711. A fifth form is delineated (fig. 66.). This I have found exceedingly
useful in experiments continued in succession for days together, and where
large quantities of indicating gas were to be collected. It is fixed on a
weighted foot, and has the form of a small retort containing the two
electrodes: the neck is narrow, and sufficiently long to deliver gas
issuing from it into a jar placed in a small pneumatic trough. The
electrode chamber, sealed hermetically at the part held in the stand, is
five inches in length, and 0.6 of an inch in diameter; the neck about nine
inches in length, and 0.4 of an inch in diameter internally. The figure
will fully indicate the construction.
712. It can hardly be requisite to remark, that in the arrangement of any
of these forms of apparatus, they, and the wires connecting them with the
substance, which is collaterally subjected to the action of the same
electric current, should be so far insulated as to ensure a certainty that
all the electricity which passes through the one shall also be transmitted
through the other.
* * * * *
713. Next to the precaution of collecting the gases, if mingled, out of
contact with the platinum, was the necessity of testing the law of a
_definite electrolytic_ action, upon water at least, under all varieties of
condition; that, with a conviction of its certainty, might also be obtained
a knowledge of those interfering circumstances which would require to be
practically guarded against.
714. The first point investigated was the influence or indifference of
extensive variations in the size of the electrodes, for which purpose
instruments like those last described (709. 710. 711.) were used. One of
these had plates 0.7 of an inch wide, and nearly four inches long; another
had plates only 0.5 of an inch wide, and 0.8 of an inch long; a third had
wires 0.02 of an inch in diameter, and three inches long; and a fourth,
similar wires only half an inch in length. Yet when these were filled with
dilute sulphuric acid, and, being placed in succession, had one common
current of electricity passed through them, very nearly the same quantity
of gas was evolved in all. The difference was sometimes in favour of one
and sometimes on the side of another; but the general result was that the
largest quantity of gases was evolved at the smallest electrodes, namely,
those consisting merely of platina wires.
715. Experiments of a similar kind were made with the single-plate,
straight tubes (707.), and also with the curved tubes (708.), with similar
consequences; and when these, with the former tubes, were arranged together
in various ways, the result, as to the equality of action of large and
small metallic surfaces when delivering and receiving the same current of
electricity, was constantly the same. As an illustration, the following
numbers are given. An instrument with two wires evolved 74.3 volumes of
mixed gases; another with plates 73.25 volumes; whilst the sum of the
oxygen and hydrogen in two separate tubes amounted to 73.65 volumes. In
another experiment the volumes were 55.3, 55.3, and 54.4.
716. But it was observed in these experiments, that in single-plate tubes
(707.) more hydrogen was evolved at the negative electrode than was
proportionate to the oxygen at the positive electrode; and generally, also,
more than was proportionate to the oxygen and hydrogen in a double-plate
tube. Upon more minutely examining these effects, I was led to refer them,
and also the differences between wires and plates (714.), to the solubility
of the gases evolved, especially at the positive electrode.
717. When the positive and negative electrodes are equal in surface, the
bubbles which rise from them in dilute sulphuric acid are always different
in character. Those from the positive plate are exceedingly small, and
separate instantly from every part of the surface of the metal, in
consequence of its perfect cleanliness (633.); whilst in the liquid they
give it a hazy appearance, from their number and minuteness; are easily
carried down by currents, and therefore not only present far greater
surface of contact with the liquid than larger bubbles would do, but are
retained a much longer time in mixture with it. But the bubbles at the
negative surface, though they constitute twice the volume of the gas at the
positive electrode, are nevertheless very inferior in number. They do not
rise so universally from every part of the surface, but seem to be evolved
at different parts; and though so much larger, they appear to cling to the
metal, separating with difficulty from it, and when separated, instantly
rising to the top of the liquid. If, therefore, oxygen and hydrogen had
equal solubility in, or powers of combining with, water under similar
circumstances, still under the present conditions the oxygen would be far
the most liable to solution; but when to these is added its well-known
power of forming a compound with water, it is no longer surprising that
such a compound should be produced in small quantities at the positive
electrode; and indeed the blenching power which some philosophers have
observed in a solution at this electrode, when chlorine and similar bodies
have been carefully excluded, is probably due to the formation there, in
this manner, of oxywater.
718. That more gas was collected from the wires than from the plates, I
attribute to the circumstance, that as equal quantities were evolved in
equal times, the bubbles at the wires having been more rapidly produced, in
relation to any part of the surface, must have been much larger; have been
therefore in contact with the fluid by a much smaller surface, and for a
much shorter time than those at the plates; hence less solution and a
greater amount collected.
719. There was also another effect produced, especially by the use of large
electrodes, which was both a consequence and a proof of the solution of
part of the gas evolved there. The collected gas, when examined, was found
to contain small portions of nitrogen. This I attribute to the presence of
air dissolved in the acid used for decomposition. It is a well-known fact,
that when bubbles of a gas but slightly soluble in water or solutions pass
through them, the portion of this gas which is dissolved displaces a
portion of that previously in union with the liquid: and so, in the
decompositions under consideration, as the oxygen dissolves, it displaces a
part of the air, or at least of the nitrogen, previously united to the
acid; and this effect takes place _most extensively_ with large plates,
because the gas evolved at them is in the most favourable condition for
solution,
720. With the intention of avoiding this solubility of the gases as much as
possible, I arranged the decomposing plates in a vertical position (707.
708.), that the bubbles might quickly escape upwards, and that the downward
currents in the fluid should not meet ascending currents of gas. This
precaution I found to assist greatly in producing constant results, and
especially in experiments to be hereafter referred to, in which other
liquids than dilute sulphuric acid, as for instance solution of potash,
were used.
721. The irregularities in the indications of the measurer proposed,
arising from the solubility just referred to, are but small, and may be
very nearly corrected by comparing the results of two or three experiments.
They may also be almost entirely avoided by selecting that solution which
is found to favour them in the least degree (728.); and still further by
collecting the hydrogen only, and using that as the indicating gas; for
being much less soluble than oxygen, being evolved with twice the rapidity
and in larger bubbles (717.), it can be collected more perfectly and in
greater purity.
722. From the foregoing and many other experiments, it results that
_variation in the size of the electrodes causes no variation in the
chemical action of a given quantity of electricity upon water_.
723. The next point in regard to which the principle of constant
electro-chemical action was tested, was _variation of intensity_. In the
first place, the preceding experiments were repeated, using batteries of an
_equal_ number of plates, _strongly_ and _weakly_ charged; but the results
were alike. They were then repeated, using batteries sometimes containing
forty, and at other times only five pairs of plates; but the results were
still the same. _Variations therefore in the intensity_, caused by
difference in the strength of charge, or in the number of alternations
used, _produced no difference as to the equal action of large and small
electrodes_.
724. Still these results did not prove that variation in the intensity of
the current was not accompanied by a corresponding variation in the
electro-chemical effects, since the actions at _all_ the surfaces might
have increased or diminished together. The deficiency in the evidence is,
however, completely supplied by the former experiments on different-sized
electrodes; for with variation in the size of these, a variation in the
intensity must have occurred. The intensity of an electric current
traversing conductors alike in their nature, quality, and length, is
probably as the quantity of electricity passing through a given sectional
area perpendicular to the current, divided by the time (360. _note_); and
therefore when large plates were contrasted with wires separated by an
equal length of the same decomposing conductor (714.), whilst one current
of electricity passed through both arrangements, that electricity must have
been in a very different state, as to _tension_, between the plates and
between the wires; yet the chemical results were the same.
725. The difference in intensity, under the circumstances described, may be
easily shown practically, by arranging two decomposing apparatus as in fig.
67, where the same fluid is subjected to the decomposing power of the same
current of electricity, passing in the vessel A. between large platina
plates, and in the vessel B. between small wires. If a third decomposing
apparatus, such as that delineated fig. 66. (711.), be connected with the
wires at _ab_, fig. 67, it will serve sufficiently well, by the degree of
decomposition occurring in it, to indicate the relative state of the two
plates as to intensity; and if it then be applied in the same way, as a
test of the state of the wires at _a'b'_, it will, by the increase of
decomposition within, show how much greater the intensity is there than at
the former points. The connexions of P and N with the voltaic battery are
of course to be continued during the whole time.
726. A third form of experiment, in which difference of intensity was
obtained, for the purpose of testing the principle of equal chemical
action, was to arrange three volta-electrometers, so that after the
electric current had passed through one, it should divide into two parts,
each of which should traverse one of the remaining instruments, and should
then reunite. The sum of the decomposition in the two latter vessels was
always equal to the decomposition in the former vessel. But the _intensity_
of the divided current could not be the same as that it had in its original
state; and therefore _variation of intensity has no influence on the
results if the quantity of electricity remain the same_. The experiment, in
fact, resolves itself simply into an increase in the size of the electrodes
(725.).
727. The _third point_, in respect to which the principle of equal
electro-chemical action on water was tested, was _variation of the strength
of the solution used_. In order to render the water a conductor, sulphuric
acid had been added to it (707.); and it did not seem unlikely that this
substance, with many others, might render the water more subject to
decomposition, the electricity remaining the same in quantity. But such did
not prove to be the case. Diluted sulphuric acid, of different strengths,
was introduced into different decomposing apparatus, and submitted
simultaneously to the action of the same electric current (714.). Slight
differences occurred, as before, sometimes in one direction, sometimes in
another; but the final result was, that _exactly the same quantity of water
was decomposed in all the solutions by the same quantity of electricity_,
though the sulphuric acid in some was seventy-fold what it was in others.
