Experimental Researches in Electricity Part 20

660. I intended to have followed this section by one on the secondary piles of Ritter, and the peculiar properties of the poles of the pile, or of metals through which electricity has pa.s.sed, which have been observed by Ritter, Van Marum, Yelin, De la Rive, Marianini, Berzelius, and others. It appears to me that all these phenomena bear a satisfactory explanation on known principles, connected with the investigation just terminated, and do not require the a.s.sumption of any new state or new property. But as the experiments advanced, especially those of Marianini, require very careful repet.i.tion and examination, the necessity of pursuing the subject of electro-chemical decomposition obliges me for a time to defer the researches to which I have just referred.

_Royal Inst.i.tution, November 30, 1833._

SEVENTH SERIES.

-- 11. _On Electro-chemical Decomposition, continued._[A] -- iv. _On some general conditions of Electro-decomposition._ -- v. _On a new Measurer of Volta-electricity._ -- vi. _On the primary or secondary character of bodies evolved in Electro-decomposition._ -- vii. _On the definite nature and extent of Electro-chemical Decompositions._ -- 13. _On the absolute quant.i.ty of Electricity a.s.sociated with the particles or atoms of Matter._

[A] Refer to the note after 1047, Series VIII.--_Dec. 1838._

Received January 9,--Read January 23, February 6 and 13, 1834.

_Preliminary._

661. The theory which I believe to be a true expression of the facts of electro-chemical decomposition, and which I have therefore detailed in a former series of these Researches, is so much at variance with those previously advanced, that I find the greatest difficulty in stating results, as I think, correctly, whilst limited to the use of terms which are current with a certain accepted meaning. Of this kind is the term _pole_, with its prefixes of positive and negative, and the attached ideas of attraction and repulsion. The general phraseology is that the positive pole _attracts_ oxygen, acids, &c., or more cautiously, that it _determines_ their evolution upon its surface; and that the negative pole acts in an equal manner upon hydrogen, combustibles, metals, and bases.

According to my view, the determining force is _not_ at the poles, but _within_ the body under decomposition; and the oxygen and acids are rendered at the _negative_ extremity of that body, whilst hydrogen, metals, &c., are evolved at the _positive_ extremity (518. 524.).

662. To avoid, therefore, confusion and circ.u.mlocution, and for the sake of greater precision of expression than I can otherwise obtain, I have deliberately considered the subject with two friends, and with their a.s.sistance and concurrence in framing them, I purpose henceforward using certain other terms, which I will now define. The _poles_, as they are usually called, are only the doors or ways by which the electric current pa.s.ses into and out of the decomposing body (556.); and they of course, when in contact with that body, are the limits of its extent in the direction of the current. The term has been generally applied to the metal surfaces in contact with the decomposing substance; but whether philosophers generally would also apply it to the surfaces of air (465.

471.) and water (493.), against which I have effected electro-chemical decomposition, is subject to doubt. In place of the term pole, I propose using that of _Electrode_[A], and I mean thereby that substance, or rather surface, whether of air, water, metal, or any other body, which bounds the extent of the decomposing matter in the direction of the electric current.

[A] [Greek: elektron], and [Greek: -odos] _a way_.

663. The surfaces at which, according to common phraseology, the electric current enters and leaves a decomposing body, are most important places of action, and require to be distinguished apart from the poles, with which they are mostly, and the electrodes, with which they are always, in contact. Wis.h.i.+ng for a natural standard of electric direction to which I might refer these, expressive of their difference and at the same time free from all theory, I have thought it might be found in the earth. If the magnetism of the earth be due to electric currents pa.s.sing round it, the latter must be in a constant direction, which, according to present usage of speech, would be from east to west, or, which will strengthen this help to the memory, that in which the sun appears to move. If in any case of electro-decomposition we consider the decomposing body as placed so that the current pa.s.sing through it shall be in the same direction, and parallel to that supposed to exist in the earth, then the surfaces at which the electricity is pa.s.sing into and out of the substance would have an invariable reference, and exhibit constantly the same relations of powers.

