question recommends itself by its beautiful simplicity, and what is still more valuable, by the great advantage of bringing back an apparently anomalous case to a general law, still there are weighty reasons stated by me elsewhere, which will hardly allow the adoption of Mr. Faraday's sagacious hypothesis.

"After having examined the action of iron upon the common sulphate of copper, I was curious to see how the same metal acts under similar circumstances upon the solutions of the nitrates of mercury. Before entering into details upon the subject, I must not omit to state, that I did not observe any essential difference of action between the protonitrate and pernitrate of mercury. A common iron wire, cleaned or not, when put into a solution of either of the neutral nitrates of mercury, does not act in the least upon the salt, that is to say, no mercury is precipitated on the iron; but what is still more surprising, the iron wire, after having been immersed only for a few seconds in such a solution, shows all the properties of inactive iron; it will, for instance, not be acted upon by common nitric acid, nor by a solution of blue vitriol. Even when a strong solution of the mercurial salt is diluted with 1,000 times its volume of water, it will still render an iron wire inactive, though in this case, as might be expected, some time is required for obtaining the effect. But if a common iron wire is first immersed in water containing so little of nitric acid as scarcely to change the colour of blue litmus paper, and afterwards plunged into the solution of mercury, it will precipitate the latter metal. It is, indeed, quite extraordinary how far this influence of the acids, favouring metallic precipitation, extends. I mixed a strong solution of neutral protonitrate of mercury with 1,000 times its volume of water, and in the same proportion I diluted common nitric acid. By putting the wire first into the acidulated water, it always acquired the property of decomposing the diluted solution of mercury on being plunged into it. Common muriatic acid, even 4,000 times diluted with water, produced the same effect. Though it is a well known fact that some free acid contained in metallic solutions favours the precipitation of one metal by another, still I am not aware that any chemist has as yet stated any particulars regarding the extent and cause of this influence. The peculiar action of acids mentioned seems to be intimately connected with the subject of my researches on the action of iron upon nitric acid, and to afford a case similar to that presented by inactive iron, in its bearing to strongly diluted nitric acid. In one of my published papers on the subject, I have stated that inactive iron loses its peculiar condition by being put into diluted nitric acid; the same thing takes place in the case before mentioned. Common iron wire is of itself inactive in a solution of a neutral salt of mercury, but is rendered active by being subjected to the action of acidulated water previously to its immersion in the solution. According to Mr. Faraday's views the acid must produce the effect spoken of, by cleaning the surface of the wire; that is to say, by dissolving some

film, with which even a common wire must be supposed to be covered; but for reasons before alluded to, I cannot entertain the opinion of this distinguished philosopher, even in this case.

"The view I have taken of the subject leads me to ascribe the effect in question to chemical excitement in the metal occasioned by the acidulated water. As iron having only for a few moments been immersed in diluted acid decomposes the neutral solution of mercury, it might be supposed that this metal should act in the same manner in a solution made somewhat acid. But I found this not to be the case. A solution of pernitrate of mercury obtained by saturating nitric acid, sp. gr. 1.35, with peroxide of mercury, was mixed with its own volume of the same acid. A common iron wire put into this acid solution had no action upon it, and assumed its peculiar condition. I could put even twenty volumes of nitric acid to it, without producing any action. But I must not omit to state the singular fact, that there is in this respect a great difference between a wire which is cleaned and one which is not. If, for instance, a common iron wire has only once passed through a piece of linen or cloth, it will be acted upon by the acid solution containing only one volume of nitric acid, whilst an uncleaned one is not affected at all. This difference is the more remarkable. as an uncleaned wire is much more violently attacked by mere nitric acid than a clean one. Another fact, still more singular, is, that different parts of the same piece of wire are sometimes differently acted upon by the same acid solution of mercury, one part being, for instance, entirely inactive whilst another contiguous to it proves to be highly active. I call this fact a very singular one, because every bit of a whole roll of iron is acted upon in common nitric acid. When an iron wire, cleaned or not, is plunged into the solution of mercury containing from thirty to fifty times its volume of nitric acid, it will be affected, and continue to be acted upon if left in the solution; but when it is again taken out of the fluid and held for hardly a second in the air, after its re-immersion it will prove entirely inactive. It is surprising, that almost the same results are obtained at very different degrees of temperature. I heated a mixture containing twenty volumes of nitric acid, and one volume of the solution of pernitrate of mercury to its boiling point. The end of an iron wire put into it was certainly acted upon, but by withdrawing it only for a few moments from the solution it was rendered inactive, so that it could afterwards be re-immersed in the nearly boiling acid fluid without being attacked by it. A certain proof that the metal acquires, even at this high degree of temperature, its peculiar inactive state is, that when put into a solution of blue vitriol or into mere common nitric acid, it does not in the least act upon these substances. In making these experiments I frequently observed the curious fact, that the iron wire immersed in the nearly boiling acid solution loses its inactive condition as soon as it is a little raised so as to expose to the air a very small part of that portion of

wire which has been immersed in the fluid; but though this is often the case, it is not invariably so.

