Chapter 30

958. There are a few circumstances connected with the production of this spark by a single pair of plates, which should be known, to ensure success to the experiment[B]. When the amalgamated surfaces of contact are quite clean and dry, the spark, on making contact, is quite as brilliant as on breaking it, if not even more so. When a film of oxide or dirt was present at either mercurial surface, then the first spark was often feeble, and often failed, the breaking spark, however, continuing very constant and bright. When a little water was put over the mercury, the spark was greatly diminished in brilliancy, but very regular both on making and breaking contact. When the contact was made between clean platina, the spark was also very small, but regular both ways. The true electric spark is, in fact, very small, and when surfaces of mercury are used, it is the combustion of the metal which produces the greater part of the light. The circumstances connected with the burning of the mercury are most favourable on breaking contact; for the act of separation exposes clean surfaces of metal, whereas, on making contact, a thin film of oxide, or soiling matter, often interferes. Hence the origin of the general opinion that it is only when the contact is broken that the spark passes.

[B] See in relation to precautions respecting a spark, 1074.--_Dec. 1838._

959. With reference to the other set of cases, namely, those of local action (947.) in which chemical affinity being exerted causes no transference of the power to a distance where no electric current is produced, it is evident that forces of the most intense kind must be active, and in some way balanced in their activity, during such combinations; these forces being directed so immediately and exclusively towards each other, that no signs of the powerful electric current they can produce become apparent, although the same final state of things is obtained as if that current had passed. It was Berzelius, I believe, who considered the heat and light evolved in cases of combustion as the consequences of this mode of exertion of the electric powers of the combining particles. But it will require a much more exact and extensive knowledge of the nature of electricity, and the manner in which it is associated with the atoms of matter, before we can understand accurately the action of this power in thus causing their union, or comprehend the nature of the great difference which it presents in the two modes of action just distinguished. We may imagine, but such imaginations must for the time be classed with the great mass of _doubtful knowledge_ (876.) which we ought rather to strive to diminish than to increase; for the very extensive contradictions of this knowledge by itself shows that but a small portion of it can ultimately prove true[A].

[A] Refer to 1738, &c. Series XIV.--_Dec. 1838._

960. Of the two modes of action in which chemical affinity is exerted, it is important to remark, that that which produces the electric current is as _definite_ as that which causes ordinary chemical combination; so that in examining the _production_ or _evolution_ of electricity in cases of combination or decomposition, it will be necessary, not merely to observe certain effects dependent upon a current of electricity, but also their _quantity_: and though it may often happen that the forces concerned in any particular case of chemical action may be partly exerted in one mode and partly in the other, it is only those which are efficient in producing the current that have any relation to voltaic action. Thus, in the combination of oxygen and hydrogen to produce water, electric powers to a most enormous amount are for the time active (861. 873.); but any mode of examining the flame which they form during energetic combination, which has as yet been devised, has given but the feeblest traces. These therefore may not, cannot, be taken as evidences of the nature of the action; but are merely incidental results, incomparably small in relation to the forces concerned, and supplying no information of the way in which the particles are active on each other, or in which their forces are finally arranged.

961. That such cases of chemical action produce no _current of electricity_, is perfectly consistent with what we know of the voltaic apparatus, in which it is essential that one of the combining elements shall form part of, or be in direct relation with, an electrolytic conductor (921. 923.). That such cases produce _no free electricity of tension_, and that when they are converted into cases of voltaic action they produce a current in which the opposite forces are so equal as to neutralize each other, prove the equality of the forces in the opposed acting particles of matter, and therefore the equality of electric power in those quantities of matter which are called _electro-chemical equivalents_ (824). Hence another proof of the definite nature of electro-chemical action (783. &c.), and that chemical affinity and electricity are forms of the same power (917. &c.).

962. The direct reference of the effects produced by the voltaic pile at the place of experimental decomposition to the chemical affinities active at the place of excitation (891. 917.), gives a very simple and natural view of the cause why the bodies (or _ions_) evolved pass in certain directions; for it is only when they pass in those directions that their forces can consist with and compensate (in direction at least) the superior forces which are dominant at the place where the action of the whole is determined. If, for instance, in a voltaic circuit, the activity of which is determined, by the attraction of zinc for the oxygen of water, the zinc move from right to left, then any other _cation_ included in the circuit, being part of an electrolyte, or forming part of it at the moment, will also move from right to left: and as the oxygen of the water, by its natural affinity for the zinc, moves from left to right, so any other body of the same class with it (i.e. any other _anion_), under its government for the time, will move from left to right.