The strengths used were of specific gravity 1.495, and downwards.
728. When an acid having a specific gravity of about 1.336 was employed,
the results were most uniform, and the oxygen and hydrogen (716.) most
constantly in the right proportion to each other. Such an acid gave more
gas than one much weaker acted upon by the same current, apparently because
it had less solvent power. If the acid were very strong, then a remarkable
disappearance of oxygen took place; thus, one made by mixing two measures
of strong oil of vitriol with one of water, gave forty-two volumes of
hydrogen, but only twelve of oxygen. The hydrogen was very nearly the same
with that evolved from acid of the specific gravity 1.232. I have not yet
had time to examine minutely the circumstances attending the disappearance
of the oxygen in this case, but imagine it is due to the formation of
oxywater, which Thenard has shown is favoured by the presence of acid.
729. Although not necessary for the practical use of the instrument I am
describing, yet as connected with the important point of constant
chemical action upon water, I now investigated the effects produced by an
electro-electric current passing through aqueous solutions of acids, salts,
and compounds, exceedingly different from each other in their nature, and
found them to yield astonishingly uniform results. But many of them which
are connected with a secondary action will be more usefully described
hereafter (778.).
730. When solutions of caustic potassa or soda, or sulphate of magnesia, or
sulphate of soda, were acted upon by the electric current, just as much
oxygen and hydrogen was evolved from them as from the diluted sulphuric
acid, with which they were compared. When a solution of ammonia, rendered a
better conductor by sulphate of ammonia (554.), or a solution of
subcarbonate of potassa was experimented with, the _hydrogen_ evolved was
in the same quantity as that set free from the diluted sulphuric acid with
which they were compared. Hence _changes in the nature of the solution do
not alter the constancy of electrolytic action upon water_.
731. I have already said, respecting large and small electrodes, that
change of order caused no change in the general effect (715.). The same was
the case with different solutions, or with different intensities; and
however the circumstances of an experiment might be varied, the results
came forth exceedingly consistent, and proved that the electro-chemical
action was still the same.
732. I consider the foregoing investigation as sufficient to prove the very
extraordinary and important principle with respect to WATER, _that when
subjected to the influence of the electric current, a quantity of it is
decomposed exactly proportionate to the quantity of electricity which has
passed_, notwithstanding the thousand variations in the conditions and
circumstances under which it may at the time be placed; and further, that
when the interference of certain secondary effects (742. &c.), together
with the solution or recombination of the gas and the evolution of air, are
guarded against, _the products of the decomposition may be collected with
such accuracy, as to afford a very excellent and valuable measurer of the
electricity concerned in their evolution_.
733. The forms of instrument which I have given, figg. 64, 65, 66. (709.
710. 711.), are probably those which will be found most useful, as they
indicate the quantity of electricity by the largest volume of gases, and
cause the least obstruction to the passage of the current. The fluid which
my present experience leads me to prefer, is a solution of sulphuric acid
of specific gravity about 1.336, or from that to 1.25; but it is very
essential that there should be no organic substance, nor any vegetable
acid, nor other body, which, by being liable to the action of the oxygen or
hydrogen evolved at the electrodes (773. &c.), shall diminish their
quantity, or add other gases to them.
734. In many cases when the instrument is used as a _comparative standard_,
or even as _a measurer_, it may be desirable to collect the hydrogen only,
as being less liable to absorption or disappearance in other ways than the
oxygen; whilst at the same time its volume is so large, as to render it a
good and sensible indicator. In such cases the first and second form of
apparatus have been used, figg. 62, 63. (707. 708.). The indications
obtained were very constant, the variations being much smaller than in
those forms of apparatus collecting both gases; and they can also be
procured when solutions are used in comparative experiments, which,
yielding no oxygen or only secondary results of its action, can give no
indications if the educts at both electrodes be collected. Such is the case
when solutions of ammonia, muriatic acid, chlorides, iodides, acetates or
other vegetable salts, &c., are employed.
735. In a few cases, as where solutions of metallic salts liable to
reduction at the negative electrode are acted upon, the oxygen may be
advantageously used as the measuring substance. This is the case, for
instance, with sulphate of copper.
736. There are therefore two general forms of the instrument which I submit
as a measurer of electricity; one, in which both the gases of the water
decomposed are collected (709. 710. 711.); and the other, in which a single
gas, as the hydrogen only, is used (707. 708.). When referred to as a
_comparative instrument_, (a use I shall now make of it very extensively,)
it will not often require particular precaution in the observation; but
when used as an _absolute measurer_, it will be needful that the barometric
pressure and the temperature be taken into account, and that the graduation
of the instruments should be to one scale; the hundredths and smaller
divisions of a cubical inch are quite fit for this purpose, and the
hundredth may be very conveniently taken as indicating a DEGREE of
electricity.
737. It can scarcely be needful to point out further than has been done how
this instrument is to be used. It is to be introduced into the course of
the electric current, the action of which is to be exerted anywhere else,
and if 60° or 70° of electricity are to be measured out, either in one or
several portions, the current, whether strong or weak, is to be continued
until the gas in the tube occupies that number of divisions or hundredths
of a cubical inch. Or if a quantity competent to produce a certain effect
is to be measured, the effect is to be obtained, and then the indication
read off. In exact experiments it is necessary to correct the volume of gas
for changes in temperature and pressure, and especially for moisture[A].
For the latter object the volta-electrometer (fig. 66.) is most accurate,
as its gas can be measured over water, whilst the others retain it over
acid or saline solutions.
[A] For a simple table of correction for moisture, I may take the
liberty of referring to my Chemical Manipulation, edition of 1830,
p. 376.
738. I have not hesitated to apply the term _degree_ (736.), in analogy
with the use made of it with respect to another most important imponderable
agent, namely, heat; and as the definite expansion of air, water, mercury,
&c., is there made use of to measure heat, so the equally definite
evolution of gases is here turned to a similar use for electricity.
739. The instrument offers the only _actual measurer_ of voltaic
electricity which we at present possess. For without being at all affected
by variations in time or intensity, or alterations in the current itself,
of any kind, or from any cause, or even of intermissions of action, it
takes note with accuracy of the quantity of electricity which has passed
through it, and reveals that quantity by inspection; I have therefore named
it a VOLTA-ELECTROMETER.
740. Another mode of measuring volta-electricity may be adopted with
advantage in many cases, dependent on the quantities of metals or other
substances evolved either as primary or as secondary results; but I refrain
from enlarging on this use of the products, until the principles on which
their constancy depends have been fully established (791. 848.);
741. By the aid of this instrument I have been able to establish the
definite character of electro-chemical action in its most general sense;
and I am persuaded it will become of the utmost use in the extensions of
the science which these views afford. I do not pretend to have made its
detail perfect, but to have demonstrated the truth of the principle, and
the utility of the application[A].
[A] As early as the year 1811, Messrs. Gay-Lussac and Thénard employed
chemical decomposition as a measure of the electricity of the voltaic
pile. See _Recherches Physico-chymiques_, p. 12. The principles and
precautions by which it becomes an exact measure were of course not
then known.--_Dec. 1838._
¶ vi. _On the primary or secondary character of the bodies evolved at the
Electrodes._
742. Before the _volta-electrometer_ could be employed in determining, as a
_general law_, the constancy of electro-decomposition, it became necessary
to examine a distinction, already recognised among scientific men, relative
to the products of that action, namely, their primary or secondary
character; and, if possible, by some general rule or principle, to decide
when they were of the one or the other kind. It will appear hereafter that
great mistakes inspecting electro-chemical action and its consequences have
arisen from confounding these two classes of results together.
743. When a substance under decomposition yields at the electrodes those
bodies uncombined and unaltered which the electric current has separated,
then they may be considered as primary results, even though themselves
compounds. Thus the oxygen and hydrogen from water are primary results; and
so also are the acid and alkali (themselves compound bodies) evolved from
sulphate of soda. But when the substances separated by the current are
changed at the electrodes before their appearance, then they give rise to
secondary results, although in many cases the bodies evolved are
elementary.
744. These secondary results occur in two ways, being sometimes due to the
mutual action of the evolved substance and the matter of the electrode, and
sometimes to its action upon the substances contained in the body itself
under decomposition. Thus, when carbon is made the positive electrode in
dilute sulphuric acid, carbonic oxide and carbonic acid occasionally appear
there instead of oxygen; for the latter, acting upon the matter of the
electrode, produces these secondary results. Or if the positive electrode,
in a solution of nitrate or acetate of lead, be platina, then peroxide of
lead appears there, equally a secondary result with the former, but now
depending upon an action of the oxygen on a substance in the solution.
Again, when ammonia is decomposed by platina electrodes, nitrogen appears
at the _anode_[A]; but though an _elementary_ body, it is a _secondary_
result in this case, being derived from the chemical action of the oxygen
electrically evolved there, upon the ammonia in the surrounding solution
(554.). In the same manner when aqueous solutions of metallic salts are
decomposed by the current, the metals evolved at the _cathode_, though
elements, are _always_ secondary results, and not immediate consequences of
the decomposing power of the electric current.
