on simultaneously with the dissolution of the gold, the result I stated did not come out so well by the use of Mr. Maclaurin's. method.*

[Footnote] * These remarks have an application to the metallic sulphides.

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And now in regard to the theory the author of that paper advances to explain the singular fact that I published as to the rapid dissolution of gold in concentrated solutions of the cyanide when the gold plate used is but partly submerged. This theory is that the dissolution of the submerged gold in these cases is due to electrolytic action produced at the surface of the cyanide next to the atmosphere; and in support of this statement he shows, and by incontrovertible evidence, that electric currents are produced under these circumstances.

Now, I am very glad to have all this evidence from Mr. Maclaurin, as it goes to sustain a statement of mine to the same effect, and which was in page form in the Government Printing Office for the Laboratory Report cited, page 44,* on the 8th of February, consequently before I saw his paper. There I stated, in effect, that an electric current is produced when gold is partly submerged in potassic cyanide. On this point, then, we agree; but when I go over his statements in regard to the direction of this current, and its relations to, or its bearings upon, the dissolution of gold, I am quite unable to agree with him, nor can I agree with him in regard to his statement that potassium is set free in any of his experiments. These three points I will take consecutively.

First, then, as to the direction of the current—that is, as to the particular part of the apparatus where this electromotive power is generated : Mr. Maclaurin states that it is generated at the surface of the liquid, and that the current will flow down to the submerged end of the gold. But I state, in the report referred to, that it is the lower part of the submerged gold where this electromotive power is generated—that is, in my own words, “the lower end being the positive pole”; and I show that the electromotive power generated by the lower end (the more deeply submerged end of the gold strip) is sufficiently strong to electro-deposit copper from its sulphate. I have now also tested the direction of the current in the apparatus figured 3 in Mr. Maclaurin's paper, and, as I expected, find that the lower end of the submerged gold strip certainly is the positive pole in this apparatus too.

Second, as to the relation of the electric current, or this electrolytic action, as it is termed, to the dissolution of gold in the potassic cyanide. Mr. Maclaurin assumes that it is this current-this action—which causes the solution of the gold in the cyanide. Now, I must contend here that it is just vice versa; that, in fact, Mr. Maclaurin has got the cart before the horse—the effect for the cause. I contend that the current (or the electromotive power) is produced by chemical action alone—that is, by the dissolution of the lower sub

[Footnote] * The Government Printer also certifies to this by letter.

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merged end of our gold strip. To be explicit : so far from the submerged gold requiring an electromotive power to pass it into solution, it produces electromotive power as it dissolves, and of sufficient intensity, as I show, to electro-deposit copper from its sulphate or gold from its cyanide. To go to particulars: taking Mr. Maclaurin's own conception as to the chemical operation that goes on in the process, viz., that the cyanogen of the cyanide attacks the submerged gold direct, while the potassium combines with oxygen at the other end of the metal (a happy conception fitting the case, and apparently true), I should maintain that the cyanodizing of the gold and the oxidizing of the potassium are the initiatory steps in the process, and the current observed is the effect of this and not the cause as assumed. The function of the interpolar connection (gold, platina, or conducting sulphide, as the case may be) is to conduct the electricity generated, and so to allow a long slip of the liquid itself to be polarized; the effect of which is that cyanodizing and oxidizing can and do go on simultaneously in the localities especially suited for each process.

Now, I have stated in the notes of mine referred to that the rapid chemical action that occurs at the gold near to or above the surface-line of the cyanide solution had a “misleading effect upon me”; and I think that Mr. Maclaurin will soon have to confess to a similar experience on his part. The gold is so rapidly dissolved at that place that the electric current observed is at first naturally assumed to have been generated there; but, though there must be electricity generated there, it does not become electromotive power, being merely that of local action of the kind we have when we use common zinc for a pole, and it takes no part in the dissolution of the submerged gold. Any way, the dissolution of this submerged end can be rapidly effected without it—that is (to keep exactly to the text), without the assistance of any electromotive action taking place at the surface, or, indeed, anywhere else. This is proved by the results of the following experiment taken from my old working-notes :—

In the following figure (Fig. 1), representing the apparatus that I used, the vessel A contains a saturated aqueous solution of potassic cyanide. B contains a weak solution of common salt. C contains an aqueous solution of potash. D, D are glass tubes filled with sodic chloride solution to make interpolar connections. E, F are strips of gold leaf glued to paper (for support), and connecting with each other through the galvanometer G. The cyanide solution is covered with a layer of oil.

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Fig. 1

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So soon as the electric connection is made through the galvanometer G, a current of electricity is produced, the direction of which shows that the gold in the vessel A (cyanide solution) is the positive pole of the voltaic pair, and in a short time the gold in that vessel will have dissolved away from its support, while that in the vessel C is not at all affected.