963. This I may illustrate by reference to fig. 83, the double circle of which may represent a complete voltaic circuit, the direction of its forces being determined by supposing for a moment the zinc _b_ and the platina _c_ as representing plates of those metals acting upon water, _d, e_, and other substances, but having their energy exalted so as to effect several decompositions by the use of a battery at _a_ (989.). This supposition may be allowed, because the action in the battery will only consist of repetitions of what would take place between _b_ and _c_, if they really constituted but a single pair. The zinc _b_, and the oxygen _d_, by their mutual affinity, tend to unite; but as the oxygen is already in association with the hydrogen _e_, and has its inherent chemical or electric powers neutralized for the time by those of the latter, the hydrogen _e_ must leave the oxygen _d_, and advance in the direction of the arrow head, or else the zinc _b_ cannot move in the same direction to unite to the oxygen _d_, nor the oxygen _d_ move in the contrary direction to unite to the zinc _b_, the relation of the _similar_ forces of _b_ and _c_, in contrary directions, to the _opposite_ forces of _d_ being the preventive. As the hydrogen _e_ advances, it, on coming against the platina _c, f_, which forms a part of the circuit, communicates its electric or chemical forces through it to the next electrolyte in the circuit, fused chloride of lead, _g, h_, where the chlorine must move in conformity with the direction of the oxygen at _d_, for it has to compensate the forces disturbed in its part of the circuit by the superior influence of those between the oxygen and zinc at _d, b_, aided as they are by those of the battery _a_; and for a similar reason the lead must move in the direction pointed out by the arrow head, that it may be in right relation to the first moving body of its own class, namely, the zinc _b_. If copper intervene in the circuit from _i_ to _k_, it acts as the platina did before; and if another electrolyte, as the iodide of tin, occur at _l, m_, then the iodine _l_, being an _anion_, must move in conformity with the exciting _anion_, namely, the oxygen _d_, and the _cation_ tin _m_ move in correspondence with the other _cations b, e_, and _h_, that the chemical forces may be in equilibrium as to their direction and quantity throughout the circuit. Should it so happen that the anions in their circulation can combine with the metals at the _anodes_ of the respective electrolytes, as would be the case at the platina _f_ and the copper _k_, then those bodies becoming parts of electrolytes, under the influence of the current, immediately travel; but considering their relation to the zinc _b_, it is evidently impossible that they can travel in any other direction than what will accord with its course, and therefore can never tend to pass otherwise than _from_ the anode and _to_ the cathode.

964. In such a circle as that delineated, therefore, all the known _anions_ may be grouped within, and all the _cations_ without. If any number of them enter as _ions_ into the constitution of _electrolytes_, and, forming one circuit, are simultaneously subject to one common current, the anions must move in accordance with each other in one direction, and the cations in the other. Nay, more than that, equivalent portions of these bodies must so advance in opposite directions: for the advance of every 32.5 parts of the zinc _b_ must be accompanied by a motion in the opposite direction of 8 parts of oxygen at _d_, of 36 parts of chlorine at _g_, of 126 parts of iodine at _l_; and in the same direction by electro-chemical equivalents of hydrogen, lead, copper and tin, at _e, h, k_. and _m_.

965. If the present paper be accepted as a correct expression of facts, it will still only prove a confirmation of certain general views put forth by Sir Humphry Davy in his Bakerian Lecture for 1806[A], and revised and re-stated by him in another Bakerian Lecture, on electrical and chemical changes, for the year 1826[B]. His general statement is, that "_chemical and electrical attractions were produced by the same cause, acting in one case on particles, in the other on masses, of matter; and that the same property, under different modifications, was the cause of all the phenomena exhibited by different voltaic combinations_[C]." This statement I believe to be true; but in admitting and supporting it, I must guard myself from being supposed to assent to all that is associated with it in the two papers referred to, or as admitting the experiments which are there quoted as decided proofs of the truth of the principle. Had I thought them so, there would have been no occasion for this investigation. It may be supposed by some that I ought to go through these papers, distinguishing what I admit from what I reject, and giving good experimental or philosophical reasons for the judgment in both cases. But then I should be equally bound to review, for the same purpose, all that has been written both for and against the necessity of metallic contact,--for and against the origin of voltaic electricity in chemical action,--a duty which I may not undertake in the present paper[D].

[A] Philosophical Transactions, 1807.

[B] Ibid. 1826, p. 383.

[C] Ibid. 1826, p. 389.