[A] Annales de Chimie, 1801, tom. li. p. 167.
745. Many of these secondary results are extremely valuable; for instance,
all the interesting compounds which M. Becquerel has obtained by feeble
electric currents are of this nature; but they are essentially chemical,
and must, in the theory of electrolytic action, be carefully distinguished
from those which are directly due to the action of the electric current.
746. The nature of the substances evolved will often lead to a correct
judgement of their primary or secondary character, but is not sufficient
alone to establish that point. Thus, nitrogen is said to be attracted
sometimes by the positive and sometimes by the negative electrode,
according to the bodies with which it may be combined (554. 555.), and it
is on such occasions evidently viewed as a primary result[A]; but I think I
shall show, that, when it appears at the positive electrode, or rather at
the _anode_, it is a secondary result (748.). Thus, also, Sir Humphry
Davy[B], and with him the great body of chemical philosophers, (including
myself,) have given the appearance of copper, lead, tin, silver, gold, &c.,
at the negative electrode, when their aqueous solutions were acted upon by
the voltaic current, as proofs that the metals, as a class, were attracted
to that surface; thus assuming the metal, in each case, to be a primary
result. These, however, I expect to prove, are all secondary results; the
mere consequence of chemical action, and no proofs either of the attraction
or of the law announced respecting their places[C].
[A] Annales de Chimie, 1804, tom. li. p. 172.
[B] Elements of Chemical Philosophy, pp. 144. 161.
[C] It is remarkable that up to 1804 it was the received opinion that
the metals were reduced by the nascent hydrogen. At that date the
general opinion was reversed by Hisinger and Berzelius (Annales de
Chimie, 1804, tom. li. p. 174,), who stated that the metals were
evolved directly by the electricity: in which opinion it appears, from
that time, Davy coincided (Philosophical Transactions, 1826, p. 388).
747. But when we take to our assistance the law of _constant
electro-chemical action_ already proved with regard to water (732.), and
which I hope to extend satisfactorily to all bodies (821.), and consider
the _quantities_ as well as the _nature_ of the substances set free, a
generally accurate judgement of the primary or secondary character of the
results may be formed: and this important point, so essential to the theory
of electrolyzation, since it decides what are the particles directly under
the influence of the current, (distinguishing them from such as are not
affected,) and what are the results to be expected, may be established with
such degree of certainty as to remove innumerable ambiguities and doubtful
considerations from this branch of the science.
748. Let us apply these principles to the case of ammonia, and the supposed
determination of nitrogen to one or the other _electrode_ (554. 555,). A
pure strong solution of ammonia is as bad a conductor, and therefore as
little liable to electrolyzation, as pure water; but when sulphate of
ammonia is dissolved in it, the whole becomes a conductor; nitrogen
_almost_ and occasionally _quite_ pure is evolved at the _anode_, and
hydrogen at the _cathode_; the ratio of the volume of the former to that of
the latter varying, but being as 1 to about 3 or 4. This result would seem
at first to imply that the electric current had decomposed ammonia, and
that the nitrogen had been determined towards the positive electrode. But
when the electricity used was measured out by the volta-electrometer (707.
736.), it was found that the hydrogen obtained was exactly in the
proportion which would have been supplied by decomposed water, whilst the
nitrogen had no certain or constant relation whatever. When, upon
multiplying experiments, it was found that, by using a stronger or weaker
solution, or a more or less powerful battery, the gas evolved at the
_anode_ was a mixture of oxygen and nitrogen, varying both in proportion
and absolute quantity, whilst the hydrogen at the _cathode_ remained
constant, no doubt could be entertained that the nitrogen at the _anode_
was a secondary result, depending upon the chemical action of the nascent
oxygen, determined to that surface by the electric current, upon the
ammonia in solution. It was the water, therefore, which was electrolyzed,
not the ammonia. Further, the experiment gives no real indication of the
tendency of the element nitrogen to either one electrode or the other; nor
do I know of any experiment with nitric acid, or other compounds of
nitrogen, which shows the tendency of this element, under the influence of
the electric current, to pass in either direction along its course.
749. As another illustration of secondary results, the effects on a
solution of acetate of potassa, may be quoted. When a very strong solution
was used, more gas was evolved at the _anode_ than at the _cathode_, in the
proportion of 4 to 3 nearly: that from the _anode_ was a mixture of
carbonic oxide and carbonic acid; that from the _cathode_ pure hydrogen.
When a much weaker solution was used, less gas was evolved at the _anode_
than at the _cathode_; and it now contained carburetted hydrogen, as well
as carbonic oxide and carbonic acid. This result of carburetted hydrogen at
the positive electrode has a very anomalous appearance, if considered as an
immediate consequence of the decomposing power of the current. It, however,
as well as the carbonic oxide and acid, is only a _secondary result_; for
it is the water alone which suffers electro-decomposition, and it is the
oxygen eliminated at the _anode_ which, reacting on the acetic acid, in the
midst of which it is evolved, produces those substances that finally appear
there. This is fully proved by experiments with the volta-electrometer
(707.); for then the hydrogen evolved from the acetate at the _cathode_ is
always found to be definite, being exactly proportionate to the electricity
which has passed through the solution, and, in quantity, the same as the
hydrogen evolved in the volta-electrometer itself. The appearance of the
carbon in combination with the hydrogen at the positive electrode, and its
non-appearance at the negative electrode, are in curious contrast with the
results which might have been expected from the law usually accepted
respecting the final places of the elements.
750. If the salt in solution be an acetate of lead, then the results at
both electrodes are secondary, and cannot be used to estimate or express
the amount of electro-chemical action, except by a circuitous process
(843.). In place of oxygen or even the gases already described (749.),
peroxide of lead now appears at the positive, and lead itself at the
negative electrode. When other metallic solutions are used, containing, for
instance, peroxides, as that of copper, combined with this or any other
decomposable acid, still more complicated results will be obtained; which,
viewed as direct results of the electro-chemical action, will, in their
proportions, present nothing but confusion, but will appear perfectly
harmonious and simple if they be considered as secondary results, and will
accord in their proportions with the oxygen and hydrogen evolved from water
by the action of a definite quantity of electricity.
751. I have experimented upon many bodies, with a view to determine whether
the results were primary or secondary. I have been surprised to find how
many of them, in ordinary cases, are of the latter class, and how
frequently water is the only body electrolyzed in instances where other
substances have been supposed to give way. Some of these results I will
give in as few words as possible.
752. _Nitric acid._--When very strong, it conducted well, and yielded
oxygen at the positive electrode. No gas appeared at the negative
electrode; but nitrous acid, and apparently nitric oxide, were formed
there, which, dissolving, rendered the acid yellow or red, and at last even
effervescent, from the spontaneous separation of nitric oxide. Upon
diluting the acid with its bulk or more of water, gas appeared at the
negative electrode. Its quantity could be varied by variations, either in
the strength of the acid or of the voltaic current: for that acid from
which no gas separated at the _cathode_, with a weak voltaic battery, did
evolve gas there with a stronger; and that battery which evolved no gas
there with a strong acid, did cause its evolution with an acid more dilute.
The gas at the _anode_ was always oxygen; that at the _cathode_ hydrogen.
When the quantity of products was examined by the volta-electrometer
(707.), the oxygen, whether from strong or weak acid, proved to be in the
same proportion as from water. When the acid was diluted to specific
gravity 1.24, or less, the hydrogen also proved to be the same in quantity
as from water. Hence I conclude that the nitric acid does not undergo
electrolyzation, but the water only; that the oxygen at the _anode_ is
always a primary result, but that the products at the _cathode_ are often
secondary, and due to the reaction of the hydrogen upon the nitric acid.
753. _Nitre._--A solution of this salt yields very variable results,
according as one or other form of tube is used, or as the electrodes are
large or small. Sometimes the whole of the hydrogen of the water decomposed
may be obtained at the negative electrode; at other times, only a part of
it, because of the ready formation of secondary results. The solution is a
very excellent conductor of electricity.
754. _Nitrate of ammonia_, in aqueous solution, gives rise to secondary
results very varied and uncertain in their proportions.
755. _Sulphurous acid._--Pure liquid sulphurous acid does not conduct nor
suffer decomposition by the voltaic current[A], but, when dissolved in
water, the solution acquires conducting power, and is decomposed, yielding
oxygen at the _anode_, and hydrogen and sulphur at the _cathode_.
[A] See also De la Rive, Bibliothèque Universelle, tom. xl. p. 205; or
Quarterly Journal of Science, vol. xxvii. p, 407.
756. A solution containing sulphuric acid in addition to the sulphurous
acid, was a better conductor. It gave very little gas at either electrode:
that at the _anode_ was oxygen, that at the _cathode_ pure hydrogen. From
the _cathode_ also rose a white turbid stream, consisting of diffused
sulphur, which soon rendered the whole solution milky. The volumes of gases
were in no regular proportion to the quantities evolved from water in the
voltameter. I conclude that the sulphurous acid was not at all affected by
the electric current in any of these cases, and that the water present was
the only body electro-chemically decomposed; that, at the _anode_, the
oxygen from the water converted the sulphurous acid into sulphuric acid,
and, at the _cathode_, the hydrogen electrically evolved decomposed the
sulphurous acid, combining with its oxygen, and setting its sulphur free. I
conclude that the sulphur at the negative electrode was only a secondary
result; and, in fact, no part of it was found combined with the small
portion of hydrogen which escaped when weak solutions of sulphurous acid
were used.