Upon this notion we purpose calling that towards the east the _anode_[A], and that towards the west the _cathode_[B]; and whatever changes may take place in our views of the nature of electricity and electrical action, as they must affect the _natural standard_ referred to, in the same direction, and to an equal amount with any decomposing substances to which these terms may at any time be applied, there seems no reason to expect that they will lead to confusion, or tend in any way to support false views. The _anode_ is therefore that surface at which the electric current, according to our present expression, enters: it is the _negative_ extremity of the decomposing body; is where oxygen, chlorine, acids, &c., are evolved; and is against or opposite the positive electrode. The _cathode_ is that surface at which the current leaves the decomposing body, and is its _positive_ extremity; the combustible bodies, metals, alkalies, and bases, are evolved there, and it is in contact with the negative electrode.

[A] [Greek: ano] _upwards_, and [Greek: -odos] _a way_; the way which the sun rises.

[B] [Greek: kata] _downwards_, and [Greek: -odos] _a way_; the way which the sun sets.

664. I shall have occasion in these Researches, also, to cla.s.s bodies together according to certain relations derived from their electrical actions (822.); and wis.h.i.+ng to express those relations without at the same time involving the expression of any hypothetical views, I intend using the following names and terms. Many bodies are decomposed directly by the electric current, their elements being set free; these I propose to call _electrolytes_.[A] Water, therefore, is an electrolyte. The bodies which, like nitric or sulphuric acids, are decomposed in a secondary manner (752.

757.), are not included under this term. Then for _electro-chemically decomposed_, I shall often use the term _electrolyzed_, derived in the same way, and implying that the body spoken of is separated into its components under the influence of electricity: it is a.n.a.logous in its sense and sound to _a.n.a.lyse_, which is derived in a similar manner. The term _electrolytical_ will be understood at once: muriatic acid is electrolytical, boracic acid is not.

[A] [Greek: elektron], and [Greek: lyo], _soluo_. N. Electrolyte, V.

Electrolyze.

665. Finally, I require a term to express those bodies which can pa.s.s to the _electrodes_, or, as they are usually called, the poles. Substances are frequently spoken of as being _electro-negative_, or _electro-positive_, according as they go under the supposed influence of a direct attraction to the positive or negative pole. But these terms are much too significant for the use to which I should have to put them; for though the meanings are perhaps right, they are only hypothetical, and may be wrong; and then, through a very imperceptible, but still very dangerous, because continual, influence, they do great injury to science, by contracting and limiting the habitual views of those engaged in pursuing it. I propose to distinguish such bodies by calling those _anions_[A] which go to the _anode_ of the decomposing body; and those pa.s.sing to the _cathode, cations_[B]; and when I have occasion to speak of these together, I shall call them _ions_. Thus the chloride of lead is an _electrolyte_, and when _electrolyzed_ evolves the two _ions_, chlorine and lead, the former being an _anion_, and the latter a _cation_.

[A] [Greek: anion] _that which goes up._ (Neuter participle.)

[B] [Greek: kation] _that which goes down._

666. These terms being once well-defined, will, I hope, in their use enable me to avoid much periphrasis and ambiguity of expression. I do not mean to press them into service more frequently than will be required, for I am fully aware that names are one thing and science another.

667. It will be well understood that I am giving no opinion respecting the nature of the electric current now, beyond what I have done on former occasions (283. 517.); and that though I speak of the current as proceeding from the parts which are positive to those which are negative (663.), it is merely in accordance with the conventional, though in some degree tacit, agreement entered into by scientific men, that they may have a constant, certain, and definite means of referring to the direction of the forces of that current.

[Since this paper was read, I have changed some of the terms which were first proposed, that I might employ only such as were at the same time simple in their nature, clear in their reference, and free from hypothesis.

-- iv. _On some general conditions of Electro-chemical Decomposition._

669. From the period when electro-chemical decomposition was first effected to the present time, it has been a remark, that those elements which, in the ordinary phenomena of chemical affinity, were the most directly opposed to each other, and combined with the greatest attractive force, were those which were the most readily evolved at the opposite extremities of the decomposing bodies (549.).

670. If this result was evident when water was supposed to be essential to, and was present in, almost every case of such decomposition (472.), it is far more evident now that it has been shown and proved that water is not necessarily concerned in the phenomena (474.), and that other bodies much surpa.s.s it in some of the effects supposed to be peculiar to that substance.