Now, here there can be no electromotive action at the upper surfaces of any of the liquids; and there is no electrical effect produced except that which goes to form the electromotive power that is given out at the surface of the gold that is in the cyanide solution A. Clearly, then, the gold in this case is not dissolved by electrolytic action, and consequently in Mr. Maclaurin's experiment (see his Fig. 3*) the gold A at the bottom was not dissolved by electrolytic action as he contends. In my experiment, for the potash in vessel C may be substituted any of the acids, even nitric acid, with thesame results. With the metal silver the results are of the same kind-that is, the silver is rapidly dissolved by the cyanide, and this even when the silver in the nitric acid is being rapidly dissolved, thus showing the great affinity of silver for cyanogen.

These results certainly appear to prove that Mr. Maclaurin has been all along upon this particular matter mistaking, as I have said, the effect for the cause.

In case these results may not be deemed sufficient proof of the incorrectness of this theory of the author's, I give the details of another experiment, with its results.

A and B are slips of gold leaf in electrical connection with the galvanometer G by insulated wires E, E. A is in a concentrated solution of potassic cyanide C, C, C, C, while the gold B is in a weak cyanide solution, or in potash solution C, C, D, D, partly separated from the strong cyanide by a diaphragm H. By this apparatus a strong electric current is generated, and the gold plate A is shown to be positive to the plate B, and very soon dissolves in the liquid.

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Fig. 2

Here we get entirely rid of all chemical action at the surface of a liquid (D) in contact with the atmosphere, consequently of all electrolytic action that could possibly be produced there; and yet the dissolution of the gold in a concentrated solution of the cyanide goes on as rapidly as it did in Mr. Maclaurin's experiments.

Now, just setting, aside galvanometric indications altogether, Mr. Maclaurin may contend here that, by allowing, as I

[Footnote] * Above, p. 703.

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have, that there is oxidation going on at the surface of the gold in vessel C, Fig. 1 (oxidation of potassium), simultaneously with the cyanodizing of the gold in vessel A, I have weakened the strength of my argument that the gold A in the strong cyanide is the positive element of the voltaic pair which is produced in this apparatus. In such a case I would answer him by stating that the free oxygen at this pole (the gold F) simply takes the place of the combined oxygen of the potash next to it, pushing that atom of oxygen on to another atom of potash to the displacement of its particular atom of oxygen, and so on till the chlorine is reached, when this radical is pushed on in like manner till cyanogen is reached, which in its turn is slid on, as it were, till the last atom of it in the line of action collides with the gold, and a chemical effect then and there takes place that is productive of the electromotive power that we find. The gold at that point thus shows itself to be the positive element of the apparatus, which is in strict accordance with the law that it is always the metal that is attacked which determines the direction of the current. But I maintain that during these actions never is any atom or fraction of an atom of either oxygen or cyanogen at any time absolutely freed from all combinations; it is, as it were, a waltz in which the dancers are ever changing partners, but not one of them is ever clear of a partner-that is, never in a state of isolation.

In regard to this matter, I would state here that lead in strong soda solutions, like gold in strong “cyanide” solutions, is oxidized and dissolved far quicker when only partly immersed than when wholly immersed therein. Like gold in cyanide solutions, lead is also more soluble in weak potash solutions than in its strong solutions. Lead in a strong potash solution is weakly positive to lead in a weak potash solution, but becomes more positive when the other piece of lead is placed in nitric acid, and so becomes rapidly dissolved. In these casee I also suppose the aërial oxygen takes the place of the oxygen of the potash next to it, and the sliding process goes on till the lead is reached, when strong chemical action takes place.

That the positive pole should be that at which the least chemical action takes place seems inexplicable until we consider that the direction of the current is here determined by intensity, not by quantity; and the current that is produced by the lead in the potash solution is more intense than that produced by the lead in the nitric-acid solution, because (in part at least) free oxygen of the air is the exciting agent in that case, while in the case of the acid solution a combination has to be broken up for the oxidation of the lead, which, of course, involves a lessening of electric intensity of the current

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thus produced.* Besides that, this free oxygen is greatly condensed as it replaces the combined oxygen of the potash.

I note here that the vice-president of the Institution of Mining and Metallurgy at the Geological Museum, Jermyn Street, London, in a paper on “selection action” in the cyanide process, and which generously deals with much old work of mine, takes the same view as Mr. Maclaurin does of electrical action as being the cause and not the effect of chemical action in the process of Macarthur and Co. So Mr. Maclaurin is in very good company. The paper itself, as may be seen, has been in a large measure anticipated by the two publications of mine referred to, in the parts dealing with the selective action of the patentees of the cyanide process.