757. _Sulphuric acid._--I have already given my reasons for concluding that
sulphuric acid is not electrolyzable, i.e. not decomposable directly by the
electric current, but occasionally suffering by a secondary action at the
_cathode_ from the hydrogen evolved there (681.). In the year 1800, Davy
considered the sulphur from sulphuric acid as the result of the action of
the nascent hydrogen[A]. In 1804, Hisinger and Berzelius stated that it was
the direct result of the action of the voltaic pile[B], an opinion which
from that time Davy seems to have adopted, and which has since been
commonly received by all. The change of my own opinion requires that I
should correct what I have already said of the decomposition of sulphuric
acid in a former series of these Researches (552.): I do not now think that
the appearance of the sulphur at the negative electrode is an immediate
consequence of electrolytic action.
[A] Nicholson's Quarterly Journal, vol. iv. pp. 280, 281.
[B] Annales de Chimie, 1804, tom. li. p. 173.
758. _Muriatic acid._--A strong solution gave hydrogen at the negative
electrode, and chlorine only at the positive electrode; of the latter, a
part acted on the platina and a part was dissolved. A minute bubble of gas
remained; it was not oxygen, but probably air previously held in solution.
759. It was an important matter to determine whether the chlorine was a
primary result, or only a secondary product, due to the action of the
oxygen evolved from water at the _anode_ upon the muriatic acid; i.e.
whether the muriatic acid was electrolyzable, and if so, whether the
decomposition was _definite_.
760. The muriatic acid was gradually diluted. One part with six of water
gave only chlorine at the _anode_. One part with eight of water gave only
chlorine; with nine of water, a little oxygen appeared with the chlorine;
but the occurrence or non-occurrence of oxygen at these strengths depended,
in part, on the strength of the voltaic battery used. With fifteen parts of
water, a little oxygen, with much chlorine, was evolved at the _anode_. As
the solution was now becoming a bad conductor of electricity, sulphuric
acid was added to it: this caused more ready decomposition, but did not
sensibly alter the proportion of chlorine and oxygen.
761. The muriatic acid was now diluted with 100 times its volume of dilute
sulphuric acid. It still gave a large proportion of chlorine at the
_anode_, mingled with oxygen; and the result was the same, whether a
voltaic battery of 40 pairs of plates or one containing only 5 pairs were
used. With acid of this strength, the oxygen evolved at the _anode_ was to
the hydrogen at the _cathode_, in volume, as 17 is to 64; and therefore the
chlorine would have been 30 volumes, had it not been dissolved by the
fluid.
762. Next with respect to the quantity of elements evolved. On using the
volta-electrometer, it was found that, whether the strongest or the weakest
muriatic acid were used, whether chlorine alone or chlorine mingled with
oxygen appeared at the _anode_, still the hydrogen evolved at the _cathode_
was a constant quantity, i.e. exactly the same as the hydrogen which the
_same quantity of electricity_ could evolve from water.
763. This constancy does not decide whether the muriatic acid is
electrolyzed or not, although it proves that if so, it must be in definite
proportions to the quantity of electricity used. Other considerations may,
however, be allowed to decide the point. The analogy between chlorine and
oxygen, in their relations to hydrogen, is so strong, as to lead almost to
the certainty, that, when combined with that element, they would perform
similar parts in the process of electro-decomposition. They both unite with
it in single proportional or equivalent quantities; and the number of
proportionals appearing to have an intimate and important relation to the
decomposability of a body (697.), those in muriatic acid, as well as in
water, are the most favourable, or those perhaps even necessary, to
decomposition. In other binary compounds of chlorine also, where nothing
equivocal depending on the simultaneous presence of it and oxygen is
involved, the chlorine is directly eliminated at the _anode_ by the
electric current. Such is the case with the chloride of lead (395.), which
may be justly compared with protoxide of lead (402.), and stands in the
same relation to it as muriatic acid to water. The chlorides of potassium,
sodium, barium, &c., are in the same relation to the protoxides of the same
metals and present the same results under the influence of the electric
current (402.).
764. From all the experiments, combined with these considerations, I
conclude that muriatic acid is decomposed by the direct influence of the
electric current, and that the quantities evolved are, and therefore the
chemical action is, _definite for a definite quantity of electricity_. For
though I have not collected and measured the chlorine, in its separate
state, at the _anode_, there can exist no doubt as to its being
proportional to the hydrogen at the _cathode_; and the results are
therefore sufficient to establish the general law of _constant
electro-chemical action_ in the case of muriatic acid.
765. In the dilute acid (761.), I conclude that a part of the water is
electro-chemically decomposed, giving origin to the oxygen, which appears
mingled with the chlorine at the _anode_. The oxygen _may_ be viewed as a
secondary result; but I incline to believe that it is not so; for, if it
were, it might be expected in largest proportion from the stronger acid,
whereas the reverse is the fact. This consideration, with others, also
leads me to conclude that muriatic acid is more easily decomposed by the
electric current than water; since, even when diluted with eight or nine
times its quantity of the latter fluid, it alone gives way, the water
remaining unaffected.
766. _Chlorides._--On using solutions of chlorides in water,--for instance,
the chlorides of sodium or calcium,--there was evolution of chlorine only
at the positive electrode, and of hydrogen, with the oxide of the base, as
soda or lime, at the negative electrode. The process of decomposition may
be viewed as proceeding in two or three ways, all terminating in the same
results. Perhaps the simplest is to consider the chloride as the substance
electrolyzed, its chlorine being determined to and evolved at the _anode_,
and its metal passing to the _cathode_, where, finding no more chlorine, it
acts upon the water, producing hydrogen and an oxide as secondary results.
As the discussion would detain me from more important matter, and is not of
immediate consequence, I shall defer it for the present. It is, however, of
_great consequence_ to state, that, on using the volta-electrometer, the
hydrogen in both cases was definite; and if the results do not prove the
definite decomposition of chlorides, (which shall be proved
elsewhere,--789. 794. 814.,) they are not in the slightest degree opposed
to such a conclusion, and do support the _general law_.
767. _Hydriodic acid._--A solution of hydriodic acid was affected exactly
in the same manner as muriatic acid. When strong, hydrogen was evolved at
the negative electrode, in definite proportion to the quantity of
electricity which had passed, i.e. in the same proportion as was evolved by
the same current from water; and iodine without any oxygen was evolved at
the positive electrode. But when diluted, small quantities of oxygen
appeared with the iodine at the _anode_, the proportion of hydrogen at the
_cathode_ remaining undisturbed.
768. I believe the decomposition of the hydriodic acid in this case to be
direct, for the reasons already given respecting muriatic acid (763. 764.).
769. _Iodides._--A solution of iodide of potassium being subjected to the
voltaic current, iodine appeared at the positive electrode (without any
oxygen), and hydrogen with free alkali at the negative electrode. The same
observations as to the mode of decomposition are applicable here as were
made in relation to the chlorides when in solution (766.).
770. _Hydro-fluoric acid and fluorides._--Solution of hydrofluoric acid did
not appear to be decomposed under the influence of the electric current: it
was the water which gave way apparently. The fused fluorides were
electrolysed (417.); but having during these actions obtained _fluorine_ in
the separate state, I think it better to refer to a future series of these
Researches, in which I purpose giving a fuller account of the results than
would be consistent with propriety here[A].
[A] I have not obtained fluorine: my expectations, amounting to
conviction, passed away one by one when subjected to rigorous
examination; some very singular results were obtained; and to one of
these I refer at 1340.--_Dec. 1838._
771. _Hydro-cyanic acid_ in solution conducts very badly. The definite
proportion of hydrogen (equal to that from water) was set free at the
_cathode_, whilst at the _anode_ a small quantity of oxygen was evolved and
apparently a solution of cyanogen formed. The action altogether
corresponded with that on a dilute muriatic or hydriodic acid. When the
hydrocyanic acid was made a better conductor by sulphuric acid, the same
results occurred.
_Cyanides._--With a solution of the cyanide of potassium, the result was
precisely the same as with a chloride or iodide. No oxygen was evolved at
the positive electrode, but a brown solution formed there. For the reasons
given when speaking of the chlorides (766.), and because a fused cyanide of
potassium evolves cyanogen at the positive electrode[A], I incline to
believe that the cyanide in solution is _directly_ decomposed.
[A] It is a very remarkable thing to see carbon and nitrogen in this
case determined powerfully towards the positive surface of the voltaic
battery; but it is perfectly in harmony with the theory of
electro-chemical decomposition which I have advanced.
772. _Ferro-cyanic acid_ and the _ferro-cyanides_, as also _sulpho-cyanic
acid_ and the _sulpho-cyanides_, presented results corresponding with those
just described (771.).