671. Water, from its const.i.tution and the nature of its elements, and from its frequent presence in cases of electrolytic action, has. .h.i.therto stood foremost in this respect. Though a compound formed by very powerful affinity, it yields up its elements under the influence of a very feeble electric current; and it is doubtful whether a case of electrolyzation can occur, where, being present, it is not resolved into its first principles.

672. The various oxides, chlorides, iodides, and salts, which I have shown are decomposable by the electric current when in the liquid state, under the same general law with water (402.), ill.u.s.trate in an equally striking manner the activity, in such decompositions, of elements directly and powerfully opposed to each other by their chemical relations.

673. On the other hand, bodies dependent on weak affinities very rarely give way. Take, for instance, gla.s.ses: many of those formed of silica, lime, alkali, and oxide of lead, may be considered as little more than solutions of substances one in another[A]. If bottle-gla.s.s be fused, and subjected to the voltaic pile, it does not appear to be at all decomposed (408.). If flint gla.s.s, which contains substances more directly opposed, be operated upon, it suffers some decomposition; and if borate of lead gla.s.s, which is a definite chemical compound, be experimented with, it readily yields up its elements (408.).

[A] Philosophical Transactions, 1830, p. 49.

674. But the result which is found to be so striking in the instances quoted is not at all borne out by reference to other cases where a similar consequence might have been expected. It may be said, that my own theory of electro-chemical decomposition would lead to the expectation that all compound bodies should give way under the influence of the electric current with a facility proportionate to the strength of the affinity by which their elements, either proximate or ultimate, are combined. I am not sure that that follows as a consequence of the theory; but if the objection is supposed to be one presented by the facts, I have no doubt it will be removed when we obtain a more intimate acquaintance with, and precise idea of, the nature of chemical affinity and the mode of action of an electric current over it (518. 524.): besides which, it is just as directly opposed to any other theory of electro-chemical decomposition as the one I have propounded; for if it be admitted, as is generally the case, that the more directly bodies are opposed to each other in their attractive forces, the more powerfully do they combine, then the objection applies with equal force to any of the theories of electrolyzation which have been considered, and is an addition to those which I have taken against them.

675. Amongst powerful compounds which are not decomposed, boracic acids stand prominent (408.). Then again, the iodide of sulphur, and the chlorides of sulphur, phosphorus, and carbon, are not decomposable under common circ.u.mstances, though their elements are of a nature which would lead to a contrary expectation. Chloride of antimony (402. 690.), the hydro-carbons, acetic acid, ammonia, and many other bodies undecomposable by the voltaic pile, would seem to be formed by an affinity sufficiently strong to indicate that the elements were so far contrasted in their nature as to sanction the expectation that, the pile would separate them, especially as in some cases of mere solution (530. 544.), where the affinity must by comparison be very weak, separation takes place[A].

[A] With regard to solution, I have met with some reasons for supposing that it will probably disappear as a cause of transference, and intend resuming the consideration at a convenient opportunity.

676. It must not be forgotten, however, that much of this difficulty, and perhaps the whole, may depend upon the absence of conducting power, which, preventing the transmission of the current, prevents of course the effects due to it. All known compounds being non-conductors when solid, but conductors when liquid, are decomposed, with _perhaps_ the single exception at present known of periodide of mercury (679. 691.)[A]; and even water itself, which so easily yields up its elements when the current pa.s.ses, if rendered quite pure, scarcely suffers change, because it then becomes a very bad conductor.

[A] See now, 1340, 1341.--_Dec. 1838._

677. If it should hereafter be proved that the want of decomposition in those cases where, from chemical considerations, it might be so strongly expected (669, 672. 674.), is due to the absence or deficiency of conducting power, it would also at the same time be proved that decomposition _depends_ upon conduction, and not the latter upon the former (413.); and in water this seems to be very nearly decided. On the other hand, the conclusion is almost irresistible, that in electrolytes the power of transmitting the electricity across the substance is _dependent_ upon their capability of suffering decomposition; taking place only whilst they are decomposing, and being proportionate to the quant.i.ty of elements separated (821.). I may not, however, stop to discuss this point experimentally at present.

