Art. XXXIX.—On the Capability of Certain Sulphides to form the Negative Pole of a Galvanic Circuit or Battery.
[Read before the Wellington Philosophical Society, October 22, 1870.]
When a piece of massive galena is placed in voltaic contact with amalgamated zinc, and immersed along with it in weak sulphuric acid, to within an inch or two of the point of contact, a galvanic current is at once established, gas in quantity being given off at the surfaces of the galena, while the zinc is rapidly oxydized.
Three or four such pairs when connected among themselves, intensity fashion, afford a current of electricity strong enough to decompose acidulated water, and manifest all the phenomena of a small galvanic battery. Such an
arrangement may properly be termed a pyritous battery—in accordance with the custom hitherto observed, of designating new forms of batteries after some distinguishing or novel feature in them.
Several other metallic sulphides manifest similar phenomena when coupled in this manner with zinc, amongst which are the following:—zinc blende, copper pyrites, vitreous copper stibnite, proto-sulphide of iron, and the sulphides of silver, mercury, platinum, and gold.
The only sulphide I have yet found any difficulty with in setting up this action, under these conditions with zinc, is mundic, or the bi-sulphide of iron; but if the point of contact between it and the zinc is submerged in the acid, gas is evolved at this point, and the area of evolution rapidly spreads around from this, until the whole surface of the specimen becomes active.
The gas given off from the surfaces of these sulphides in the foregoing experiments, was sulphuretted hydrogen. The effects, therefore, upon these sulphides when thus made to form the negative pole of a galvanic pair, is to desulphurize them; in some cases the ultimate effect is to reduce the metal to the metallic state. At any rate this obtains for the sulphides of mercury, lead, silver, platinum, and gold. With common yellow copper pyrites, a beautiful iridescence is communicated to it in a few seconds after the liberation of gas commences, owing, of course, to desulphurization.
Saline solutions appear to produce the same effects in these instances as sulphuric acid, but they take place much slower.
The fact is thus directly established, that several of the metallic sulphides are capable of performing all the functions of the negative pole of a galvanic pair. From this, and the manner in which these sulphides have been connected with each other, it is clearly demonstrated that they are pretty good conductors of electricity. To a certain extent, indeed, all bodies are conductors of electricity, the terms conductor and non-conductor being only expressive, as Faraday affirms, “of extreme degrees of one common condition,” there being no complete conductor, nor any absolute non-conductor; but these results show, I think, these sulphides are conductors to a degree not before recognized —a circumstance which renders a comparison of their conducting power with that of other conductors necessary.
Indirectly it has appeared, that nascent hydrogen is able to decompose these sulphides at common temperatures, by combining directly with their sulphur, thus accomplishing at a low temperature that which would require a very high one, in case it (the hydrogen) were presented to the sulphide in its ordinary form.
In relation to the amalgamating processes used for the extraction of gold at our batteries, these results prove that zinc amalgam, in contact with acid solution, has precisely the same effect in decomposing the sulphides of gold, mercury, iron, etc., as sodium amalgam; like this amalgam, therefore, it keeps
perfectly bright and mobile in presence of sulphides, or the products of their natural metamorphoses.
Whether or no such an amalgam could ever be profitably substituted for sodium amalgam in our gold batteries, has yet to be determined. I am afraid that the continual addition of sulphuric acid to the water conveying the auriferous stuff to the plates, etc., which the use of this amalgam necessitates, would increase so much the cost of extraction as to render it unprofitable; still, as the water need only be very slightly acidified, it may be well to keep the fact thus arrived at in mind, when projecting intended improvements in the amalgamation of the gold from auriferous reefs.
In conclusion, I will only now remark, that the kind of phenomena just described appear to have some relation to the formation and decomposition of metalliferous lodes.
It is pretty certain, analogically considered,* that these sulphides should be able to form among themselves a series of voltaic pairs in presence of saline solutions, as they differ from each other in respect to their affinities for oxygen. Galena and copper pyrites, for instance, should form a voltaic pair, in which the galena would be the negative element; sulphide of silver and galena again should furnish another pair, in which the galena would have its function reversed, and so on for the rest, according to their relative proneness to change.
In a natural way, therefore, the contact of dissimilar sulphides generally should set up galvanic action and chemical decomposition, and by setting up this action, we might have a sulphide decomposed by saline solutions, which it would be able to resist if it stood alone; or, on the other hand, we might have an easily decomposable sulphide preserved by the association with it of one still more ready to decompose.
Since the results just detailed were arrived at, I have been referred by Dr. Hector to a paper by Mr. Robert Hunt, entitled, “Researches on the Influence of Magnetism and Voltaic Electricity on Crystallization and Conditions of Matter,” given in Memoirs of Geological Survey of Great Britain, Vol. i.
The subject of that paper is similar in part to this under consideration, but I do not see that the author has anticipated any portion of the results stated here; he certainly does not demonstrate the actual and continuous production of electricity by the contact of sulphides with positive bodies in saline solutions; nor does he show that copper pyrites conducts electricity, the water line being, as you will observe by reference to his diagram 14, page 457, above the point of contact between the pyrites and the battery, so that the change or decomposition of the ore need not involve the necessity of conducting power in the pyrites, as in the case of that connected with the positive end of the
[Footnote] * Since determined to be the case.—See Art. XLI.
battery, the conducting power necessary for this decomposition might well have progressed around, from this point or line of contact by the liberation of copper; while, in case of the other piece of pyrites, all the conducting power necessary for the production of the phenomena described may, with propriety, be referred to the wire bound around it.
I would also state, that in repeating this experiment of Mr. Hunt's I find that, different to his own observations as stated, both the pieces of pyrites are chemically affected, while it is not that in contact with the copper of the battery which displays such marked iridescence, but that communicating with the zinc; and it passes into this state not by an oxydation process, but by a desulphurizing one, brought about by the liberation of hydrogen upon its surfaces; this gas, when freshly liberated, having a desulphurizing effect upon sulphides generally, as I have clearly shown above.