Christian Friedrich Schoenbein to Faraday   12 September 1836

Sir

I feel much obliged to you for the kind manner in which you mentioned my late researches on iron in the philoso‑phical Magazine¹. It is this kindness which encourages me to address to you a second letter on the same subject. First allow me to make some observations regarding the explanation, you give about the cause of the peculiar voltaic condition of iron. If I have not misunderstood it, you account for the inactivity of this metall by two suppositions; one of which is, that a film of oxide is formed round the iron similar to that produced by heating the metall; the second, that this oxide has the property of not being dissolved by nitric acid of a certain strength. Now the formation of such an oxide takes place perhaps, when iron is in voltaic association for instance with platina; in this case water may be decomposed and the oxigen resulting from the decomposition combine with iron to form the supposed oxide. But how is it, when iron is made inactive by plunging it into strong nitric acid? I should not think, that in this case either nitric acid or water is decomposed; that is to say, that an oxide is formed. There is another fact of a similar kind, which makes me doubt of the existence of the film in question. This fact is, that iron wire turns inactive even in common nitric acid by repeated immersions, that is to say, after this metall has been acted upon in the usual way, after deut‑oxide of iron and a nitrate has been formed. Now I think, we may ask, why in this instance the common chemical action is, at once, s[t]opt and how it happens, that, on a sudden an oxide is formed of a description quite different from what the first one (deutoxide) was. I confess, I have not the slightest idea about the cause of such a change of action. There is another fact, which clearly shows, that under some circumstances at least, the inactive state of iron has nothing to do with a peculiar strength of nitric acid. In my last letter² I had the honour to state to you, that iron serving as the positive electrode of a pile proves to be inactive in nitric acid, whatever its degree of aqueous dilution may be, whilst a wire made inactive either by immersion in strong nitric acid or by association with oxide of iron (produced by heating) is acted upon in the common way by nitric acid containing water beyond a certain quantity. Now if one of the circumstances determining the peculiar con‑dition of iron were a peculiar strength of nitric acid, there should be common chemical action in one case as well, as there is in the other, but the contrary being the fact, we are entitled to draw the conclusion, that the inactivity of iron is not always dependent upon a peculiar strength of the acid. As we must evidently give up one part of the explanation in one case, I am afraid, there is sufficient reason to make us doubtful of its holding good in other ones. As to the film of oxide, which you think to be formed round the iron in all cases, where this metall shows its peculiar condition, I have also observed, that a coating of a blackish substance is produced round an iron wire, when, being connected with the anode of the pile, it was plunged into common nitric acid, closing at the same time the circuit; but I could never remark the least change in the metallic state of the surface of the wire, in case it was immersed in dilute acid (containing about 10 times its volume of water) under the circumstances mentioned. In the very moment, when I brought the iron wire into the fluid, the evolution of oxigen began at the metall. The same experiment made in a solution of potash, showed the same phenomenon. If for instance in the latter case a film were produced, I should think the wire provided with it and put into common nitric acid was to prove as inactive, as a wire, whose end had previously been oxidized by heating; such, however, is not the case. According to your experiments, there always dissolves some iron in nitric acid, even whilst this metall renders the function of the positive electrode. I found the same, but only when I made use of common nitric acid, never when it was considerably dilute. I, therefore, strongly suspect, that the nitrate to be met with in the first case, is not produced within the acid. I ascribe its formation to the vapours rising out of the acid, which corrode the iron laid bare to them. The salt thus produced and first deposited on the superior part of the iron wire is afterwards carried down into the fluid by capillary action. As it appeared to me a point of importance to know, whether iron, under the circumstances mentioned, is or is not dissolved in nitric acid, I took particular care of ascertaining the fact. For this purpose I made in one instance use of an acid containing 10 times its volume of water, left in it the wire (being connected with the pile) for fully six hours and afterwards saturated the acid with ammonia. Not the smallest quantity of oxide was precipitated, though the volume of oxigen evolved at the iron during the time had comparatively been considerable. From a second experiment I obtained the same result. I put a solution of potash into a tumbler, and dilute nitric acid upon it in such a manner, as to prevent the fluids from mixing with one another. An iron wire serving as the positive electrode and reaching down to the bottom of the tumbler, was left in it for three hours. Not the slightest bit of oxide made its appearance; as soon however, as the circuit was broken, greenish flakes of oxide of iron were precipitated; and the same substance was seen forming, in case the circuit had not been closed in the manner required for causing the evolution of oxigen. From these facts I think it may safely be inferred, that nitric acid does not dissolve iron, whilst this metall is placed under the influence of a current moving through it in a certain direction[.] As the existence of such a relation of a current to chemical action would be of the greatest importance to science, I lively hope, you will pay a particular attention of this subject and enter into a close investigation of it. I pass now to another subject. During my researches on iron, I often made the observation, that nitric acid, remaining the same with regard