a continuation of that provided with an oxidized end. But no action occurs, even after the wires have been separated from each other. If the second wire, having become indifferent, be now taken out of the acid, and made to touch at any of its parts, not having been immersed, a third wire, and both again introduced into the acid so as to make that part of the second wire which had previously been in the fluid enter first, either of the wires will be acted upon either during their contact or after their separation. In this manner the third wire can make indifferent or passive a fourth one, and so on.

"Another fact, which has as yet, as far as I know, not been observed, is the following one. A wire made indifferent by any of the means before mentioned is immersed in nitric acid of sp. gr. 1.35, so as to have a considerable part of it remaining out of the fluid; another common wire is put into the same acid, likewise having one of its ends rising above the level of the fluid. The part immersed of this wire will, of course, be acted upon in a lively manner. If the ends of the wires which are out of the acid be now made to touch one another, the indifferent wire will instantly be turned into an active one, whatever may be the lengths of the parts of the wires not immersed. (If there is any instance of chemical affinity being transmitted in the form of a current by means of conducting bodies, I think the fact just stated may be considered as such.) It is a matter of course, that direct contact between the two wires in question is not an indispensably necessary condition for communicating chemical activity from the active wire to the passive one; for any metal connecting the two ends of the wires renders the same service.

"Before passing to another subject, I must mention a fact, which seems to be one of some importance, An iron wire curved into a fork is made to touch at its bend a wire provided with an oxidized end; in this state of contact both are introduced into nitric acid of sp. gr. 1.35 and 30°, so as first to immerse in the acid the oxidized end; the fork will, of course, not be affected. If now a common iron wire be put into the acid, and one of the ends of the fork touched by it, this end will immediately be acted upon, whilst the other end remains passive; but as soon as the iron wire with the oxidized end is put out of contact with the bend of the fork, its second end is also turned active. If the parts of the fork rising above the level of the acid be touched by an iron wire, part of which is immersed and active in the acid, no communication of chemical activity will take place, and both ends of the fork remain passive; but by the removal of the iron wire (with the oxidized end) from the bend of the fork, this will be thrown into chemical action.

"As all the phenomena spoken of in the preceding lines are, no doubt, in some way or other dependent upon a peculiar electrical state of the wires, I was very curious to see in what manner iron would be acted upon by nitric acid when used as an electrode. For this purpose I made use of that form of the pile called couronne des tasses, consisting of fifteen pairs of zinc and copper. A platina wire

was connected with (what we call) the negative pole of the pile: an iron wire with the positive one. The free end of the platina wire was first plunged into nitric acid sp. gr. 1.35, and by the free end of the iron wire the circuit closed. Under these circumstances the iron was not in the least affected by the acid; and it remained indifferent to the fluid not only as long as the current was passing through it, but even after it had ceased to perform the function of the positive electrode. The iron wire proved, in fact, to be possessed of all the properties of what we have called a passive one. If such a wire is made to touch the negative electrode, it instantaneously becomes an active one, and a nitrate of iron is formed, whether it be separate from the positive pole or still connected with it, and the acid be strong or weak.

"But another phenomenon is dependent upon the passive state of the iron, which phenomenon is in direct to contradiction all the assertions hitherto made by philosophical experimenters. The oxygen at the anode arising from the decomposition of water contained in the acid, does not combine with the iron serving as the electrode, but is evolved at it, just in the same manner as if it were platina, and to such a volume as to bear the ratio of 1: 2 to the quantity of hydrogen evolved at the cathode. To obtain this result I made use of an acid containing twenty times its volume of water; I found, however, that an acid containing 400 times its volume of water still shows the phenomenon in a very obvious manner. But I must repeat it, the indispensable condition for causing the evolution of the oxygen at the iron wire is to close the circuit exactly in the same manner as above mentioned. For if, exempli gratia, the circuit be closed with the negative platina wire, not one single bubble of oxygen gas makes its appearance at the positive iron; neither is oxygen given out at it when the circuit is closed, by plunging first one end of the iron wire into the nitric acid, and by afterwards putting its other end in connexion with the positive pole of the pile. In both cases a nitrate of iron is formed, even in an acid containing 400 times its volume of water; which salt may be easily observed descending from the iron wire in the shape of brownish-yellow coloured streaks.

