§14. _On the Electricity of the Voltaic Pile; its source, quantity,
intensity, and general characters._ ¶ i. _On simple Voltaic Circles._ ¶ ii.
_On the intensity necessary for Electrolyzation._ ¶ iii. _On associated
Voltaic Circles, or the Voltaic Battery._ ¶ iv. _On the resistance of an
Electrolyte to Electrolytic action._ ¶ v. _General remarks on the active
Voltaic Battery._
Received April 7,--Read June 5, 1831.
¶ i. _On simple Voltaic Circles._
875. The great question of the source of electricity, in the voltaic pile
has engaged the attention of so many eminent philosophers, that a man of
liberal mind and able to appreciate their powers would probably conclude,
although he might not have studied the question, that the truth was
somewhere revealed. But if in pursuance of this impression he were induced
to enter upon the work of collating results and conclusions, he would find
such contradictory evidence, such equilibrium of opinion, such variation
and combination of theory, as would leave him in complete doubt respecting
what he should accept as the true interpretation of nature: he would be
forced to take upon himself the labour of repeating and examining the
facts, and then use his own judgement on them in preference to that of
others.
876. This state of the subject must, to those who have made up their minds
on the matter, be my apology for entering upon its investigation. The views
I have taken of the definite action of electricity in decomposing bodies
(783.), and the identity of the power so used with the power to be overcome
(855.), founded not on a mere opinion or general notion, but on facts
which, being altogether new, were to my mind precise and conclusive, gave
me, as I conceived, the power of examining the question with advantages not
before possessed by any, and which might compensate, on my part, for the
superior clearness and extent of intellect on theirs. Such are the
considerations which have induced me to suppose I might help in deciding
the question, and be able to render assistance in that great service of
removing _doubtful knowledge_. Such knowledge is the early morning light of
every advancing science, and is essential to its development; but the man
who is engaged in dispelling that which is deceptive in it, and revealing
more clearly that which is true, is as useful in his place, and as
necessary to the general progress of the science, as he who first broke
through the intellectual darkness, and opened a path into knowledge before
unknown to man.
877. The identity of the force constituting the voltaic current or
electrolytic agent, with that which holds the elements of electrolytes
together (855.), or in other words with chemical affinity, seemed to
indicate that the electricity of the pile itself was merely a mode of
exertion, or exhibition, or existence of _true chemical action_, or rather
of its cause; and I have consequently already said that I agree with those
who believe that the _supply_ of electricity is due to chemical powers
(857.).
878. But the great question of whether it is originally due to metallic
contact or to chemical action, i.e. whether it is the first or the second
which _originates_ and determines the current, was to me still doubtful;
and the beautiful and simple experiment with amalgamated zinc and platina,
which I have described minutely as to its results (863, &c.), did not
decide the point; for in that experiment the chemical action does not take
place without the contact of the metals, and the metallic contact is
inefficient without the chemical action. Hence either might be looked upon
as the _determining_ cause of the current.
879. I thought it essential to decide this question by the simplest
possible forms of apparatus and experiment, that no fallacy might be
inadvertently admitted. The well-known difficulty of effecting
decomposition by a single pair of plates, except in the fluid exciting them
into action (863.), seemed to throw insurmountable obstruction in the way
of such experiments; but I remembered the easy decomposability of the
solution of iodide of potassium (316.), and seeing no theoretical reason,
if metallic contact was not _essential_, why true electro-decomposition
should not be obtained without it, even in a single circuit, I persevered
and succeeded.
880. A plate of zinc, about eight inches long and half an inch wide, was
cleaned and bent in the middle to a right angle, fig. 73 _a_, Plate VI. A
plate of platina, about three inches long and half an inch wide, was
fastened to a platina wire, and the latter bent as in the figure, _b_.
These two pieces of metal were arranged together as delineated, but as yet
without the vessel _c_, and its contents, which consisted of dilute
sulphuric acid mingled with a little nitric acid. At _x_ a piece of folded
bibulous paper, moistened in a solution of iodide of potassium, was placed
on the zinc, and was pressed upon by the end of the platina wire. When
under these circumstances the plates were dipped into the acid of the
vessel _c_, there was an immediate effect at _x_, the iodide being
decomposed, and iodine appearing at the _anode_ (663.), i.e. against the
end of the platina wire.
881. As long as the lower ends of the plates remained in the acid the
electric current continued, and the decomposition proceeded at _x_. On
removing the end of the wire from place to place on the paper, the effect
was evidently very powerful; and on placing a piece of turmeric paper
between the white paper and zinc, both papers being moistened with the
solution of iodide of potassium, alkali was evolved at the _cathode_ (663.)
against the zinc, in proportion to the evolution of iodine at the _anode_.
Hence the decomposition was perfectly polar, and decidedly dependent upon a
current of electricity passing from the zinc through the acid to the
platina in the vessel _c_, and back from the platina through the solution
to the zinc at the paper _x_.
