Christian Friedrich Schoenbein to Faraday   31 December 1837

My dear Sir

From a series of experiments lately made by me with the view of ascertaining the voltaic relations of some per‑oxides, platina and inactive iron to one another I have obtained some results, which, to my opinion, are such as to throw some additional light upon the cause of voltaic electricity and modify, to a certain degree at least, the notions we have, hither to, enter‑tained about that interesting subject. You will recollect that the voltaic relation of peroxide of lead to Iron engaged my attention some time ago and you are, perhaps also aware of the fact stated by me in "Poggendorff's Annalen"¹ that the peroxide in question if voltaically associated with iron disappears by degrees when plunged into nitric acid of any strength. Now, as we know, that no chemical action whatever takes place in the circumstances mentioned, iron being in its peculiar condition and having, in a voltaic point of view, all the properties of platina I could not but be very much surprised at the dis‑appearance of the peroxide of lead. Although I was not able of tracing at the time any voltaic current and of accounting for any disturbance of the electric equilibrium of the arrangement alluded to, I nevertheless suspected, that the dissolution of the mentioned substance was effected under the influence of current electricity. Having now at my disposal a galvanometer, which is provided with 2000 coils and made in other respects very delicate, I have taken up that subject again and attempted first to ascertain, whether there is any voltaic relation of platina to inactive iron. In contradiction to the results which You and I obtained some time ago², I have found out by means of my galvanometer, that iron being in its peculiar condition and associated with platina gives rise to a sensible current if put into nitric acid, be the latter ever so strong or somewhat diluted with water. Making use of an acid of sp. gr. 1.4 the deviation of the needle, (on putting the iron and platina wires in connection with the galvanometer) amounted to about 90°. I must not omit to state, that the current excited in the circumstances mentioned is not a momentary but a continuous one and at the same time quite independent of any oxidation of the iron. The direction of the current in question is such as it would be, if the latter metal was attacked by the acid, that is to say, inactive iron is positive to platina. Another fact as curious and interesting as that just stated is the following one. Two platina wires being connected by one set of their ends with the galvanometer and by the other set with nitric acid or an aqueous solution of sulfate of copper, excite a current, provided one of the ends (immersing in the fluid) of one of the platina wires be covered with a film of peroxide of lead. The current passes from the platina through the fluid to the peroxide. When the said film is so thin as to produce, what is called "Nobili's colours"³, it disappears within a very few seconds after having been immersed into nitric acid and the whole arrangement connected with the wire of the galvanometer. From the facts stated, it appears that platina is positive with regard to peroxide of lead, and that the disappearance of that compound is caused by a current, which eliminates hydrogen at the negative peroxide, by which means the latter is reduced to protoxide of lead and rendered soluble in nitric acid. In a similar manner I have ascertained that the voltaic relation of inactive iron to peroxide of lead is exactly the same, as that of platina to the said peroxide. In using peroxide of silver instead of lead voltaic effects are produced quite the same as those which were just spoken of; that is to say, a continuous current is excited to which the peroxide acts the part of the cathode and either of the metals in question that of the anode. As to the voltaic relation, which one of the mentioned peroxides bears to the other my experiments have shown, that peroxide of silver is always negative with regard to the peroxide of lead, be the fluid made use of nitric acid or a solution of blue vitriol. Now from all the facts above stated, I think, we are allowed to draw two important inferences: 1.) that peroxide of silver, peroxide of lead, platina and inactive iron represent a series of substances, in which the preceding one is always negative with regard to that which follows in the list. 2.) that any two of the four substances mentioned being voltaically associated with one another and put either into nitric acid or a solution of sulfate of copper excite a continuous current, which is not due to oxidation or any chemical change. It is hardly necessary to add, that the currents produced in the said circumstances are extremely feeble being only indicated by most delicate galvanometers. You will agree with me, that the facts spoken of are highly important in a scientific point of view, as they do produce evidence in favor of that theory, which asserts, that by the mere contact of heterogeneous substances their electrical equilibrium can be disturbed, quite independent of any chemical action taking place between them. All chemists certainly maintain, that pure nitric acid for instance, does not chemically affect at all neither platina nor peroxide of lead; and inactive iron too, as we now well know, is not the least attacked by the said acid. Now I ask, whence does the current originate, which is produced, when we combine the substances in question in such a manner, as to form with them a voltaic arrangement. I have attempted to answer that puzzling question in a paper, which, before long, will be published in Poggendorff's Annales⁴ as well as in the Biblioth. univ.