§ 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 quantity
of Electricity associated 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 circumlocution, 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
assistance 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
passes 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. Wishing 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 passing 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 passing 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 passing 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 class bodies
together according to certain relations derived from their electrical
actions (822.); and wishing 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 analogous in its sense and sound
to _analyse_, 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 pass 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 passing 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: aniôn] _that which goes up._ (Neuter participle.)
[B] [Greek: katiôn] _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
surpass it in some of the effects supposed to be peculiar to that
substance.
671. Water, from its constitution and the nature of its elements, and from
its frequent presence in cases of electrolytic action, has hitherto 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.), illustrate 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, glasses: 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-glass be fused, and
subjected to the voltaic pile, it does not appear to be at all decomposed
(408.). If flint glass, which contains substances more directly opposed, be
operated upon, it suffers some decomposition; and if borate of lead glass,
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 circumstances, 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 passes, 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 quantity 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 pass 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 illustrate 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,
arsenic, 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. _Arsenic 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 arsenic 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 illustration 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.
687. _Chloride of sulphur_ does not conduct, nor is it decomposed. It
consists of single proportionals of its elements, but is not on that
account an exception to the rule (679.), which does not affirm that _all_
compounds of single proportionals of elements are decomposable, but that
such as are decomposable are so constituted.