773. _Acetic acid._--Glacial acetic acid, when fused (405.), is not
decomposed by, nor does it conduct, electricity. On adding a little water
to it, still there were no signs of action; on adding more water, it acted
slowly and about as pure water would do. Dilute sulphuric acid was added to
it in order to make it a better conductor; then the definite proportion of
hydrogen was evolved at the _cathode_, and a mixture of oxygen in very
deficient quantity, with carbonic acid, and a little carbonic oxide, at the
_anode_. Hence it appears that acetic acid is not electrolyzable, but that
a portion of it is decomposed by the oxygen evolved at the _anode_,
producing secondary results, varying with the strength of the acid, the
intensity of the current, and other circumstances.
774. _Acetates._--One of these has been referred to already, as affording
only secondary results relative to the acetic acid (749.). With many of the
metallic acetates the results at both electrodes are secondary (746. 750.).
Acetate of soda fused and anhydrous is directly decomposed, being, as I
believe, a true electrolyte, and evolving soda and acetic acid at the
_cathode_ and _anode_. These however have no sensible duration, but are
immediately resolved into other substances; charcoal, sodiuretted hydrogen,
&c., being set free at the former, and, as far as I could judge under the
circumstances, acetic acid mingled with carbonic oxide, carbonic acid, &c.
at the latter.
775. _Tartaric acid._--Pure solution of tartaric acid is almost as bad a
conductor as pure water. On adding sulphuric acid, it conducted well, the
results at the positive electrode being primary or secondary in different
proportions, according to variations in the strength of the acid and the
power of the electric current (752.). Alkaline tartrates gave a large
proportion of secondary results at the positive electrode. The hydrogen at
the negative electrode remained constant unless certain triple metallic
salts were used.
776. Solutions, of salts containing other vegetable acids, as the
benzoates; of sugar, gum, &c., dissolved in dilute sulphuric acid; of
resin, albumen, &c., dissolved in alkalies, were in turn submitted to the
electrolytic power of the voltaic current. In all these cases, secondary
results to a greater or smaller extent were produced at the positive
electrode.
777. In concluding this division of these Researches, it cannot but occur
to the mind that the final result of the action of the electric current
upon substances, placed between the electrodes, instead of being simple may
be very complicated. There are two modes by which these substances may be
decomposed, either by the direct force of the electric current, or by the
action of bodies which that current may evolve. There are also two modes by
which new compounds may be formed, i.e. by combination of the evolving
substances whilst in their nascent state (658.), directly with the matter
of the electrode; or else their combination with those bodies, which being
contained in, or associated with, the body suffering decomposition, are
necessarily present at the _anode_ and _cathode_. The complexity is
rendered still greater by the circumstance that two or more of these
actions may occur simultaneously, and also in variable proportions to each
other. But it may in a great measure be resolved by attention to the
principles already laid down (747.).
778. When _aqueous_ solutions of bodies are used, secondary results are
exceedingly frequent. Even when the water is not present in large quantity,
but is merely that of combination, still secondary results often ensue: for
instance, it is very possible that in Sir Humphry Davy's decomposition of
the hydrates of potassa and soda, a part of the potassium produced was the
result of a secondary action. Hence, also, a frequent cause for the
disappearance of the oxygen and hydrogen which would otherwise be evolved:
and when hydrogen does _not_ appear at the _cathode_ in an _aqueous
solution_, it perhaps always indicates that a secondary action has taken
place there. No exception to this rule has as yet occurred to my
observation.
779. Secondary actions are _not confined to aqueous solutions_, or cases
where water is present. For instance, various chlorides acted upon, when
fused (402.), by platina electrodes, have the chlorine determined
electrically to the _anode_. In many cases, as with the chlorides of lead,
potassium, barium, &c., the chlorine acts on the platina and forms a
compound with it, which dissolves; but when protochloride of tin is used,
the chlorine at the _anode_ does not act upon the platina, but upon the
chloride already there, forming a perchloride which rises in vapour (790.
804.). These are, therefore, instances of secondary actions of both kinds,
produced in bodies containing no water.
780. The production of boron from fused borax (402. 417.) is also a case of
secondary action; for boracic acid is not decomposable by electricity
(408.), and it was the sodium evolved at the _cathode_ which, re-acting on
the boracic acid around it, took oxygen from it and set boron free in the
experiments formerly described.
781. Secondary actions have already, in the hands of M. Becquerel, produced
many interesting results in the formation of compounds; some of them new,
others imitations of those occurring naturally[A]. It is probable they may
prove equally interesting in an opposite direction, i.e. as affording cases
of analytic decomposition. Much information regarding the composition, and
perhaps even the arrangement, of the particles of such bodies as the
vegetable acids and alkalies, and organic compounds generally, will
probably be obtained by submitting them to the action of nascent oxygen,
hydrogen, chlorine, &c. at the electrodes; and the action seems the more
promising, because of the thorough command which we possess over attendant
circumstances, such as the strength of the current, the size of the
electrodes, the nature of the decomposing conductor, its strength, &c., all
of which may be expected to have their corresponding influence upon the
final result.
782. It is to me a great satisfaction that the extreme variety of secondary
results has presented nothing opposed to the doctrine of a constant and
definite electro-chemical action, to the particular consideration of which
I shall now proceed.
¶ vii. _On the definite nature and extent of Electro-chemical
Decomposition._
783. In the third series of these Researches, after proving the identity of
electricities derived from different sources, and showing, by actual
measurement, the extraordinary quantity of electricity evolved by a very
feeble voltaic arrangement (371. 376.), I announced a law, derived from
experiment, which seemed to me of the utmost importance to the science of
electricity in general, and that branch of it denominated electro-chemistry
in particular. The law was expressed thus: _The chemical power of a current
of electricity is in direct proportion to the absolute quantity of
electricity which passes_ (377.).
[A] Annales de Chimie, tom, xxxv. p. 113.
784. In the further progress of the successive investigations, I have had
frequent occasion to refer to the same law, sometimes in circumstances
offering powerful corroboration of its truth (456. 504. 505.); and the
present series already supplies numerous new cases in which it holds good
(704. 722. 726. 732.). It is now my object to consider this great principle
more closely, and to develope some of the consequences to which it leads.
That the evidence for it may be the more distinct and applicable, I shall
quote cases of decomposition subject to as few interferences from secondary
results as possible, effected upon bodies very simple, yet very definite in
their nature.
785. In the first place, I consider the law as so fully established with
respect to the decomposition of _water_, and under so many circumstances
which might be supposed, if anything could, to exert an influence over it,
that I may be excused entering into further detail respecting that
substance, or even summing up the results here (732.). I refer, therefore,
to the whole of the subdivision of this series of Researches which contains
the account of the _volta-electrometer_ (704. &c.).
786. In the next place, I also consider the law as established with respect
to _muriatic acid_ by the experiments and reasoning already advanced, when
speaking of that substance, in the subdivision respecting primary and
secondary results (758. &c.).
787. I consider the law as established also with regard to _hydriodic acid_
by the experiments and considerations already advanced in the preceding
division of this series of Researches (767. 768.).
788. Without speaking with the same confidence, yet from the experiments
described, and many others not described, relating to hydro-fluoric,
hydro-cyanic, ferro-cyanic, and sulpho-cyanic acids (770. 771. 772.), and
from the close analogy which holds between these bodies and the hydracids
of chlorine, iodine, bromine, &c., I consider these also as coming under
subjection to the law, and assisting to prove its truth.
789. In the preceding cases, except the first, the water is believed to be
inactive; but to avoid any ambiguity arising from its presence, I sought
for substances from which it should be absent altogether; and, taking
advantage of the law of conduction already developed (380. &c.), I soon
found abundance, amongst which _protochloride of tin_ was first subjected
to decomposition in the following manner. A piece of platina wire had one
extremity coiled up into a small knob, and, having been carefully weighed,
was sealed hermetically into a piece of bottle-glass tube, so that the knob
should be at the bottom of the tube within (fig. 68.). The tube was
suspended by a piece of platina wire, so that the heat of a spirit-lamp
could be applied to it. Recently fused protochloride of tin was introduced
in sufficient quantity to occupy, when melted, about one-half of the tube;
the wire of the tube was connected with a volta-electrometer (711.), which
was itself connected with the negative end of a voltaic battery; and a
platina wire connected with the positive end of the same battery was dipped
into the fused chloride in the tube; being however so bent, that it could
not by any shake of the hand or apparatus touch the negative electrode at
the bottom of the vessel. The whole arrangement is delineated in fig. 69.
790. Under these circumstances the chloride of tin was decomposed: the
chlorine evolved at the positive electrode formed bichloride of tin (779.),
which passed away in fumes, and the tin evolved at the negative electrode
combined with the platina, forming an alloy, fusible at the temperature to
which the tube was subjected, and therefore never occasioning metallic
communication through the decomposing chloride. When the experiment had
been continued so long as to yield a reasonable quantity of gas in the
volta-electrometer, the battery connexion was broken, the positive
electrode removed, and the tube and remaining chloride allowed to cool.
When cold, the tube was broken open, the rest of the chloride and the glass
being easily separable from the platina wire and its button of alloy. The
latter when washed was then reweighed, and the increase gave the weight of
the tin reduced.