⁵ and in which you will besides find a detailed account of all the experiments made by me upon the subject spoken of. If my time was not so much taken up with a variety of business I would have drawn up a memoir in English and sent it to the Editors of your excellent philosophical Magazine for insertion; but those gentlemen will, perhaps, give a translation of the paper⁶. Before closing my letter allow me to communicate to you in a general manner the view, which I have taken of the subject in question. In the first place, I must tell you, that I am by no means inclined to consider mere contact in any case as the cause of excitement of even the most feeble current. I maintain, on the contrary, in accordance with the principles of the chemical theory, that any current produced in a hydroelectric voltaic circle is always due to some chemical action. But as to the idea, which I attach to the term "chemical action" I go farther, than You and Mr. de la Rive⁷ seem to go; for I maintain, that any tendency of two different substances to unite chemically with one another must be considered as a chemical action, be that tendency followed up by the actual combination of those substances or be it not and that such a tendency is capable of putting electricity into circulation. I will try to render this idea of mine somewhat clearer by applying it to some particular cases. Supposing a voltaic circle be composed of platina, peroxide of lead and nitric acid, I say, that the current excited in such an arrangement is due first to the tendency of the acid to unite with the protoxide of lead or, what is the same, to the tendency of one proportion of the oxigen to separate from the peroxide; secondly to the tendency of water to combine with the same protoxide to form a hydrate and thirdly to the tendency of water to withdraw a proportion of oxigen from the peroxide to produce peroxide of hydrogen, wh<<ich>> tendency, from very well known chemical reasons, is yet increased by the presence of the acid. It is true, none of the said tendencies do lead to any chemical results; for no nitrate of lead, no hydrate, no peroxide of hydrogen is actually produced; but are we allowed to infer from the want of a practical result, that no chemical action whatever takes place, when nitric acid and peroxide of lead are put in contact with one another; I ask, are we to suppose, that the chemical affinities alluded to are entirely dormant and incapable of any exertion? The results from my late experiments induce me to answer the question in the negative. Being quite of your opinion, that chemical affinity and current electricity are but different forms of the same thing, I cannot help thinking that any sort of chemical action or tendency must be capable of being trans‑formed into the shape of a current. For that current, which is produced by inactive iron (being voltaically associated with platina) I likewise account by a chemical tendency on the part of the former metal. Though inactive iron be not in the least attacked by nitric acid, its affinity for the oxigen of the latter is, on that account, by no means, entirely destroyed; the metal, whilst surrounded by the acid is continually tending to oxidize itself and the current excited in such a case is nothing else but as it were the electrical translation of a chemical exertion.

All the cases above stated, where currents are observed, independent of any chemical change, can easily be explained by applying to them the same principle, by means of which we have accounted for the current produced by nitric acid and peroxide of lead &c. Having already passed the usual limits of a letter, I add only one more observation to my former, and I have done. According to my experiments peroxide of silver proves to be the most powerful means for exciting in iron its peculiar voltaic condition. It surpassed in this respect even the peroxide of lead. An iron wire, for instance, one end of which is covered with only a small particle of the first‑-mentioned substance will not be attacked either by nitric acid of any degree of dilution or by a solution of blue vitriol. The voltaic association of one substance with the other is easily effected by connecting one end of an iron wire with the positive electrode of a pile and by plunging for a few minutes the other end of the wire into a solution of nitrate of silver. I am just about to write a paper on that interesting subject.

I am my dear Sir | Your's very truly | C.F. Schoenbein

Bâle Dec 31. 1837.

Address: Dr. Michael Faraday | &c &c &c | Royal Institution | London