882. That the decomposition at _x_ was a true electrolytic action, due to a
current determined by the state of things in the vessel _c_, and not
dependent upon any mere direct chemical action of the zinc and platina on
the iodide, or even upon any _current_ which the solution of iodide might
by its action on those metals tend to form at _x_, was shown, in the first
place, by removing the vessel _c_ and its acid from the plates, when all
decomposition at _x_ ceased, and in the next by connecting the metals,
either in or out of the acid, together, when decomposition of the iodide at
_x_ occurred, but in a _reverse order_; for now alkali appeared against the
end of the platina wire, and the iodine passed to the zinc, the current
being the contrary of what it was in the former instance, and produced
directly by the difference of action of the solution in the paper on the
two metals. The iodine of course _combined_ with the zinc.
883. When this experiment was made with pieces of zinc amalgamated over the
whole surface (863.), the results were obtained with equal facility and in
the same direction, even when only dilute sulphuric acid was contained in
the vessel _c_ (fig. 73.). Whichsoever end of the zinc was immersed in the
acid, still the effects were the same: so that if, for a moment, the
mercury might be supposed to supply the metallic contact, the inversion of
the amalgamated piece destroys that objection. The use of _unamalgamated
zinc_ (880.) removes all possibility of doubt[A].
[A] The following is a more striking mode of making the above
elementary experiment. Prepare a plate of zinc, ten or twelve inches
long and two inches wide, and clean it thoroughly: provide also two
discs of clean platina, about one inch and a half in diameter:--dip
three or four folds of bibulous paper into a strong solution of iodide
of potassium, place them on the clean zinc at one end of the plate,
and put on them one of the platina discs: finally dip similar folds of
paper or a piece of linen cloth into a mixture of equal parts nitric
acid and water, and place it at the other end of the zinc plate with
the second platina disc upon it. In this state of things no change at
the solution of the iodide will be perceptible; but if the two discs
be connected by a platina (or any other) wire for a second or two, and
then that over the iodide be raised, it will be found that the _whole_
of the surface beneath is deeply stained with _evolved iodine_.--_Dec.
1838._
884 When, in pursuance of other views (930.), the vessel _c_ was made to
contain a solution of caustic potash in place of acid, still the same
results occurred. Decomposition of the iodide was effected freely, though
there was no metallic contact of dissimilar metals, and the current of
electricity was in the _same direction_ as when acid was used at the place
of excitement.
885. Even a solution of common salt in the glass _c_ could produce all
these effects.
886. Having made a galvanometer with platina wires, and introduced it into
the course of the current between the platina plate and the place of
decomposition _x_, it was affected, giving indications of currents in the
same direction as those shown to exist by the chemical action.
887. If we consider these results generally, they lead to very important
conclusions. In the first place, they prove, in the most decisive manner,
that _metallic contact is not necessary for the production of the voltaic
current._ In the next place, they show a most extraordinary mutual relation
of the chemical affinities of the fluid which _excites_ the current, and
the fluid which is _decomposed_ by it.
888. For the purpose of simplifying the consideration, let us take the
experiment with amalgamated zinc. The metal so prepared exhibits no effect
until the current can pass: it at the same time introduces no new action,
but merely removes an influence which is extraneous to those belonging
either to the production or the effect of the electric current under
investigation (1000.); an influence also which, when present, tends only to
confuse the results.
889. Let two plates, one of amalgamated zinc and the other of platina, be
placed parallel to each other (fig. 74.), and introduce a drop of dilute
sulphuric acid, _y_, between them at one end: there will be no sensible
chemical action at that spot unless the two plates are connected somewhere
else, as at PZ, by a body capable of conducting electricity. If that body
be a metal or certain forms of carbon, then the current passes, and, as it
circulates through the fluid at _y_, decomposition ensues.
890. Then remove the acid from _y_, and introduce a drop of the solution of
iodide of potassium at _x_ (fig. 75.). Exactly the same set of effects
occur, except that when the metallic communication is made at PZ, the
electric current is in the opposite direction to what it was before, as is
indicated by the arrows, which show the courses of the currents (667.).
891. Now _both_ the solutions used are conductors, but the conduction in
them is essentially connected with decomposition (858.) in a certain
constant order, and therefore the appearance of the elements in certain
places _shows_ in what direction a current has passed when the solutions
are thus employed. Moreover, we find that when they are used at opposite
ends of the plates, as in the last two experiments (889. 890.), metallic
contact being allowed at the other extremities, the currents are in
opposite directions. We have evidently, therefore, the power of opposing
the actions of the two fluids simultaneously to each other at the opposite
ends of the plates, using each one as a conductor for the discharge of the
current of electricity, which the other tends to generate; in fact,
substituting them for metallic contact, and combining both experiments into
one (fig. 76.). Under these circumstances, there is an opposition of
forces: the fluid, which brings into play the stronger set of chemical
affinities for the zinc, (being the dilute acid,) overcomes the force of
the other, and determines the formation and direction of the electric
current; not merely making that current pass through the weaker liquid, but
actually reversing the tendency which the elements of the latter have in
relation to the zinc and platina if not thus counteracted, and forcing them
in the contrary direction to that they are inclined to follow, that its own
current may have free course. If the dominant action at _y_ be removed by
making metallic contact there, then the liquid at _x_ resumes its power; or
if the metals be not brought into contact at _y_ but the affinities of the
solution there weakened, whilst those active _x_ are strengthened, then the
latter gains the ascendency, and the decompositions are produced in a
contrary order.