791. I will give the particular results of one experiment, in illustration
of the mode adopted in this and others, the results of which I shall have
occasion to quote. The negative electrode weighed at first 20 grains; after
the experiment, it, with its button of alloy, weighed 23.2 grains. The tin
evolved by the electric current at the _cathode_: weighed therefore 3.2
grains. The quantity of oxygen and hydrogen collected in the
volta-electrometer = 3.85 cubic inches. As 100 cubic inches of oxygen and
hydrogen, in the proportions to form water, may be considered as weighing
12.92 grains, the 3.85 cubic inches would weigh 0.49742 of a grain; that
being, therefore, the weight of water decomposed by the same electric
current as was able to decompose such weight of protochloride of tin as
could yield 3.2 grains of metal. Now 0.49742 : 3.2 :: 9 the equivalent of
water is to 57.9, which should therefore be the equivalent of tin, if the
experiment had been made without error, and if the electro-chemical
decomposition _is in this case also definite_. In some chemical works 58 is
given as the chemical equivalent of tin, in others 57.9. Both are so near
to the result of the experiment, and the experiment itself is so subject to
slight causes of variation (as from the absorption of gas in the
volta-electrometer (716.), &c.), that the numbers leave little doubt of the
applicability of the _law of definite action_ in this and all similar cases
of electro-decomposition.
792. It is not often I have obtained an accordance in numbers so near as
that I have just quoted. Four experiments were made on the protochloride of
tin, the quantities of gas evolved in the volta-electrometer being from
2.05 to 10.29 cubic inches. The average of the four experiments gave 58.53
as the electro-chemical equivalent for tin.
793. The chloride remaining after the experiment was pure protochloride of
tin; and no one can doubt for a moment that the equivalent of chlorine had
been evolved at the _anode_, and, having formed bichloride of tin as a
secondary result, had passed away.
794. _Chloride of lead_ was experimented upon in a manner exactly similar,
except that a change was made in the nature of the positive electrode; for
as the chlorine evolved at the _anode_ forms no perchloride of lead, but
acts directly upon the platina, it produces, if that metal be used, a
solution of chloride of platina in the chloride of lead; in consequence of
which a portion of platina can pass to the _cathode_, and would then
produce a vitiated result. I therefore sought for, and found in plumbago,
another substance, which could be used safely as the positive electrode in
such bodies as chlorides, iodides, &c.
The chlorine or iodine does not act upon it, but is evolved in the free
state; and the plumbago has no re-action, under the circumstances, upon the
fused chloride or iodide in which it is plunged. Even if a few particles of
plumbago should separate by the heat or the mechanical action of the
evolved gas, they can do no harm in the chloride.
795. The mean of three experiments gave the number of 100.85 as the
equivalent for lead. The chemical equivalent is 103.5. The deficiency in my
experiments I attribute to the solution of part of the gas (716.) in the
volta-electrometer; but the results leave no doubt on my mind that both the
lead and the chlorine are, in this case, evolved in _definite quantities_
by the action of a given quantity of electricity (814. &c.).
796. _Chloride of antimony._--It was in endeavouring to obtain the
electro-chemical equivalent of antimony from the chloride, that I found
reasons for the statement I have made respecting the presence of water in
it in an earlier part of these Researches (690. 693. &c.).
797. I endeavoured to experiment upon the _oxide of lead_ obtained by
fusion and ignition of the nitrate in a platina crucible, but found great
difficulty, from the high temperature required for perfect fusion, and the
powerful fluxing qualities of the substance. Green-glass tubes repeatedly
failed. I at last fused the oxide in a small porcelain crucible, heated
fully in a charcoal fire; and, as it is was essential that the evolution of
the lead at the _cathode_ should take place beneath the surface, the
negative electrode was guarded by a green-glass tube, fused around it in
such a _manner as to expose only the knob of platina_ at the lower end
(fig. 70.), so that it could be plunged beneath the surface, and thus
exclude contact of air or oxygen with the lead reduced there. A platina
wire was employed for the positive electrode, that metal not being subject
to any action from the oxygen evolved against it. The arrangement is given
in fig. 71.
798. In an experiment of this kind the equivalent for the lead came out
93.17, which is very much too small. This, I believe, was because of the
small interval between the positive and negative electrodes in the oxide of
lead; so that it was not unlikely that some of the froth and bubbles formed
by the oxygen at the _anode_ should occasionally even touch the lead
reduced at the _cathode_, and re-oxidize it. When I endeavoured to correct
this by having more litharge, the greater heat required to keep it all
fluid caused a quicker action on the crucible, which was soon eaten
through, and the experiment stopped.
799. In one experiment of this kind I used borate of lead (408. 673.). It
evolves lead, under the influence of the electric current, at the _anode_,
and oxygen at the _cathode_; and as the boracic acid is not either directly
(408.) or incidentally decomposed during the operation, I expected a result
dependent on the oxide of lead. The borate is not so violent a flux as the
oxide, but it requires a higher temperature to make it quite liquid; and if
not very hot, the bubbles of oxygen cling to the positive electrode, and
retard the transfer of electricity. The number for lead came out 101.29,
which is so near to 103.5 as to show that the action of the current had
been definite.
800. _Oxide of bismuth._--I found this substance required too high a
temperature, and acted too powerfully as a flux, to allow of any experiment
being made on it, without the application of more time and care than I
could give at present.
801. The ordinary _protoxide of antimony_, which consists of one
proportional of metal and one and a half of oxygen, was subjected to the
action of the electric current in a green-glass tube (789.), surrounded by
a jacket of platina foil, and heated in a charcoal fire. The decomposition
began and proceeded very well at first, apparently indicating, according to
the general law (679. 697.), that this substance was one containing such
elements and in such proportions as made it amenable to the power of the
electric current. This effect I have already given reasons for supposing
may be due to the presence of a true protoxide, consisting of single
proportionals (696. 693.). The action soon diminished, and finally ceased,
because of the formation of a higher oxide of the metal at the positive
electrode. This compound, which was probably the peroxide, being infusible
and insoluble in the protoxide, formed a crystalline crust around the
positive electrode; and thus insulating it, prevented the transmission of
the electricity. Whether, if it had been fusible and still immiscible, it
would have decomposed, is doubtful, because of its departure from the
required composition (697.). It was a very natural secondary product at the
positive electrode (779.). On opening the tube it was found that a little
antimony had been separated at the negative electrode; but the quantity was
too small to allow of any quantitative result being obtained[A].
[A] This paragraph is subject to the corrective note now appended to
paragraph 696.--_Dec. 1838._
802. _Iodide of lead._--This substance can be experimented with in tubes
heated by a spirit-lamp (789.); but I obtained no good results from it,
whether I used positive electrodes of platina or plumbago. In two
experiments the numbers for the lead came out only 75.46 and 73.45, instead
of 103.5. This I attribute to the formation of a periodide at the positive
electrode, which, dissolving in the mass of liquid iodide, came in contact
with the lead evolved at the negative electrode, and dissolved part of it,
becoming itself again protiodide. Such a periodide does exist; and it is
very rarely that the iodide of lead formed by precipitation, and
well-washed, can be fused without evolving much iodine, from the presence
of this percompound; nor does crystallization from its hot aqueous solution
free it from this substance. Even when a little of the protiodide and
iodine are merely rubbed together in a mortar, a portion of the periodide
is formed. And though it is decomposed by being fused and heated to dull
redness for a few minutes, and the whole reduced to protiodide, yet that is
not at all opposed to the possibility, that a little of that which is
formed in great excess of iodine at the _anode_, should be carried by the
rapid currents in the liquid into contact with the _cathode_.
803. This view of the result was strengthened by a third experiment, where
the space between the electrodes was increased to one third of an inch; for
now the interfering effects were much diminished, and the number of the
lead came out 89.04; and it was fully confirmed by the results obtained in
the cases of _transfer_ to be immediately described (818.).
The experiments on iodide of lead therefore offer no exception to the
_general law_ under consideration, but on the contrary may, from general
considerations, be admitted as included in it.
804. _Protiodide of tin._--This substance, when fused (402.), conducts and
is decomposed by the electric current, tin is evolved at the _anode_, and
periodide of tin as a secondary result (779. 790.) at the _cathode_. The
temperature required for its fusion is too high to allow of the production
of any results fit for weighing.
805. _Iodide of potassium_ was subjected to electrolytic action in a tube,
like that in fig. 68. (789.). The negative electrode was a globule of lead,
and I hoped in this way to retain the potassium, and obtain results that
could be weighed and compared with the volta-electrometer indication; but
the difficulties dependent upon the high temperature required, the action
upon the glass, the fusibility of the platina induced by the presence of
the lead, and other circumstances, prevented me from procuring such
results. The iodide was decomposed with the evolution of iodine at the
_anode_, and of potassium at the _cathode_, as in former cases.
806. In some of these experiments several substances were placed in
succession, and decomposed simultaneously by the same electric current:
thus, protochloride of tin, chloride of lead, and water, were thus acted on
at once. It is needless to say that the results were comparable, the tin,
lead, chlorine, oxygen, and hydrogen evolved being _definite in quantity_
and electro-chemical equivalents to each other.
* * * * *
807. Let us turn to another kind of proof of the _definite chemical action
of electricity_. If any circumstances could be supposed to exert an
influence over the quantity of the matters evolved during electrolytic
action, one would expect them to be present when electrodes of different
substances, and possessing very different chemical affinities for such
matters, were used. Platina has no power in dilute sulphuric acid of
combining with the oxygen at the _anode_, though the latter be evolved in
the nascent state against it. Copper, on the other hand, immediately unites
with the oxygen, as the electric current sets it free from the hydrogen;
and zinc is not only able to combine with it, but can, without any help
from the electricity, abstract it directly from the water, at the same time
setting torrents of hydrogen free. Yet in cases where these three
substances were used as the positive electrodes in three similar portions
of the same dilute sulphuric acid, specific gravity 1.336, precisely the
same quantity of water was decomposed by the electric current, and
precisely the same quantity of hydrogen set free at the _cathodes_ of the
three solutions.