"The results which I have obtained from experiments made with iron wire and an acid solution of mercury much diluted by water, are likewise worthy of being stated. One volume of a very strong solution of neutral protonitrate of mercury, five volumes of nitric acid, sp. gr. 1.35, and 200 volumes of water were mixed together. A piece of cleaned iron wire put into this solution did not precipitate mercury. By plunging such a wire into water slightly acidulated, its power of acting upon the salt of mercury, as above mentioned, is instantaneously called forth. The wire having once acquired this power retains it; provided, however, it be called into play at intervals of time not much exceeding a second or so. But if the wire after having been active in the solution is taken out of it, cleaned from the adhering mercury, and left exposed to the air only for a few seconds, it will have lost its property of precipitating the lastnamed metal, and rest entirely inactive in the solution, whatever length of time it may remain immersed in it. This remarkable and sudden change of the condition of iron is most likely due to some action of the air; for if the wire, being still in its active state with regard to the solution of mercury, is put into water or hydrogen gas, it preserves its precipitating power. I have not yet put iron into other mediums than those mentioned, nor have I examined whether moisture has anything to do with the phenomenon. At any rate, this subject seems to me in many respects sufficiently interesting to deserve further investigation. Before passing to another subject, I have still to mention some facts connected with those just spoken of. An iron wire which proves to be entirely inactive in the last mentioned solution of mercury, is not so with regard to a solution of blue vitriol or to common nitric acid; for a wire which does not throw down mercury, precipitates copper, or is violently acted upon by the said acid. From chemical reasons we are led to expect that the very contrary should take place: the affinity of copper for oxygen being much greater than that of mercury; that is to say, we should think the mercury salt easier to be decomposed by iron than the copper salt. It seems, therefore, as if the anomalous fact does not result from the action of common affinity. Another fact worthy of remark is, that iron acts quite differently upon the neutral nitrates of mercury dissolved in alcohol or ether, from what it does upon the aqueous solutions of the same salts. In the former case iron always precipitates mercury and never turns inactive, whilst, as above stated, in the latter case the contrary takes place. If an iron wire, having been rendered inactive by immersion in an aqueous solution of the mercury salt, is put into alcohol or ether containing the same salt, it loses its peculiar condition and returns into its active state.

"I think it not quite irrelevant to the subject treated of in this paper, if I produce a new case bearing evidence in favour of the

theory, according to which voltaic electricity is due to chemical action. It is true that the beautiful researches of Mr. Faraday, as well as those of Mr. de la Rive, have led to results which remove from an unbiassed mind even a shadow of a doubt on the subject, and which prove in the most satisfactory manner, that mere contact of heterogeneous metals is not capable of disturbing their electrical equilibrium. Still, as the number of philosophers who maintain the hypothesis of Volta is as yet rather considerable, I think it not quite useless to increase the body of evidence against it.

"If an iron wire rendered inactive by immersion in nitric acid is associated with a platina wire, and two of their ends put into a solution of blue vitriol, not the smallest quantity of copper will be precipitated on the platina; but if the inactive iron is thrown into chemical action, by being touched within the solution, either with a common iron wire, or by any other metal which chemically acts upon the copper salt, at the very moment of contact a film of copper makes its appearance on the platina. Now, if according to the views of Volta, electricity be excited by the mere contact of different metals, in the case in question a current should be produced, and in consequence of such a current chemical decomposition should take place, that is to say, copper should be eliminated at the platina. But from such not being the case, it follows that there is no current, consequently no electricity, produced by the contact of iron and platina. By having recourse to the galvanometer, the absence of a current under the circumstances mentioned, is placed beyond doubt. If the inactive iron wire is connected with one end of the wire of the galvanometer, the platina wire with the other one, and if the two free ends of the iron and platina wires are plunged into a solution of blue vitriol, not the least deflection of the magnetic needle takes place; but as soon as the part of the inactive iron wire immersed in the solution is touched with a metal capable of causing chemical action, the needle becomes agitated, and at the same time a deposition of copper takes place on both wires. From this fact it appears that the oxidation of iron has no sooner been occasioned than two effects of a current are produced: chemical decomposition of an electrolyte and affection of the needle. Now, as previously to oxidation no such effects are obtained, we are fully entitled to draw the inference that the phenomenon of oxidation bears to that of a current the relation of cause to effect, or generally speaking, that voltaic electricity is due to chemical action, and by no means to contact.

"I am quite confident that inactive iron can be used in a great number of cases for obtaining results similar to that just spoken of, and that the peculiar state of this metal offers to philosophers in many other respects a most valuable means for making electrochemical researches."

Professor Schöenbein's next paper is entitled "On the peculiar Voltaic Condition of Iron as excited by Peroxide of Lead."

The author commences by stating "that the most powerful vol

taic 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 my oxidized iron wire, or one associated with platinum, &c., is acted upon by that acid, if much diluted, in precisely 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 with peroxide of lead) into an aqueous solution of common sulphate of copper. Under these circumstances not the smallest particle of copper will be precipitated on any part of the wire immersed in the solution. This peculiar state of the wire, however, lasts only as long as both ends of it are in the solution; for no sooner is the protected one removed from the liquid, than the other left immersed turns active, and 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 another, which is the impossibility of transferring, within the copper solution, the peculiar state from wire to wire, which is so easily accomplished in nitric acid. I must not omit here to state the remarkable fact, that by mixing the solution of the sulphate with a comparative small quantity of common salt, the calling forth of the peculiar state is prevented, not only in the foregoing case, but in all that will be mentioned hereafter. This fact is by no means an insulated one, and depends upon the same cause as that which proves to the disengagement of oxygen at the iron whilst in the capacity of the positive terminal of the pile, from 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 in the same direction as that produced by calling forth the peculiar state of this metal in 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 employed in my experiments, though indicating rather feeble currents, certainly does not possess the greatest possible sensibility. It consists of 100 convolutions of wire. Should it be discovered, by the employment of more delicate galvanometers, that no current is called into existence during the process of iron being rendered inactive, it would prove that the inactivity of iron has, as to its origin, nothing to do with what we call a current."

"The best way," says Professor Schöenbein, "of associating iron with peroxide of lead, is to make it the positive electrode of a couronne des tasses, of about a dozen pairs of copper and zinc, and to put the free end of this wire into a solution of the common acetate of lead for about eight or ten minutes. By the action of the pile