Part 20 (1/2)

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.