892. Before drawing a _final_ conclusion from this mutual dependence and
state of the chemical affinities of two distant portions of acting fluids
(916.), I will proceed to examine more minutely the various circumstances
under which the re-action of the body suffering decomposition is rendered
evident upon the action of the body, also undergoing decomposition, which
produces the voltaic current.
893. The use of _metallic contact_ in a single pair of plates, and the
cause of its great superiority above contact made by other kinds of matter,
become now very evident. When an amalgamated zinc plate is dipped into
dilute sulphuric acid, the force of chemical affinity exerted between the
metal and the fluid is not sufficiently powerful to cause sensible action
at the surfaces of contact, and occasion the decomposition of water by the
oxidation of the metal, although it _is_ sufficient to produce such a
condition of the electricity (or the power upon which chemical affinity
depends) as would produce a current if there were a path open for it (916.
956.); and that current would complete the conditions necessary, under the
circumstances, for the decomposition of the water.
894. Now the presence of a piece of platina touching both the zinc and the
fluid to be decomposed, opens the path required for the electricity. Its
_direct communication_ with the zinc is effectual, far beyond any
communication made between it and that metal, (i.e. between the platina
and zinc,) by means of decomposable conducting bodies, or, in other words,
_electrolytes_, as in the experiment already described (891.); because,
when _they_ are used, the chemical affinities between them and the zinc
produce a contrary and opposing action to that which is influential in the
dilute sulphuric acid; or if that action be but small, still the affinity
of their component parts for each other has to be overcome, for they cannot
conduct without suffering decomposition; and this decomposition is found
_experimentally_ to re-act back upon the forces which in the acid tend to
produce the current (904. 910. &c.), and in numerous cases entirely to
neutralize them. Where direct contact of the zinc and platina occurs, these
obstructing forces are not brought into action, and therefore the
production and the circulation of the electric current and the concomitant
action of decomposition are then highly favoured.
895. It is evident, however, that one of these opposing actions may be
dismissed, and yet an electrolyte be used for the purpose of completing the
circuit between the zinc and platina immersed separately into the dilute
acid; for if, in fig. 73, the platina wire be retained in metallic contact
with the zinc plate _a_, at _x_, and a division of the platina be made
elsewhere, as at _s_, then the solution of iodide placed there, being in
contact with platina at both surfaces, exerts no chemical affinities for
that metal; or if it does, they are equal on both sides. Its power,
therefore, of forming a current in opposition to that dependent upon the
action of the acid in the vessel _c_, is removed, and only its resistance
to decomposition remains as the obstacle to be overcome by the affinities
exerted in the dilute sulphuric acid.
896. This becomes the condition of a single pair of active plates where
_metallic contact_ is allowed. In such cases, only one set of opposing
affinities are to be overcome by those which are dominant in the vessel
_c_; whereas, when metallic contact is not allowed, two sets of opposing
affinities must be conquered (894.).
897. It has been considered a difficult, and by some an impossible thing,
to decompose bodies by the current from a single pair of plates, even when
it was so powerful as to heat bars of metal red-hot, as in the case of
Hare's calorimeter, arranged as a single voltaic circuit, or of Wollaston's
powerful single pair of metals. This difficulty has arisen altogether from
the antagonism of the chemical affinity engaged in producing the current
with the chemical affinity to be overcome, and depends entirely upon their
relative intensity; for when the sum of forces in one has a certain degree
of superiority over the sum of forces in the other, the former gain the
ascendency, determine the current, and overcome the latter so as to make
the substance exerting them yield up its elements in perfect accordance,
both as to direction and quantity, with the course of those which are
exerting the most intense and dominant action.
898. Water has generally been the substance, the decomposition of which has
been sought for as a chemical test of the passage of an electric current.
But I now began to perceive a reason for its failure, and for a fact which
I had observed long before (315. 316.) with regard to the iodide of
potassium, namely, that bodies would differ in facility of decomposition by
a given electric current, according to the condition and intensity of their
ordinary chemical affinities. This reason appeared in their _re-action upon
the affinities_ tending to cause the current; and it appeared probable,
that many substances might be found which could be decomposed by the
current of a single pair of zinc and platina plates immersed in dilute
sulphuric acid, although water resisted its action. I soon found this to be
the case, and as the experiments offer new and beautiful proofs of the
direct relation and opposition of the chemical affinities concerned in
producing and in resisting the stream of electricity, I shall briefly
describe them.
899. The arrangement of the apparatus was as in fig. 77. The vessel _v_
contained dilute sulphuric acid; Z and P are the zinc and platina plates;
_a_, _b_, and _c_ are platina wires; the decompositions were effected at
_x_, and occasionally, indeed generally, a galvanometer was introduced into
the circuit at _g_: its place only is here given, the circle at _g_ having
no reference to the size of the instrument. Various arrangements were made
at _x_, according to the kind of decomposition to be effected. If a drop of
liquid was to be acted upon, the two ends were merely dipped into it; if a
solution contained in the pores of paper was to be decomposed, one of the
extremities was connected with a platina plate supporting the paper, whilst
the other extremity rested on the paper, _e_, fig. 81: or sometimes, as
with sulphate of soda, a plate of platina sustained two portions of paper,
one of the ends of the wires resting upon each piece, _c_, fig. 86. The
darts represent the direction of the electric current (667.).