to its strength and temperature acts with a different degree of inten‑sity upon the metall mentioned. If for instance iron wire, being in the same state, as it is sold, be plunged into nitric acid spec. grav. 1.35 and 12° the metall is violently acted upon and continues to be so, as long as there is any particle of iron left. But if the wire be taken out of the acid after a few moments' action, held for a second or two in the air, and replunged into the acid, the degree of intensity of action will already be a little diminished; and having four, at most, five times repeated the same operation, the metall will cease altogether to be affected by the acid, in fact it will then be in its well-known peculiar voltaic condition. Between the greatest violence of action and complete inactivity, there are certainly an infinite number of intermediate degrees of intensity of action. But we may distinguish two principal ones; one which is superior and another which is inferior to the degree of that influence of platina, which tends to stop chemical action. Indeed, if a platina wire is made to touch an uncleaned iron wire after having for the first time been immersed in nitric acid of the strength above mentioned, it cannot interrupt chemical action, but it will stop it, after the second or third immersion of the iron. An‑other fact worthy of remark is, that the degree of stability of the inactive state of iron called forth by immersing this metall several times in nitric acid spec. grav. 1.35, is much greater than that produced by voltaic association or by immersion in strong nitric acid. For if an iron wire made inactive by one of the latter means is turned again into the active state, it will be much more violently affected by the acid, than a wire brought into its peculiar condition by the way of immersions in common nitric acid. That the latter wire is more strongly inactive than one brought into this peculiar state by any other method is still more obviously indicated by the fact, that in most cases it turns by touching within the acid another wire, (which has been made slowly active in this acid) into an inactive one; whilst a common wire made inactive by immersion in strong nitric acid, or by transference or by a direct voltaic association is always thrown into a violent action by being touched by a slowly active wire. I must not omit to mention, that an iron wire having once been made inactive by repeated immersions, will under no circumstances whatever be so lively acted upon by nitric acid, as a common one; and the action commenced at it can always be stopt by platina, provided the acid be not too dilute or too hot. The peculiar lustre, the white colour and the soft touch of the surface of a wire turned inactive by immersions in common nitric acid likewise deserve to my opinion to engage attention of scientific men. Such a wire is, indeed, as to its appearance so like platina, that it can hardly be dis‑tinguished from this metall. Even after the file had several times passed over its surface, I thought the colour of the metall to be still whiter, than that of a common wire's surface (likewise produced by filing). Several persons to whom I showed such wires, were of the same opinion. If my observations should happen to be correct, it would prove, that the action of the acid upon the iron spoken of occasions a very remarkable change of aggregation of its particles; and it is perhaps by such a change, that we may account for some of the anomalous bearings of iron. Before I conclude I must mention a fact I find already alluded to in a paper of Herschels³, a paper which by the bye I saw but a little while ago in looking over in periodicals the articles treating on nitric acid. This fact consists in a sort of action of nitric acid upon iron, which for its extra‑ordinary character highly merits farther scientific inquiry. The best way of calling forth the phenomenon is the following one. A common iron wire having been made inactive by simple immersions in nitric acid sp. gr. 1.35 is touched within this fluid by a piece of copper; the wire will by this means be thrown into action which action, however, is not continuous, but takes place, as it were, by pulsations, in other words the wire will alternately be active and inactive. Sometimes it happens that the wire relapsed into its inactive state after the first touch of the copper; in such a case, it must repeatedly be retouched by this metall in order to obtain the effect desired. Temperature and the degree of dilution of the acid, within which the action occurs, remaining the same, the number of pulsations performed in equal spaces of time remain likewise the same. By augmenting the temperature and the quantity of water of the acid the inter‑vals of action and inaction are made shorter and at last these two states follow each other in such quick succession as to pass into a continuous action. Different wires separate from one another, being in the same acid and exhibiting the phenomenon in question, do not pulsate together; but as soon as they are put in contact with each other either within or without the acid, the whole set of wires pulsate at the same time, a fact, which is worthy of remark. Herschel's assertion according to which only an acid, having already been made use of for inducing in iron wires the inactive state, is capable of exhibiting the pheno‑menon spoken of, does not agree with what I have observed; for I found, that quite pure nitric acid renders the service, when an iron wire made inactive by immersions is plunged into it and turned active again by the way described. This fact, I think, proves, that the cause of the pulsation like action lies rather in the state of the wire, than in that of the acid.

As the "Bibliotheque universelle" will before long publish some papers of mine⁴, written on the same subjects, with which I have taken the liberty to entertain you in the preceding lines, you will, perhaps, think it worth your while to have a look at them, and excuse me, when I do at present not enter into further details.

Recommending my humble individual to your kind indulgence I am, Sir, your most obedient Servant

Prof. Schoenbein

Bale Septb. 12th 1836.

Address: Michael Faraday Esq. D.C.L., F.R.S. | at the Royal Institution | London.