"I have still to state the remarkable fact, that if the evolution of oxygen at the anode be ever so rapidly going on, and the iron wire made to touch the negative electrode within the acid, the disengagement of oxygen is discontinued, not only during the time of contact of the wires, but after the electrodes have been separated from each other. A few moments holding the iron wire out of the acid is, however, sufficient to recommunicate to it the property of letting oxygen gas evolve at its surface. By the same method the wire acquires its evolving power again, whatever may have been the cause of its loss. The evolution of oxygen also takes place in dilute sulphuric and phosphoric acids, provided, however, the circuit be closed in the manner above described. It is worthy of remark that the disengagement of oxygen at the iron in the last-named acid is much easier stopped,

and much more difficult to be caused again, than is the case in nitric acid. In an aqueous solution of caustic potash, oxygen is evolved at the positive iron, in whatever manner the circuit may be closed, but no such disengagement takes place in aqueous solutions of hydracids, chlorides, bromides, iodides, fluorides. The oxygen, resulting in these cases from the decomposition of water, and the anion (chlorine, bromine, &c.) of the other electrolyte decomposed combine at the same time with the iron.

"To generalize these facts, it may be be said, that independently of the manner of closing the circuit, oxygen is always disengaged at the positive iron, provided the aqueous fluid in which it is immersed do not (in a sensible manner) chemically act upon it; and that no evolution of oxygen at the anode in contact with iron under any circumstances takes place, if besides oxygen another anion is set free possessed of a strong affinity for iron. This metal having once had oxygen evolved at itself, proves always to be indifferent to nitric acid of a certain strength, whatever may be the chemical nature of the fluid in which the phenomenon has taken place.

"I have made a series of experiments upon the silver, copper, tin, lead, cadmium, bismuth, zinc, mercury, but none showed any resemblance to iron, for all of them were oxidized when serving as positive electrodes. Having at this present moment neither cobalt nor nickel at my command, I could not try these magnetic metals, which I strongly suspect to act in the same manner as iron does.

"It appears from what I have just stated that the anomalous bearing of the iron has nothing to do with its degree of affinity for oxygen, but must be founded upon something else. Your sagacity, which has already penetrated into so many mysteries of nature, will easily put away the veil which as yet covers the phenomenon stated in my letter, in case you should think it worth while to make it the object of your researches.

"Before I finish I must beg of you the favour of overlooking with indulgence the many faults I have, no doubt, committed in my letter. Formerly I was tolerably well acquainted with your native tongue; but now, having been out of practice in writing or speaking it, it is rather hard work to me to express myself in English.

"It is hardly necessary to say that you may privately or publicly make any use of the contents of this letter.

"I am, Sir, your most obedient Servant,

"C. T. SCHOENBEIN,

"Prof. of Chem. in the University of Bâle."

"Bâle, May 17, 1836."

Further Observations on the Action of Nitric Acid upon Iron. By DR. SCHOENbein.

I HAVE already remarked that nitric acid, which generally attacks iron with violence, has no action upon an iron wire, one end of which, before its immersion in the acid, has been heated to dullness. From experiments since made, I find that the action is dependent on the quantity of water combined with the acid. This quantity I have not yet accurately ascertained, but I find that acid of the sp. gr. of 1.36 diluted with 15, 30, 60, 120, 240, 480, and 960 times its volume of water, attacks an iron wire heated at one end in the same manner as when not heated, and that the oxidated iron falls off by degrees into the acid without being dissolved in it.