900. Solution of _iodide of potassium_, in moistened paper, being placed at
the interruption of the circuit at _x_, was readily decomposed. Iodine was
evolved at the _anode_, and alkali at the _cathode_, of the decomposing
body.
901. _Protochloride of tin_, when fused and placed at _x_, was also readily
decomposed, yielding perchloride of tin at the _anode_ (779.), and tin at
the _cathode_.
902. Fused chloride of silver, placed at _x_, was also easily decomposed;
chlorine was evolved at the _anode_, and brilliant metallic silver, either
in films upon the surface of the liquid, or in crystals beneath, evolved at
the _cathode_.
903. Water acidulated with sulphuric acid, solution of muriatic acid,
solution of sulphate of soda, fused nitre, and the fused chloride and
iodide of lead were not decomposed by this single pair of plates, excited
only by dilute sulphuric acid.
904. These experiments give abundant proofs that a single pair of plates
can electrolyze bodies and separate their elements. They also show in a
beautiful manner the direct relation and opposition of the chemical
affinities concerned at the two points of action. In those cases where the
sum of the opposing affinities at _x_ was sufficiently beneath the sum of
the acting affinities in _v_, decomposition took place; but in those cases
where they rose higher, decomposition was effectually resisted and the
current ceased to pass (891.).
905. It is however, evident, that the sum of acting affinities in _v_ may
be increased by using other fluids than dilute sulphuric acid, in which
latter case, as I believe, it is merely the affinity of the zinc for the
oxygen already combined with hydrogen in the water that is exerted in
producing the electric current (919.): and when the affinities are so
increased, the view I am supporting leads to the conclusion, that bodies
which resisted in the preceding experiments would then be decomposed,
because of the increased difference between their affinities and the acting
affinities thus exalted. This expectation was fully confirmed in the
following manner.
906. A little nitric acid was added to the liquid in the vessel _r_, so as
to make a mixture which I shall call diluted nitro-sulphuric acid. On
repeating the experiments with this mixture, all the substances before
decomposed again gave way, and much more readily. But, besides that, many
which before resisted electrolyzation, now yielded up their elements. Thus,
solution of sulphate of soda, acted upon in the interstices of litmus and
turmeric paper, yielded acid at the _anode_ and alkali at the _cathode_;
solution of muriatic acid tinged by indigo yielded chlorine at the _anode_
and hydrogen at the _cathode_; solution of nitrate of silver yielded silver
at the _cathode_. Again, fused nitre and the fused iodide and chloride of
lead were decomposable by the current of this single pair of plates, though
they were not by the former (903.).
907. A solution of acetate of lead was apparently not decomposed by this
pair, nor did water acidulated by sulphuric acid seem at first to give way
(973.).
908. The increase of intensity or power of the current produced by a simple
voltaic circle, with the increase of the force of the chemical action at
the exciting place, is here sufficiently evident. But in order to place it
in a clearer point of view, and to show that the decomposing effect was not
at all dependent, in the latter cases, upon the mere capability of evolving
_more_ electricity, experiments were made in which the quantity evolved
could be increased without variation in the intensity of the exciting
cause. Thus the experiments in which dilute sulphuric acid was used (899.),
were repeated, using large plates of zinc and platina in the acid; but
still those bodies which resisted decomposition before, resisted it also
under these new circumstances. Then again, where nitro-sulphuric acid was
used (906.), mere wires of platina and zinc were immersed in the exciting
acid; yet, notwithstanding this change, those bodies were now decomposed
which resisted any current tending to be formed by the dilute sulphuric
acid. For instance, muriatic acid could not be decomposed by a single pair
of plates when immersed in dilute sulphuric acid; nor did making the
solution of sulphuric acid strong, nor enlarging the size of the zinc and
platina plates immersed in it, increase the power; but if to a weak
sulphuric acid a very little nitric acid was added, then the electricity
evolved had power to decompose the muriatic acid, evolving chlorine at the
_anode_ and hydrogen at the _cathode_, even when mere wires of metals were
used. This mode of increasing the intensity of the electric current, as it
excludes the effect dependent upon many pairs of plates, or even the effect
of making any one acid stronger or weaker, is at once referable to the
condition and force of the chemical affinities which are brought into
action, and may, both in principle and practice, be considered as perfectly
distinct from any other mode.
909. The direct reference which is thus experimentally made in the simple
voltaic circle of the _intensity_ of the electric current to the
_intensity_ of the chemical action going on at the place where the
existence and direction of the current is determined, leads to the
conclusion that by using selected bodies, as fused chlorides, salts,
solutions of acids, &c., which may act upon the metals employed with
different degrees of chemical force; and using also metals in association
with platina, or with each other, which shall differ in the degree of
chemical action exerted between them and the exciting fluid or electrolyte,
we shall be able to obtain a series of comparatively constant effects due
to electric currents of different intensities, which will serve to assist
in the construction of a scale competent to supply the means of determining
relative degrees of intensity with accuracy in future researches[A].