[D] I at one time intended to introduce here, in the form of a note, a table of reference to the papers of the different philosophers who have referred the origin of the electricity in the voltaic pile to contact, or to chemical action, or to both; but on the publication of the first volume of M. Becquerel's highly important and valuable Traité de l'Electricité et du Magnétisme, I thought it far better to refer to that work for these references, and the views held by the authors quoted. See pages 86, 91, 104, 110, 112, 117, 118, 120, 151, 152, 224, 227, 228, 232, 233, 252, 255, 257, 258, 290, &c.--July 3rd, 1834.

¶ ii. _On the Intensity necessary for Electrolyzation._

966. It became requisite, for the comprehension of many of the conditions attending voltaic action, to determine positively, if possible, whether electrolytes could resist the action of an electric current when beneath a certain intensity? whether the intensity at which the current ceased to act would be the same for all bodies? and also whether the electrolytes thus resisting decomposition would conduct the electric current as a metal does, after they ceased to conduct as electrolytes, or would act as perfect insulators?

967. It was evident from the experiments described (904. 906.) that different bodies were decomposed with very different facilities, and apparently that they required for their decomposition currents of different intensities, resisting some, but giving way to others. But it was needful, by very careful and express experiments, to determine whether a current could really pass through, and yet not decompose an electrolyte (910.).

968. An arrangement (fig. 84.) was made, in which two glass vessels contained the same dilute sulphuric acid, sp. gr. 1.25. The plate _z_ was amalgamated zinc, in connexion, by a platina wire _a_, with the platina plate _e_; _b_ was a platina wire connecting the two platina plates PP'; _c_ was a platina wire connected with the platina plate P". On the plate _e_ was placed a piece of paper moistened in solution of iodide of potassium: the wire _c_ was so curved that its end could be made to rest at pleasure on this paper, and show, by the evolution of iodine there, whether a current was passing; or, being placed in the dotted position, it formed a direct communication with the platina plate _e_, and the electricity could pass without causing decomposition. The object was to produce a current by the action of the acid on the amalgamated zinc in the first vessel A; to pass it through the acid in the second vessel B by platina electrodes, that its power of decomposing water might, if existing, be observed; and to verify the existence of the current at pleasure, by decomposition at _e_, without involving the continual obstruction to the current which would arise from making the decomposition there constant. The experiment, being arranged, was examined and the existence of a current ascertained by the decomposition at _e_; the whole was then left with the end of the wire _c_ resting on the plate _e_, so as to form a constant metallic communication there.

969. After several hours, the end of the wire _c_ was replaced on the test-paper at _e_: decomposition occurred, and _the proof_ of a passing current was therefore complete. The current was very feeble compared to what it had been at the beginning of the experiment, because of a peculiar state acquired by the metal surfaces in the second vessel, which caused them to oppose the passing current by a force which they possess under these circumstances (1040.). Still it was proved, by the decomposition, that this state of the plates in the second vessel was not able entirely to stop the current determined in the first, and that was all that was needful to be ascertained in the present inquiry.

970. This apparatus was examined from time to time, and an electric current always found circulating through it, until twelve days had elapsed, during which the water in the second vessel had been constantly subject to its action. Notwithstanding this lengthened period, not the slightest appearance of a bubble upon either of the plates in that vessel occurred. From the results of the experiment, I conclude that a current _had_ passed, but of so low an intensity as to fall beneath that degree at which the elements of water, unaided by any secondary force resulting from the capability of combination with the matter of the electrodes, or of the liquid surrounding them, separated from each other.

971. It may be supposed, that the oxygen and hydrogen had been evolved in such small quantities as to have entirely dissolved in the water, and finally to have escaped at the surface, or to have reunited into water. That the hydrogen can be so dissolved was shown in the first vessel; for after several days minute bubbles of gas gradually appeared upon a glass rod, inserted to retain the zinc and platina apart, and also upon the platina plate itself, and these were hydrogen. They resulted principally in this way:--notwithstanding the amalgamation of the zinc, the acid exerted a little direct action upon it, so that a small stream of hydrogen bubbles was continually rising from its surface; a little of this hydrogen gradually dissolved in the dilute acid, and was in part set free against the surfaces of the rod and the plate, according to the well-known action of such solid bodies in solutions of gases (623. &c.).

972. But if the gases had been evolved in the second vessel by the decomposition of water, and had tended to dissolve, still there would have been every reason to expect that a few bubbles should have appeared on the electrodes, especially on the negative one, if it were only because of its action as a nucleus on the solution supposed to be formed; but none appeared even after twelve days.

