Christian Friedrich Schoenbein to Faraday   27 April 1837

Dear Sir

I take once more the liberty to address to you by writing a short account of the results of my latest researches on the peculiar condition of iron. To my opinion, these results, though they do not yet solve the riddle of the subject, are such as to excite scientific curiosity, at least, as much, as the facts did, a description of which I had the honor to communicate to you last year¹. The space allotted to a letter being so small, I am obliged to be as concise as possible in describing the phenomena, recently observed by me; but if you should be interested with the details of the subject, I take the liberty of referring you to a paper of mine, which in some time will be published in "Poggendorff's Annalen"². In the first place I must tell you, that the most powerful voltaic association, into which iron can be brought, in order to excite its peculiar condition, is that with peroxide of lead. A common iron wire, one of the ends of which is covered with this substance proves to be inactive not only towards nitric acid of a given strength, but towards nitric acid containing any quantity of water; whilst as you know, my oxidized iron wire, or one associated with platina &c is acted upon by this acid if much diluted just in the same manner as unprotected iron. But the superiority of the association mentioned to any other at present known is exhibited in a still more striking manner by putting the two ends of an iron wire, (one of which is covered by, peroxide of lead) into an aqueous solution of the common sulphate of copper in the same way as the two ends of the oxidized wire &c are plunged into common nitric acid. Under these circumstances not the smallest particle of copper will be precipitated on any part of the wire immersing into the said solution; this peculiar state of the wire, however, lasts only so long, as both ends of it are plunging in the solution; for no sooner the protected one is removed from the liquid, than the other one left immersing turns active, that is to say throws down copper. In this respect, therefore, there is a great difference between the action of the wire in question upon the solution of blue vitriol, and that of the oxidized one upon common nitric acid. This difference of action implies an‑other, namely the impossibility of transferring within the copper solution the peculiar state from wire to wire, which to do is so easy within nitric acid. I must not omit here to state the remarkable fact, that by mixing the solution of the sulphate with a comparatively small quantity of chloride of sodium (common salt) the calling forth of the peculiar condition is prevented, not only in the foregoing case, but in all that will be mentioned afterwards. This fact is by no means an insulated one, and depends upon the same cause, which prevents the disengagement of oxigen at the iron (whilst constituting the positive electrode of the pile) out of a solution of haloid salts &c. Presuming, that by rendering iron inactive towards sulphate of copper in the way described, a current would be excited as to its direction equal to that, produced by calling forth the peculiar state of this metal within nitric acid, and having had recourse to the galvanometer, I was very much struck on finding that the needle was not in the least affected. The instrument I made use of in my experiments, though indicating rather weak currents, does certainly not possess the highest degree of sensibility possible; (it contains about 100 coils) but as in a scientific point of view it is of very great importance to know, whether the peculiar condition of iron can in any way be called forth with‑out exciting at the same time a current, I beg you to decide this question by means of your most delicate galvanometer. If your experiments should happen to place beyond doubt the absence of any current under the before mentioned circum‑stance, why such a result would allow of drawing very curious inferences from it, and prove, in the first place, that the in‑activity of iron has as to its origin nothing to do with what we call a current. A series of phenomena, regarding the action of iron wire (associated with peroxide of lead) upon a solution of sulphate of copper may be called forth, which exhibits a beautiful analogy to that set of facts, communicated to you in my letter, you had the kindness to have inserted in the Phil. Magazine No. 59³. To obtain with iron in the said solution results similar to those mentioned in my letter, with regard to the action of this metal upon nitric acid, the following conditions must be fulfilled. In the first and second case the oxidized iron wire EF is to be replaced by a wire, whose end E is covered with peroxide of lead, every other thing remains the same as stated in my letter. As to the third fact, CPD is to be an iron wire having its end D associated with peroxide of lead; EF is to be a common iron wire. With respect to the fourth case, E instead of being oxidized must he covered with peroxide of lead. To obtain a result analogous to the fifth fact, the iron wire CPD its end D being associated with the men‑tioned substance, must first be put into the vessels. Supposing D to be within B, the end E of an iron wire EF is to be plunged into A and F afterwards into B. As the third case shows it, F will turn under these circumstances inactive. Things being in this state, put the one end of a third common iron wire into B and afterwards its other end into A, and F will cease to be in its peculiar condition. Now, whatever the number of wires similar to that of EF may be, all their inactive ends being within B turn active under the circumstances mentioned, though they do not touch each other anywhere. Concerning the sixth case, it is obtained exactly in the same manner as stated in my letter, provided the oxidized end be replaced by one covered with peroxide of lead. Bending up the common end of the experimental wire, is, however, not required. The best way of associating an iron wire with peroxide of lead is, to make it the positive electrode of a crown of cups (containing about a dozen of pairs of copper and zi<<nc>>) and to put the free end of this wire into a solution of the common acetate of lead (Saccharum Saturni) for about 8-10 minutes. By the action of the pile the peroxide is deposited on the positive iron wire. En passant I must tell you, that many reasons lead me to be‑lieve, that iron associated with this substance will form the most powerful voltaic element known, and I am just about to construct out of such couples a pile. As to the chemical nature of the matter producing the colours of Nobili's chromatic scale⁴ a notice of mine will shortly be published in Poggendorff's Annals⁵, from which you will see, that your idea about the subject is entirely correct⁶ and consequently the view of the italian Philosopher wrong. Some of the facts regarding the peculiar condition of iron and observed by Mr. Noad⁷ are, as you will easily perceive, quite the same which were already stated in my letter above mentioned. In publishing them as new ones, the gentleman was most likely not aware of my observations⁸. I cannot close these lines without expressing you my sincere thanks for the service, which you so kindly rendered me by forwarding my last paper to the editors of the Phil. Magazine⁹.

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

Bâle April 27th 37

Address: Dr. Faraday | Royal Institution | London