[A] In relation to this difference and its probable cause, see
considerations on inductive polarization, 1354, &c.--_Dec. 1838._
910. I have already expressed the view which I take of the decomposition in
the experimental place, as being the direct consequence of the superior
exertion at some other spot of the same kind of power as that to be
overcome, and therefore as the result of an antagonism of forces of the
_same_ nature (891. 904.). Those at the place of decomposition have a
re-action upon, and a power over, the exerting or determining set
proportionate to what is needful to overcome their own power; and hence a
curious result of _resistance_ offered by decompositions to the original
determining force, and consequently to the current. This is well shown in
the cases where such bodies as chloride of lead, iodide of lead, and water
would not decompose with the current produced by a single pair of zinc and
platina plates in sulphuric acid (903.), although they would with a current
of higher intensity produced by stronger chemical powers. In such cases no
sensible portion of the current passes (967.); the action is stopped; and I
am now of opinion that in the case of the law of conduction which I
described in the Fourth Series of these Researches (413.), the bodies which
are electrolytes in the fluid state cease to be such in the solid form,
because the attractions of the particles by which they are retained in
combination and in their relative position, are then too powerful for the
electric current[A]. The particles retain their places; and as
decomposition is prevented, the transmission of the electricity is
prevented also; and although a battery of many plates may be used, yet if
it be of that perfect kind which allows of no extraneous or indirect action
(1000.), the whole of the affinities concerned in the activity of that
battery are at the same time also suspended and counteracted.
[A] Refer onwards to 1705.--_Dec. 1838._
911. But referring to the _resistance_ of each single case of
decomposition, it would appear that as these differ in force according to
the affinities by which the elements in the substance tend to retain their
places, they also would supply cases constituting a series of degrees by
which to measure the initial intensities of simple voltaic or other
currents of electricity, and which, combined with the scale of intensities
determined by different degrees of _acting force_ (909.), would probably
include a sufficient set of differences to meet almost every important case
where a reference to intensity would be required.
912. According to the experiments I have already had occasion to make, I
find that the following bodies are electrolytic in the order in which I
have placed them, those which are first being decomposed by the current of
lowest intensity. These currents were always from a single pair of plates,
and may be considered as elementary _voltaic forces_.
Iodide of potassium (solution).
Chloride of silver (fused).
Protochloride of tin (fused).
Chloride of lead (fused).
Iodide of lead (fused).
Muriatic acid (solution).
Water, acidulated with sulphuric acid.
913. It is essential that, in all endeavours to obtain the relative
electrolytic intensity necessary for the decomposition of different bodies,
attention should be paid to the nature of the electrodes and the other
bodies present which may favour secondary actions (986.). If in
electro-decomposition one of the elements separated has an affinity for the
electrode, or for bodies present in the surrounding fluid, then the
affinity resisting decomposition is in part balanced by such power, and the
true place of the electrolyte in a table of the above kind is not obtained:
thus, chlorine combines with a positive platina electrode freely, but
iodine scarcely at all, and therefore I believe it is that the fused
chlorides stand first in the preceding Table. Again, if in the
decomposition of water not merely sulphuric but also a little nitric acid
be present, then the water is more freely decomposed, for the hydrogen at
the _cathode_ is not ultimately expelled, but finds oxygen in the nitric
acid, with which it can combine to produce a secondary result; the
affinities opposing decomposition are in this way diminished, and the
elements of the water can then be separated by a current of lower
intensity.
914. Advantage may be taken of this principle to interpolate more minute
degrees into the scale of initial intensities already referred to (909.
911.) than is there spoken of; for by combining the force of a current
_constant_ in its intensity, with the use of electrodes consisting of
matter, having more or less affinity for the elements evolved from the
decomposing electrolyte, various intermediate degrees may be obtained.
* * * * *
915. Returning to the consideration of the source of electricity (878.
&c.), there is another proof of the most perfect kind that metallic contact
has nothing to do with the _production_ of electricity in the voltaic
circuit, and further, that electricity is only another mode of the exertion
of chemical forces. It is, the production of the _electric spark_ before
any contact of metals is made, and by the exertion of _pure and unmixed
chemical forces_. The experiment, which will be described further on
(956.), consists in obtaining the spark upon making contact between a plate
of zinc and a plate of copper plunged into dilute sulphuric acid. In order
to make the arrangement as elementary as possible, mercurial surfaces were
dismissed, and the contact made by a copper wire connected with the copper
plate, and then brought to touch a clean part of the zinc plate. The
electric spark appeared, and it must of necessity have existed and passed
_before the zinc and the copper were in contact_.
916. In order to render more distinct the principles which I have been
endeavouring to establish, I will restate them in their simplest form,
according to my present belief. The electricity of the voltaic pile (856.
note) is not dependent either in its origin or its continuance upon the
contact of the metals with each other (880. 915.). It is entirely due to
chemical action (882.), and is proportionate in its intensity to the
intensity of the affinities concerned in its production (908.); and in its
quantity to the quantity of matter which has been chemically active during
its evolution (869.). This definite production is again one of the
strongest proofs that the electricity is of chemical origin.