Thirdly, in regard to the assumption that potassium is liberated from potassic cyanide in the experiment figured 3 in his paper, I would observe that Mr. Maclaurin omits to give me any proof of this, and that therefore it is impossible for me to think otherwise than that this assumption is altogether an unreasonable one. It is true, however, that Professor Gladstone, Ph.D., F.R.S., and Mr. Alfred Tribe, in a paper read before the Royal Society in 1875. gave the sanction of their names to this idea of a liberation of potassium under circumstances somewhat similar to those we have in Mr. Maclaurin's experiment, but, as these investigators have not replied to the strictures I made thereon, I suppose they have abandoned that idea. However it is, I would, in answer to Mr. Maclaurin's assumption, refer him to these strictures of mine,§ and supplement them by the following remarks :—

As Mr. Maclaurin well-knows, the affinities of this metal (potassium) for oxygen are of extraordinary intensity; for cyanogen also they are very intense, as a white heat alone does not decompose either compound. To this I think Mr. Maclaurin will agree; and, if so, how can he assume that the feeble electric current that he supposes to be produced at the surface of the liquid can overturn affinities like

[Footnote] * More investigations are required in this direction before the reason of this can be thoroughly understood. This action of metals in strong and weak solutions of alkalies is anomalous. Iron in strong HCl coupled with itself in the weak acids did not give me a current, the intensity of the electricity generated in each cell being alike. Copper in sulphuric-acid is positive to copper in nitric acid.

[Footnote] † “On the So-called Selective Action of Very Dilute Solutions of Cyanide of Potassium used in obtaining Gold and Silver from Ores and Compounds,” by James Mactear. Read before that institution, November, 1895, and first seen by me 17th March, 1896.

[Footnote] ‡ “On the Replacement of Electro-positive by Electro-negative Metals in a Voltaic Cell.” Proc. Roy. Soc., Lond., 1875.

[Footnote] § “Notes on the alleged ‘Replacement of Electro-positive by Electronegative Metals in a Voltaic Cell,’” by William Skey. Trans. N.Z. Inst., vol. viii., 1875, pp. 343–345.

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these to give us the potassium in a free state? But supposing even that he had an electric battery laid on of sufficient power theoretically to liberate potassium from its cyanide, how can he expect even then to have potassium set free for the minutest portion of time when there is water, also free oxygen, in contact with the potassium compound during the whole time that chemical action is going on? No; I consider that in the experiments of all these investigators the potassie cyanide or the potassic chloride is simply passed to the oxide by a substitution so direct that never was any of the metal set free. A person travelling over the boundary of two Shires has never any part of his person clear of both; so here the atom of potassium in exchanging companions is never clear-that is, never free in any part of it of one or the other. This is the position I have long taken on the subject generally, and I cannot retire from it before I have something more than mere assertions, whatever direction they come from.

I would like to make here a few observations in connection with Mr. Maclaurin's important discovery of the great insolubility of oxygen in strong solutions of potassic cyanide. In the Laboratory Notes of mine already referred to I state, in affirmation No. 2, “That, generally, any salt added to a good working solution of the cyanide acts the same as an equal quantity of the cyanide in retarding or preventing dissolution of gold.”* Now, these facts lead me to suppose that it is a general law that all aqueous solutions of salts, fixed alkalies, and alkaline earths are also solvent of oxygen in a proportion inversely to that of their strength; that, in fact, Mr. Maclaurin's law for the cyanide solution is really a general law for all saline solutions. This receives some support from the fact that a great number of solid salts, when dissolved in ordinary water at a common temperature, liberate gas, while if the water has been boiled for a considerable time, then rapidly cooled, and then immediately placed over any of these salts, not a trace of gas can be seen to escape therefrom, as the salt dissolves. I formerly considered that the gas escaping when crystallized salts were placed in water was simply the air they had oecluded in the act of crystallization; but it appears pretty clear to me now that this was one of my misconceptions.

In the light that our recent discoveries throw on the exact chemical nature and reactions involved in the cyanide process, I now venture to state here my opinion on the question as to whether in that process as conducted at the mines the gold is first oxidized or cyanodized; and it is this: In a plentiful

[Footnote] * The fixed alkalies act similarly, but not ammonia or chloride of ammonium.

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supply of free oxygen a part of the gold is directly oxidized, while another part, and that the larger part, is cyanodized, while, if free oxygen is scarce, the less gold is oxidized and the more is cyanodized, until in those extreme cases where free oxygen is entirely absent, while at the same time the gold is in contact with minerals conductive of electricity, or is but only partly submerged in the cyanide, all the gold is then won by being directly cyanodized.

In conclusion, I again express my regret that at the time I prepared my notes on the cyanide process for the Mines Department I was unacquainted, with Mr. Maelaurin's communication to the Chemical Society on this subject. I had no chance to ignore it, as I am supposed to have done; and if I had had that chance Mr. Maclaurin may be sure that it would have been the furthest from my thought to take it. Not to say anything about the dictates of common honesty, which may or may not rule my dealings with the world, my own native caution and hereditary prudence would have told me that a paper prepared so skilfully, so laboriously, and with such a wealth of illustration and of argument as this was, is not one to be “ignored” by anybody except to his own disadvantage and dishonour. On the other hand, I would assure Mr. Maclaurin that I am very much pleased to learn that one of our university colleges has turned out of its roomy and well-fitted halls a native-born New-Zealander who has entered the wide and fascinating field of original research, among the physical sciences, and made his début among our brother scientists of the Old World in the strikingly successful and honourable manner that he has. And I shall be glad to see his example followed by other New-Zealanders, and also to have the valuable assistance continued of a friendly but full criticism of any other communications to the scientific world that I may have the opportunity of laying before it.