808. The experiment was made thus. Portions of the dilute sulphuric acid
were put into three basins. Three volta-electrometer tubes, of the form
figg. 60. 62. were filled with the same acid, and one inverted in each
basin (707.). A zinc plate, connected with the positive end of a voltaic
battery, was dipped into the first basin, forming the positive electrode
there, the hydrogen, which was abundantly evolved from it by the direct
action of the acid, being allowed to escape. A copper plate, which dipped
into the acid of the second basin, was connected with the negative
electrode of the _first_ basin; and a platina plate, which dipped into the
acid of the third basin, was connected with the negative electrode of the
_second_ basin. The negative electrode of the third basin was connected
with a volta-electrometer (711.), and that with the negative end of the
voltaic battery.
809. Immediately that the circuit was complete, the _electro-chemical
action_ commenced in all the vessels. The hydrogen still rose in,
apparently, undiminished quantities from the positive zinc electrode in the
first basin. No oxygen was evolved at the positive copper electrode in the
second basin, but a sulphate of copper was formed there; whilst in the
third basin the positive platina electrode evolved pure oxygen gas, and was
itself unaffected. But in _all_ the basins the hydrogen liberated at the
_negative_ platina electrodes was the _same in quantity_, and the same with
the volume of hydrogen evolved in the volta-electrometer, showing that in
all the vessels the current had decomposed an equal quantity of water. In
this trying case, therefore, the _chemical action of electricity_ proved to
be _perfectly definite_.
810. A similar experiment was made with muriatic acid diluted with its bulk
of water. The three positive electrodes were zinc, silver, and platina; the
first being able to separate and combine with the chlorine _without_ the
aid of the current; the second combining with the chlorine only after the
current had set it free; and the third rejecting almost the whole of it.
The three negative electrodes were, as before, platina plates fixed within
glass tubes. In this experiment, as in the former, the quantity of hydrogen
evolved at the _cathodes_ was the same for all, and the same as the
hydrogen evolved in the volta-electrometer. I have already given my reasons
for believing that in these experiments it is the muriatic acid which is
directly decomposed by the electricity (764.); and the results prove that
the quantities so decomposed are _perfectly definite_ and proportionate to
the quantity of electricity which has passed.
811. In this experiment the chloride of silver formed in the second basin
retarded the passage of the current of electricity, by virtue of the law of
conduction before described (394.), so that it had to be cleaned off four
or five times during the course of the experiment; but this caused no
difference between the results of that vessel and the others.
812. Charcoal was used as the positive electrode in both sulphuric and
muriatic acids (808. 810.); but this change produced no variation of the
results. A zinc positive electrode, in sulphate of soda or solution of
common salt, gave the same constancy of operation.
813. Experiments of a similar kind were then made with bodies altogether in
a different state, i.e. with _fused_ chlorides, iodides, &c. I have already
described an experiment with fused chloride of silver, in which the
electrodes were of metallic silver, the one rendered negative becoming
increased and lengthened by the addition of metal, whilst the other was
dissolved and eaten away by its abstraction. This experiment was repeated,
two weighed pieces of silver wire being used as the electrodes, and a
volta-electrometer included in the circuit. Great care was taken to
withdraw the negative electrodes so regularly and steadily that the
crystals of reduced silver should not form a _metallic_ communication
beneath the surface of the fused chloride. On concluding the experiment the
positive electrode was re-weighed, and its loss ascertained. The mixture of
chloride of silver, and metal, withdrawn in successive portions at the
negative electrode, was digested in solution of ammonia, to remove the
chloride, and the metallic silver remaining also weighed: it was the
reduction at the _cathode_, and exactly equalled the solution at the
_anode_; and each portion was as nearly as possible the equivalent to the
water decomposed in the volta-electrometer.
814. The infusible condition of the silver at the temperature used, and the
length and ramifying character of its crystals, render the above experiment
difficult to perform, and uncertain in its results. I therefore wrought
with chloride of lead, using a green-glass tube, formed as in fig. 72. A
weighed platina wire was fused into the bottom of a small tube, as before
described (789.). The tube was then bent to an angle, at about half an inch
distance from the closed end; and the part between the angle and the
extremity being softened, was forced upward, as in the figure, so as to
form a bridge, or rather separation, producing two little depressions or
basins _a, b_, within the tube. This arrangement was suspended by a platina
wire, as before, so that the heat of a spirit-lamp could be applied to it,
such inclination being given to it as would allow all air to escape during
the fusion of the chloride of lead. A positive electrode was then provided,
by bending up the end of a platina wire into a knot, and fusing about
twenty grains of metallic lead on to it, in a small closed tube of glass,
which was afterwards broken away. Being so furnished, the wire with its
lead was weighed, and the weight recorded.
815. Chloride of lead was now introduced into the tube, and carefully
fused. The leaded electrode was also introduced; after which the metal, at
its extremity, soon melted. In this state of things the tube was filled up
to _c_ with melted chloride of lead; the end of the electrode to be
rendered negative was in the basin _b_, and the electrode of melted lead
was retained in the basin _a_, and, by connexion with the proper conducting
wire of a voltaic battery, was rendered positive. A volta-electrometer was
included in the circuit.
816. Immediately upon the completion of the communication with the voltaic
battery, the current passed, and decomposition proceeded. No chlorine was
evolved at the positive electrode; but as the fused chloride was
transparent, a button of alloy could be observed gradually forming and
increasing in size at _b_, whilst the lead at _a_ could also be seen
gradually to diminish. After a time, the experiment was stopped; the tube
allowed to cool, and broken open; the wires, with their buttons, cleaned
and weighed; and their change in weight compared with the indication of the
volta-electrometer.
817. In this experiment the positive electrode had lost just as much lead
as the negative one had gained (795.), and the loss and gain were very
nearly the equivalents of the water decomposed in the volta-electrometer,
giving for lead the number 101.5. It is therefore evident, in this
instance, that causing a _strong affinity_, or _no affinity_, for the
substance evolved at the _anode_, to be active during the experiment
(807.), produces no variation in the definite action of the electric
current.
818. A similar experiment was then made with iodide of lead, and in this
manner all confusion from the formation of a periodide avoided (803.). No
iodine was evolved during the whole action, and finally the loss of lead at
the _anode_ was the same as the gain at the _cathode_, the equivalent
number, by comparison with the result in the volta-electrometer, being
103.5.
819. Then protochloride of tin was subjected to the electric current in the
same manner, using of course, a tin positive electrode. No bichloride of
tin was now formed (779. 790.). On examining the two electrodes, the
positive had lost precisely as much as the negative had gained; and by
comparison with the volta-electrometer, the number for tin came out 59.
820. It is quite necessary in these and similar experiments to examine the
interior of the bulbs of alloy at the ends of the conducting wires; for
occasionally, and especially with those which have been positive, they are
cavernous, and contain portions of the chloride or iodide used, which must
be removed before the final weight is ascertained. This is more usually the
case with lead than tin.
821. All these facts combine into, I think, an irresistible mass of
evidence, proving the truth of the important proposition which I at first
laid down, namely, _that the chemical power of a current of electricity is
in direct proportion to the absolute quantity of electricity which passes_
(377. 783.). They prove, too, that this is not merely true with one
substance, as water, but generally with all electrolytic bodies; and,
further, that the results obtained with any _one substance_ do not merely
agree amongst themselves, but also with those obtained from _other
substances_, the whole combining together into _one series of definite
electro-chemical actions_ (505.). I do not mean to say that no exceptions
will appear: perhaps some may arise, especially amongst substances existing
only by weak affinity; but I do not expect that any will seriously disturb
the result announced. If, in the well-considered, well-examined, and, I may
surely say, well-ascertained doctrines of the definite nature of ordinary
chemical affinity, such exceptions occur, as they do in abundance, yet,
without being allowed to disturb our minds as to the general conclusion,
they ought also to be allowed if they should present themselves at this,
the opening of a new view of electro-chemical action; not being held up as
obstructions to those who may be engaged in rendering that view more and
more perfect, but laid aside for a while, in hopes that their perfect and
consistent explanation will ultimately appear.
* * * * *
822. The doctrine of _definite electro-chemical action_ just laid down,
and, I believe, established, leads to some new views of the relations and
classifications of bodies associated with or subject to this action. Some
of these I shall proceed to consider.
823. In the first place, compound bodies may be separated into two great
classes, namely, those which are decomposable by the electric current, and
those which are not: of the latter, some are conductors, others
non-conductors, of voltaic electricity[A]. The former do not depend for
their decomposability upon the nature of their elements only; for, of the
same two elements, bodies may be formed, of which one shall belong to one
class and another to the other class; but probably on the proportions also
(697.). It is further remarkable, that with very few, if any, exceptions
(414. 691.), these decomposable bodies are exactly those governed by the
remarkable law of conduction I have before described (694.); for that law
does not extend to the many compound fusible substances that are excluded
from this class. I propose to call bodies of this, the decomposable class,
_Electrolytes_ (664.).