Diluted nitric acid acts upon iron wire protected by platina or gold in the same manner as when the end of the wire is heated. Many chemists state that iron is not acted upon by ordinary nitric acid when diluted with three times its volume of water: according to my experiments, this metal is sensibly dissolved by nitric acid diluted with 1000 times its volume of water. As it is evident that the different action of the same nitric acid on iron is caused by a certain electrical state of the metal, I endeavoured to ascertain its nature by making an iron wire the positive pole of a voltaic battery set in action by nitric acid. I experimented in the following manner :—

Nitric acid, of sp. gr. 1.36, at the ordinary temperature, was used in a circle of fifteen plates, with a voltaic cup apparatus; at the positive pole an iron wire, and at the negative a platina wire, dipped into the acid. When I closed the circuit with the negative wire, the iron wire was acted upon as usual; when I closed it with the iron wire, by first dropping one end in the nitric acid, and then making the other end the positive pole of the battery, the same effect also took place; but when I closed the circuit so that one end of the iron wire was first united with the positive pole, and the other end afterwards dipped into the acid, no action took place on the iron, and it possessed, after its separation from the positive pole, all the properties which it had by heating, or when protected by gold and platina, precisely those which I have already so fully stated in my former paper. I heated the nitric acid used in the circuit to nearly its boiling point before it acted upon the positive iron wire. It follows, of course, that under these circumstances the water contained in the nitric acid was decomposed. No hydrogen gas is given out at the negative pole from strong nitric acid, for instance, of sp. gr. 1.36, but it combines with a part of the oxygen of the acid, and converts the latter into nitrous acid.

At a temperature of 70° centigrade, a gas is given out at the negative platina wire, which I have not yet particularly examined, but which is probably deutoxide of nitrogen. It has hitherto been

considered that the other element of the water, the oxygen, combines with the positive iron wire, and forms a hydrate with the nitric acid. If the circuit is so formed that the nitric acid has no action upon the iron wire, the free oxygen does not combine with the metal, but is given off in a gaseous state, precisely as when silver, gold, or platina wires are used. This is not only the case with acid of the above strength, but also with acid diluted with 1, 10, 100, and even 400 times its volume of water. That the iron is not partially oxidated is evident from its unchanged metallic lustre, as also from the proportions of gas given off at both wires, which I found according to several measurements to be as 1 to 2. If the two wires where the water is being decomposed are brought for a few seconds into contact (the nitric acid being diluted with about ten parts of water), and then again separated, oxygen gas is no longer given out at the positive iron wire, but a yellow nitrate is formed, which sinks to the bottom. If, however, the end of the iron wire which has been dipped into the nitric acid is exposed to the air for a moment, and the circuit then closed, oxygen gas is again given off from the iron. If the communication is broken by means of the negative wire, and again made, after a few seconds oxygen continues to be given out at the positive iron wire. When the acid is very much diluted, it requires some time after closing the circuit before the oxygen gas appears. In whatever manner the giving out of the oxygen gas is interrupted, it always recommences if the iron wire is exposed for a short time to the air, and the circuit then formed by it in the usual way. Exactly the same effects take place when diluted sulphuric or phosphoric acid is substituted for nitric acid.

In order that oxygen gas should be given off by these acids at the positive iron wire, it is first necessary that the negative pole should be in communication, by means of a wire, with the decomposing fluid; that one end of the iron wire should be in contact with the positive pole, and the other end with the acid. By any other mode of closing the circuit, oxygen gas is not given out, even when the iron wire has been previously dipped in diluted sulphuric or phosphoric acid. The giving out of oxygen gas will be interrupted upon bringing the wires of the poles into contact when the last-mentioned acids are used, and will not return upon exposure of the iron wire to the air. No oxygen gas is given out at the positive iron wire when subphosphorous or phosphorous acid dissolved in water is substituted, probably because it combines with the acid: when diluted alkalies are used, oxygen gas is given out at the positive end, in whatever way the circuit may be closed. During my experiments I have observed many other singular phenomena, which I shall communicate hereafter, when the circumstances under which they occurred are better understood. One of these, however, I will mention in conclusion: in the same nitric acid in which a platina wire served for the negative pole, a quantity of hydrogen gas was given off, but when an iron wire was substituted for the platina, no gas