678. When a compound contains such elements as are known to pa.s.s towards the opposite extremities of the voltaic pile, still the proportions in which they are present appear to be intimately connected with capability in the compound of suffering or resisting decomposition. Thus, the protochloride of tin readily conducts, and is decomposed (402.), but the perchloride neither conducts nor is decomposed (406.). The protiodide of tin is decomposed when fluid (402.); the periodide is not (405.). The periodide of mercury when fused is not decomposed (691.), even though it does conduct. I was unable to contrast it with the protiodide, the latter being converted into mercury and periodide by heat.

679. These important differences induced me to look more closely to certain binary compounds, with a view of ascertaining whether a _law_ regulating the _decomposability_ according to some _relation of the proportionals or equivalents_ of the elements, could be discovered. The proto compounds only, amongst those just referred to, were decomposable; and on referring to the substances quoted to ill.u.s.trate the force and generality of the law of conduction and decomposition which I discovered (402.), it will be found that all the oxides, chlorides, and iodides subject to it, except the chloride of antimony and the periodide of mercury, (to which may now perhaps be added corrosive sublimate,) are also decomposable, whilst many per compounds of the same elements, not subject to the law, were not so (405. 406.).

680. The substances which appeared to form the strongest exceptions to this general result were such bodies as the sulphuric, phosphoric, nitric, a.r.s.enic, and other acids.

681. On experimenting with sulphuric acid, I found no reason to believe that it was by itself a conductor of, or decomposable by, electricity, although I had previously been of that opinion (552.). When very strong it is a much worse conductor than if diluted[A]. If then subjected to the action of a powerful battery, oxygen appears at the _anode_, or positive electrode, although much is absorbed (728.), and hydrogen and sulphur appear at the _cathode_, or negative electrode. Now the hydrogen has with me always been pure, not sulphuretted, and has been deficient in proportion to the sulphur present, so that it is evident that when decomposition occurred water must have been decomposed. I endeavoured to make the experiment with anhydrous sulphuric acid; and it appeared to me that, when fused, such acid was not a conductor, nor decomposed; but I had not enough of the dry acid in my possession to allow me to decide the point satisfactorily. My belief is, that when sulphur appears during the action of the pile on sulphuric acid, it is the result of a secondary action, and that the acid itself is not electrolyzable (757.).

[A] De la Rive.

682. Phosphoric acid is, I believe, also in the same condition; but I have found it impossible to decide the point, because of the difficulty of operating on fused anhydrous phosphoric acid. Phosphoric acid which has once obtained water cannot be deprived of it by heat alone. When heated, the hydrated acid volatilizes. Upon subjecting phosphoric acid, fused upon the ring end of a wire (401.), to the action of the voltaic apparatus, it conducted, and was decomposed; but gas, which I believe to be hydrogen, was always evolved at the negative electrode, and the wire was not affected as would have happened had phosphorus been separated. Gas was also evolved at the positive electrode. From all the facts, I conclude it was the water and not the acid which was decomposed.

683. _a.r.s.enic acid_. This substance conducted, and was decomposed; but it contained water, and I was unable at the time to press the investigation so as to ascertain whether a fusible anhydrous a.r.s.enic acid could be obtained.

It forms, therefore, at present no exception to the general result.

684. Nitrous acid, obtained by distilling nitrate of lead, and keeping it in contact with strong sulphuric acid, was found to conduct and decompose slowly. But on examination there were strong reasons for believing that water was present, and that the decomposition and conduction depended upon it. I endeavoured to prepare a perfectly anhydrous portion, but could not spare the time required to procure an unexceptionable result.

685. Nitric acid is a substance which I believe is not decomposed directly by the electric current. As I want the facts in ill.u.s.tration of the distinction existing between primary and secondary decomposition, I will merely refer to them in this place (752.).

686. That these mineral acids should confer facility of conduction and decomposition on water, is no proof that they are competent to favour and suffer these actions in themselves. Boracic acid does the same thing, though not decomposable. M. de la Rive has pointed out that chlorine has this power also; but being to us an elementary substance, it cannot be due to its capability of suffering decomposition.