[A] I mean here by voltaic electricity, merely electricity from a most
abundant source, but having very small intensity.
824. Then, again, the substances into which these divide, under the
influence of the electric current, form an exceedingly important general
class. They are combining bodies; are directly associated with the
fundamental parts of the doctrine of chemical affinity; and have each a
definite proportion, in which they are always evolved during electrolytic
action. I have proposed to call these bodies generally _ions_, or
particularly _anions_ and _cations_, according as they appear at the
_anode_ or _cathode_ (665.); and the numbers representing the proportions
in which they are evolved _electro-chemical equivalents_. Thus hydrogen,
oxygen, chlorine, iodine, lead, tin are _ions_; the three former are
_anions_, the two metals are _cations_, and 1, 8, 3, 125, 104, 58, are
their _electro-chemical equivalents_ nearly.
825. A summary of certain points already ascertained respecting
_electrolytes, ions_, and _electro-chemical equivalents_, may be given in
the following general form of propositions, without, I hope, including any
serious error.
826. i. A single _ion_, i.e. one not in combination with another, will have
no tendency to pass to either of the electrodes, and will be perfectly
indifferent to the passing current, unless it be itself a compound of more
elementary _ions_, and so subject to actual decomposition. Upon this fact
is founded much of the proof adduced in favour of the new theory of
electro-chemical decomposition, which I put forth in a former series of
these Researches (518. &c.).
827. ii. If one _ion_ be combined in right proportions (697.) with another
strongly opposed to it in its ordinary chemical relations, i.e. if an
_anion_ be combined with a _cation_, then both will travel, the one to the
_anode_, the other to the _cathode_, of the decomposing body (530, 542.
547.).
828. iii. If, therefore, an _ion_ pass towards one of the electrodes,
another _ion_ must also be passing simultaneously to the other electrode,
although, from secondary action, it may not make its appearance (743.).
829. iv. A body decomposable directly by the electric current, i.e. an
_electrolyte_, must consist of two _ions_, and must also render them up
during the act of decomposition.
830. v. There is but one _electrolyte_ composed of the same two elementary
_ions_; at least such appears to be the fact (697.), dependent upon a law,
that _only single electro-chemical equivalents of elementary ions can go to
the electrodes, and not multiples_.
831. vi. A body not decomposable when alone, as boracic acid, is not
directly decomposable by the electric current when in combination (780.).
It may act as an _ion_ going wholly to the _anode_ or _cathode_, but does
not yield up its elements, except occasionally by a secondary action.
Perhaps it is superfluous for me to point out that this proposition has _no
relation_ to such cases as that of water, which, by the presence of other
bodies, is rendered a better conductor of electricity, and _therefore_ is
more freely decomposed.
832. vii. The nature of the substance of which the electrode is formed,
provided it be a conductor, causes no difference in the
electro-decomposition, either in kind or degree (807. 813.): but it
seriously influences, by secondary action (714.), the state in which the
finally appear. Advantage may be taken of this principle in combining and
_ions_ collecting such _ions_ as, if evolved in their _free_ state, would
be unmanageable[A].
[A] It will often happen that the electrodes used may be of such a
nature as, with the fluid in which they are immersed, to produce an
electric current, either according with or opposing that of the
voltaic arrangement used, and in this way, or by direct chemical
action, may sadly disturb the results. Still, in the midst of all
these confusing effects, the electric current, which actually passes
in any direction through the body suffering decomposition, will
produce its own definite electrolytic action.
833. viii. A substance which, being used as the electrode, can combine with
the _ion_ evolved against it, is also, I believe, an _ion_, and combines,
in such cases, in the quantity represented by its _electro-chemical
equivalent_. All the experiments I have made agree with this view; and it
seems to me, at present, to result as a necessary consequence. Whether, in
the secondary actions that take place, where the _ion_ acts, not upon the
matter of the electrode, but on that which is around it in the liquid
(744.), the same consequence follows, will require more extended
investigation to determine.
834. ix. Compound _ions_ are not necessarily composed of electro-chemical
equivalents of simple _ions_. For instance, sulphuric acid, boracic acid,
phosphoric acid, are _ions_, but not _electrolytes_, i.e. not composed of
electro-chemical equivalents of simple _ions_.
835. x. Electro-chemical equivalents are always consistent; i.e. the same
number which represents the equivalent of a substance A when it is
separating from a substance B, will also represent A when separating from a
third substance C. Thus, 8 is the electro-chemical equivalent of oxygen,
whether separating from hydrogen, or tin, or lead; and 103.5 is the
electrochemical equivalent of lead, whether separating from oxygen, or
chlorine, or iodine.
836. xi. Electro-chemical equivalents coincide, and are the same, with
ordinary chemical equivalents.
837. By means of experiment and the preceding propositions, a knowledge of
_ions_ and their electro-chemical equivalents may be obtained in various
ways.
838. In the first place, they may be determined directly, as has been done
with hydrogen, oxygen, lead, and tin, in the numerous experiments already
quoted.
839. In the next place, from propositions ii. and iii., may be deduced the
knowledge of many other _ions_, and also their equivalents. When chloride
of lead was decomposed, platina being used for both electrodes (395.),
there could remain no more doubt that chlorine was passing to the _anode_,
although it combined with the platina there, than when the positive
electrode, being of plumbago (794.), allowed its evolution in the free
state; neither could there, in either case, remain any doubt that for every
103.5 parts of lead evolved at the _cathode_, 36 parts of chlorine were
evolved at the _anode_, for the remaining chloride of lead was unchanged.
So also, when in a metallic solution one volume of oxygen, or a secondary
compound containing that proportion, appeared at the _anode_, no doubt
could arise that hydrogen, equivalent to two volumes, had been determined
to the _cathode_, although, by a secondary action, it had been employed in
reducing oxides of lead, copper, or other metals, to the metallic state. In
this manner, then, we learn from the experiments already described in these
Researches, that chlorine, iodine, bromine, fluorine, calcium, potassium,
strontium, magnesium, manganese, &c., are _ions_ and that their
_electro-chemical equivalents_ are the same as their _ordinary chemical
equivalents_.
840. Propositions iv. and v. extend our means of gaining information. For
if a body of known chemical composition is found to be decomposable, and
the nature of the substance evolved as a primary or even a secondary result
(743. 777.) at one of the electrodes, be ascertained, the electro-chemical
equivalent of that body may be deduced from the known constant composition
of the substance evolved. Thus, when fused protiodide of tin is decomposed
by the voltaic current (804.), the conclusion may be drawn, that both the
iodine and tin are _ions_, and that the proportions in which they combine
in the fused compound express their electro-chemical equivalents. Again,
with respect to the fused iodide of potassium (805.), it is an electrolyte;
and the chemical equivalents will also be the electro-chemical equivalents.
841. If proposition viii. sustain extensive experimental investigation,
then it will not only help to confirm the results obtained by the use of
the other propositions, but will give abundant original information of its
own.
842. In many instances, the _secondary results_ obtained by the action of
the evolved _ion_ on the substances present in the surrounding liquid or
solution, will give the electro-chemical equivalent. Thus, in the solution
of acetate of lead, and, as far as I have gone, in other proto-salts
subjected to the reducing action of the nascent hydrogen at the _cathode_,
the metal precipitated has been in the same quantity as if it had been a
primary product, (provided no free hydrogen escaped there,) and therefore
gave accurately the number representing its electro-chemical equivalent.
843. Upon this principle it is that secondary results may occasionally be
used as measurers of the volta-electric current (706. 740.); but there are
not many metallic solutions that answer this purpose well: for unless the
metal is easily precipitated, hydrogen will be evolved at the _cathode_ and
vitiate the result. If a soluble peroxide is formed at the _anode_, or if
the precipitated metal crystallize across the solution and touch the
positive electrode, similar vitiated results are obtained. I expect to find
in some salts, as the acetates of mercury and zinc, solutions favourable
for this use.
844. After the first experimental investigations to establish the definite
chemical action of electricity, I have not hesitated to apply the more
strict results of chemical analysis to correct the numbers obtained as
electrolytic results. This, it is evident, may be done in a great number of
cases, without using too much liberty towards the due severity of
scientific research. The series of numbers representing electro-chemical
equivalents must, like those expressing the ordinary equivalents of
chemically acting bodies, remain subject to the continual correction of
experiment and sound reasoning.
845. I give the following brief Table of _ions_ and their electro-chemical
equivalents, rather as a specimen of a first attempt than as anything that
can supply the want which must very quickly be felt, of a full and complete
tabular account of this class of bodies. Looking forward to such a table as
of extreme utility (if well-constructed) in developing the intimate
relation of ordinary chemical affinity to electrical actions, and
identifying the two, not to the imagination merely, but to the conviction
of the senses and a sound judgement, I may be allowed to express a hope,
that the endeavour will always be to make it a table of _real_, and not
_hypothetical_, electro-chemical equivalents; for we shall else overrun the
facts, and lose all sight and consciousness of the knowledge lying directly
in our path.
846. The equivalent numbers do not profess to be exact, and are taken
almost entirely from the chemical results of other philosophers in whom I
could repose more confidence, as to these points, than in myself.