917. As _volta-electro-generation_ is a case of mere chemical action, so
_volta-electro-decomposition_ is simply a case of the preponderance of one
set of chemical affinities more powerful in their nature, over another set
which are less powerful: and if the instance of two opposing sets of such
forces (891.) be considered, and their mutual relation and dependence borne
in mind, there appears no necessity for using, in respect to such cases,
any other term than chemical affinity, (though that of electricity may be
very convenient,) or supposing any new agent to be concerned in producing
the results; for we may consider that the powers at the two places of
action are in direct communion and balanced against each other through the
medium of the metals (891.), fig. 76, in a manner analogous to that in
which mechanical forces are balanced against each other by the intervention
of the lever (1031.).
918. All the facts show us that that power commonly called chemical
affinity, can be communicated to a distance through the metals and certain
forms of carbon; that the electric current is only another form of the
forces of chemical affinity; that its power is in proportion to the
chemical affinities producing it; that when it is deficient in force it may
be helped by calling in chemical aid, the want in the former being made up
by an equivalent of the latter; that, in other words, _the forces termed
chemical affinity and electricity are one and the same._
919. When the circumstances connected with the production of electricity in
the ordinary voltaic circuit are examined and compared, it appears that the
source of that agent, always meaning the electricity which circulates and
completes the current in the voltaic apparatus, and gives that apparatus
power and character (947. 996.), exists in the chemical action which takes
place directly between the metal and the body with which it combines, and
not at all in the subsequent action of the substance so produced with the
acid present[A]. Thus, when zinc, platina, and dilute sulphuric acid are
used, it is the union of the zinc with the oxygen of the water which
determines the current; and though the acid is essential to the removal of
the oxide so formed, in order that another portion of zinc may act on
another portion of water, it does not, by combination with that oxide,
produce any sensible portion of the current of electricity which
circulates; for the quantity of electricity is dependent upon the quantity
of zinc oxidized, and in definite proportion to it: its intensity is in
proportion to the intensity of the chemical affinity of the zinc for the
oxygen under the circumstances, and is scarcely, if at all, affected by the
use of either strong or weak acid (908.).
[A] Wollaston, Philosophical Transactions, 1801, p. 427.
920. Again, if zinc, platina, and muriatic acid are used, the electricity
appears to be dependent upon the affinity of the zinc for the chlorine, and
to be circulated in exact proportion to the number of particles of zinc and
chlorine which unite, being in fact an equivalent to them.
921. But in considering this oxidation, or other direct action upon the
METAL itself, as the cause and source of the electric current, it is of the
utmost importance to observe that the oxygen or other body must be in a
peculiar condition, namely, in the state of _combination_; and not only so,
but limited still further to such a state of combination and in such
proportions as will constitute an _electrolyte_ (823.). A pair of zinc and
platina plates cannot be so arranged in oxygen gas as to produce a current
of electricity, or act as a voltaic circle, even though the temperature may
be raised so high as to cause oxidation of the zinc far more rapidly than
if the pair of plates were plunged into dilute sulphuric acid; for the
oxygen is not part of an electrolyte, and cannot therefore conduct the
forces onwards by decomposition, or even as metals do by itself. Or if its
gaseous state embarrass the minds of some, then liquid chlorine may be
taken. It does not excite a current of electricity through the two plates
by combining with the zinc, for its particles cannot transfer the
electricity active at the point of combination across to the platina. It is
not a conductor of itself, like the metals; nor is it an electrolyte, so as
to be capable of conduction during decomposition, and hence there is simple
chemical action at the spot, and no electric current[A].
[A] I do not mean to affirm that no traces of electricity ever appear
in such cases. What I mean is, that no electricity is evolved in any
way, due or related to the causes which excite voltaic electricity, or
proportionate to them. That which does appear occasionally is the
smallest possible fraction of that which the acting matter could
produce if arranged so as to act voltaically, probably not the one
hundred thousandth, or even the millionth part, and is very probably
altogether different in its source.
922. It might at first be supposed that a conducting body not electrolytic,
might answer as the third substance between the zinc and the platina; and
it is true that we have some such capable of exerting chemical action upon
the metals. They must, however, be chosen from the metals themselves, for
there are no bodies of this kind except those substances and charcoal. To
decide the matter by experiment, I made the following arrangement. Melted
tin was put into a glass tube bent into the form of the letter V, fig. 78,
so as to fill the half of each limb, and two pieces of thick platina wire,
_p_, _w_, inserted, so as to have their ends immersed some depth in the
tin: the whole was then allowed to cool, and the ends _p_ and _w_ connected
with a delicate galvanometer. The part of the tube at _x_ was now reheated,
whilst the portion _y_ was retained cool. The galvanometer was immediately
influenced by the thermo-electric current produced. The heat was steadily
increased at _x_, until at last the tin and platina combined there; an
effect which is known to take place with strong chemical action and high
ignition; but not the slightest additional effect occurred at the
galvanometer. No other deflection than that due to the thermo-electric
current was observable the whole time. Hence, though a conductor, and one
capable of exerting chemical action on the tin, was used, yet, not being an
_electrolyte_, not the slightest effect of an electrical current could be
observed (947.).