973. When a few drops only of nitric acid were added to the vessel A, fig. 84, then the results were altogether different. In less than five minutes bubbles of gas appeared on the plates P' and P" in the second vessel. To prove that this was the effect of the electric current (which by trial at _c_ was found at the same time to be passing,) the connexion at _c_ was broken, the plates P'P" cleared from bubbles and left in the acid of the vessel B, for fifteen minutes: during that time no bubbles appeared upon them; but on restoring the communication at _c_, a minute did not elapse before gas appeared in bubbles upon the plates. The proof, therefore, is most full and complete, that the current excited by dilute sulphuric acid with a little nitric acid in vessel A, has intensity enough to overcome the chemical affinity exerted between the oxygen and hydrogen of the water in the vessel B, whilst that excited by dilute sulphuric acid alone has _not_ sufficient intensity.

974. On using a strong solution of caustic potassa in the vessel A, to excite the current, it was found by the decomposing effects at _e_, that the current passed. But it had not intensity enough to decompose the water in the vessel B; for though left for fourteen days, during the whole of which time the current was found to be passing, still not the slightest appearance of gas appeared on the plates P'P", nor any other signs of the water having suffered decomposition.

975. Sulphate of soda in solution was then experimented with, for the purpose of ascertaining with respect to it, whether a certain electrolytic intensity was also required for its decomposition in this state, in analogy with the result established with regard to water (974). The apparatus was arranged as in fig. 85; P and Z are the platina and zinc plates dipping into a solution of common salt; _a_ and _b_ are platina plates connected by wires of platina (except in the galvanometer _g_) with P and Z; _c_ is a connecting wire of platina, the ends of which can be made to rest either on the plates _a, b_, or on the papers moistened in solutions which are placed upon them; so that the passage of the current without decomposition, or with one or two decompositions, was under ready command, as far as arrangement was concerned. In order to change the _anodes_ and _cathodes_ at the places of decomposition, the form of apparatus fig. 86, was occasionally adopted. Here only one platina plate, _c_, was used; both pieces of paper on which decomposition was to be effected were placed upon it, the wires from P and Z resting upon these pieces of paper, or upon the plate _c_, according as the current with or without decomposition of the solutions was required.

976. On placing solution of iodide of potassium in paper at one of the decomposing localities, and solution of sulphate of soda at the other, so that the electric current should pass through both at once, the solution of iodide was slowly decomposed, yielding iodine at the _anode_ and alkali at the _cathode_; but the solution of sulphate of soda exhibited no signs of decomposition, neither acid nor alkali being evolved from it. On placing the wires so that the iodide alone was subject to the action of the current (900.), it was quickly and powerfully decomposed; but on arranging them so that the sulphate of soda alone was subject to action, it still refused to yield up its elements. Finally, the apparatus was so arranged under a wet bell-glass, that it could be left for twelve hours, the current passing during the whole time through a solution of sulphate of soda, retained in its place by only two thicknesses of bibulous litmus and turmeric paper. At the end of that time it was ascertained by the decomposition of iodide of potassium at the second place of action, that the current was passing and had passed for the twelve hours, and yet no trace of acid or alkali from the sulphate of soda appeared.

977. From these experiments it may, I think, be concluded, that a solution of sulphate of soda can conduct a current of electricity, which is unable to decompose the neutral salt present; that this salt in the state of solution, like water, requires a certain electrolytic intensity for its decomposition; and that the necessary intensity is much higher for this substance than for the iodide of potassium in a similar state of solution.

978. I then experimented on bodies rendered decomposable by fusion, and first on _chloride of lead_. The current was excited by dilute sulphuric acid without any nitric acid between zinc and platina plates, fig. 87, and was then made to traverse a little chloride of lead fused upon glass at _a_, a paper moistened in solution of iodide of potassium at _b_, and a galvanometer at _g_. The metallic terminations at _a_ and _b_ were of platina. Being thus arranged, the decomposition at _b_ and the deflection at _g_ showed that an electric current was passing, but there was no appearance of decomposition at _a_, not even after a _metallic_ communication at _b_ was established. The experiment was repeated several times, and I am led to conclude that in this case the current has not intensity sufficient to cause the decomposition of the chloride of lead; and further, that, like water (974.), fused chloride of lead can conduct an electric current having an intensity below that required to effect decomposition.

979. _Chloride of silver_ was then placed at _a_, fig. 87, instead of chloride of lead. There was a very ready decomposition of the solution of iodide of potassium at _b_, and when metallic contact was made there, very considerable deflection of the galvanometer needle at _g_. Platina also appeared to be dissolved at the anode of the fused chloride at _a_, and there was every appearance of a decomposition having been effected there.