923. From this it seems apparent that the peculiar character and condition
of an electrolyte is _essential_ in one part of the voltaic circuit; and
its nature being considered, good reasons appear why it and it alone should
be effectual. An electrolyte is always a compound body: it can conduct, but
only whilst decomposing. Its conduction depends upon its decomposition and
the _transmission of its particles_ in directions parallel to the current;
and so intimate is this connexion, that if their transition be stopped, the
current is stopped also; if their course be changed, its course and
direction change with them; if they proceed in one direction, it has no
power to proceed in any other than a direction invariably dependent on
them. The particles of an electrolytic body are all so mutually connected,
are in such relation with each other through their whole extent in the
direction of the current, that if the last is not disposed of, the first is
not at liberty to take up its place in the new combination which the
powerful affinity of the most active metal tends to produce; and then the
current itself is stopped; for the dependencies of the current and the
decomposition are so mutual, that whichsoever be originally determined,
i.e. the motion of the particles or the motion of the current, the other is
invariable in its concomitant production and its relation to it.
924. Consider, then, water as an electrolyte and also as an oxidizing body.
The attraction of the zinc for the oxygen is greater, under the
circumstances, than that of the oxygen for the hydrogen; but in combining
with it, it tends to throw into circulation a current of electricity in a
certain direction. This direction is consistent (as is found by innumerable
experiments) with the transfer of the hydrogen from the zinc towards the
platina, and the transfer in the opposite direction of fresh oxygen from
the platina towards the zinc; so that the current _can pass_ in that one
line, and, whilst it passes, can consist with and favour the renewal of the
conditions upon the surface of the zinc, which at first determined both the
combination and circulation. Hence the continuance of the action there, and
the continuation of the current. It therefore appears quite as essential
that there should be an electrolyte in the circuit, in order that the
action may be transferred forward, in a _certain constant direction,_ as
that there should be an oxidizing or other body capable of acting directly
on the metal; and it also appears to be essential that these two should
merge into one, or that the principle directly active on the metal by
chemical action should be one of the _ions_ of the electrolyte used.
Whether the voltaic arrangement be excited by solution of acids, or
alkalies, or sulphurets, or by fused substances (476.), this principle has
always hitherto, as far as I am aware, been an _anion_ (943.); and I
anticipate, from a consideration of the principles of electric action, that
it must of necessity be one of that class of bodies.
925. If the action of the sulphuric acid used in the voltaic circuit be
considered, it will be found incompetent to produce any sensible portion of
the electricity of the current by its combination with the oxide formed,
for this simple reason, it is deficient in a most essential condition: it
forms no part of an electrolyte, nor is it in relation with any other body
present in the solution which will permit of the mutual transfer of the
particles and the consequent transfer of the electricity. It is true, that
as the plane at which the acid is dissolving the oxide of zinc formed by
the action of the water, is in contact with the metal zinc, there seems no
difficulty in considering how the oxide there could communicate an
electrical state, proportionate to its own chemical action on the acid, to
the metal, which is a conductor without decomposition. But on the side of
the acid there is no substance to complete the circuit: the water, as
water, cannot conduct it, or at least only so small a proportion that it is
merely an incidental and almost inappreciable effect (970.); and it cannot
conduct it as an electrolyte, because an electrolyte conducts in
consequence of the _mutual_ relation and action of its particles; and
neither of the elements of the water, nor even the water itself, as far as
we can perceive, are _ions_ with respect to the sulphuric acid (848.)[A].
[A] It will be seen that I here agree with Sir Humphry Davy, who has
experimentally supported the opinion that acids and alkalies in
combining do not produce any current of electricity. Philosophical
Transactions, 1826, p. 398.
926. This view of the secondary character of the sulphuric acid as an agent
in the production of the voltaic current, is further confirmed by the fact,
that the current generated and transmitted is directly and exactly
proportional to the quantity of water decomposed and the quantity of zinc
oxidized (868. 991.), and is the same as that required to decompose the
same quantity of water. As, therefore, the decomposition of the water shows
that the electricity has passed by its means, there remains no other
electricity to be accounted for or to be referred to any action other than
that of the zinc and the water on each other.
927. The general case (for it includes the former one (924.),) of acids and
bases, may theoretically be stated in the following manner. Let _a_, fig.
79, be supposed to be a dry oxacid, and _b_ a dry base, in contact at _c_,
and in electric communication at their extremities by plates of platina
_pp_, and a platina wire _w_. If this acid and base were fluid, and
combination took place at _c_, with an affinity ever so vigorous, and
capable of originating an electric current, the current could not circulate
in any important degree; because, according to the experimental results,
neither _a_ nor _b_ could conduct without being decomposed, for they are
either electrolytes or else insulators, under all circumstances, except to
very feeble and unimportant currents (970. 986.). Now the affinities at _c_
are not such as tend to cause the _elements_ either of _a_ or _b_ to
separate, but only such as would make the two bodies combine together as a
whole; the point of action is, therefore, insulated, the action itself
local (921. 947.), and no current can be formed.
928. If the acid and base be dissolved in water, then it is possible that a
small portion of the electricity due to chemical action may be conducted by
the water without decomposition (966. 984.); but the quantity will be so
small as to be utterly disproportionate to that due to the equivalents of
chemical force; will be merely incidental; and, as it does not involve the
essential principles of the voltaic pile, it forms no part of the phenomena
at present under investigation[A].
[A] It will I trust be fully understood, that in these investigations
I am not professing to take an account of every small, incidental, or
barely possible effect, dependent upon slight disturbances of the
electric fluid during chemical action, but am seeking to distinguish
and identify those actions on which the power of the voltaic battery
essentially depends.
929. If for the oxacid a hydracid be substituted (927.),--as one analogous
to the muriatic, for instance,--then the state of things changes
altogether, and a current due to the chemical action of the acid on the
base is possible. But now both the bodies act as electrolytes, for it is
only one principle of each which combine mutually,--as, for instance, the
chlorine with the metal,--and the hydrogen of the acid and the oxygen of
the base are ready to traverse with the chlorine of the acid and the metal
of the base in conformity with the current and according to the general
principles already so fully laid down.
930. This view of the oxidation of the metal, or other _direct_ chemical
action upon it, being the sole cause of the production of the electric
current in the ordinary voltaic pile, is supported by the effects which
take place when alkaline or sulphuretted solutions (931. 943.) are used for
the electrolytic conductor instead of dilute sulphuric acid. It was in
elucidation of this point that the experiments without metallic contact,
and with solution of alkali as the exciting fluid, already referred to
(884.), were made.
931. Advantage was then taken of the more favourable condition offered,
when metallic contact is allowed (895.), and the experiments upon the
decomposition of bodies by a single pair of plates (899.) were repeated,
solution of caustic potassa being employed in the vessel _v_, fig. 77. in
place of dilute sulphuric acid. All the effects occurred as before: the
galvanometer was deflected; the decompositions of the solutions of iodide
of potassium, nitrate of silver, muriatic acid, and sulphate of soda ensued
at _x_; and the places where the evolved principles appeared, as well as
the deflection of the galvanometer, indicated a current in the _same
direction_ as when acid was in the vessel _v_; i.e. from the zinc through
the solution to the platina, and back by the galvanometer and substance
suffering decomposition to the zinc.
932. The similarity in the action of either dilute sulphuric acid or
potassa goes indeed far beyond this, even to the proof of identity in
_quantity_ as well as in _direction_ of the electricity produced. If a
plate of amalgamated zinc be put into a solution of potassa, it is not
sensibly acted upon; but if touched in the solution by a plate of platina,
hydrogen is evolved on the surface of the latter metal, and the zinc is
oxidized exactly as when immersed in dilute sulphuric acid (863.). I
accordingly repeated the experiment before described with weighed plates of
zinc (864. &c.), using however solution of potassa instead of dilute
sulphuric acid. Although the time required was much longer than when acid
was used, amounting to three hours for the oxidizement of 7.55 grains of
zinc, still I found that the hydrogen evolved at the platina plate was the
equivalent of the metal oxidized at the surface of the zinc. Hence the
whole of the reasoning which was applicable in the former instance applies
also here, the current being in the same direction, and its decomposing
effect in the same degree, as if acid instead of alkali had been used
(868.).
933. The proof, therefore, appears to me complete, that the combination of
the acid with the oxide, in the former experiment, had nothing to do with
the production of the electric current; for the same current is here
produced when the action of the acid is absent, and the reverse action of
an alkali is present. I think it cannot be supposed for a moment, that the
alkali acted chemically as an acid to the oxide formed; on the contrary,
our general chemical knowledge leads to the conclusion, that the ordinary
metallic oxides act rather as acids to the alkalies; yet that kind of
action would tend to give a reverse current in the present case, if any
were due to the union of the oxide of the exciting metal with the body
which combines with it. But instead of any variation of this sort, the
direction of the electricity was constant, and its quantity also directly
proportional to the water decomposed, or the zinc oxidized. There are
reasons for believing that acids and alkalies, when in contact with metals
upon which they cannot act directly, still have a power of influencing
their attractions for oxygen (941.); but all the effects in these
experiments prove, I think, that it is the oxidation of the metal
necessarily dependent upon, and associated as it is with, the
electrolyzation of the water (921. 923.) that produces the current; and
that the acid or alkali merely acts as solvents, and by removing the
oxidized zinc, allows other portions to decompose fresh water, and so
continues the evolution or determination of the current.
934. The experiments were then varied by using solution of ammonia instead
of solution of potassa; and as it, when pure, is like water, a bad
conductor (554.), it was occasionally improved in that power by adding
sulphate of ammonia to it. But in all the cases the results were the same
as before; decompositions of the same kind were effected, and the electric
current producing these was in the same direction as in the experiments
just described.
935. In order to put the equal and similar action of acid and alkali to
stronger proof, arrangements were made as in fig. 80.; the glass vessel A
contained dilute sulphuric acid, the corresponding glass vessel B solution
of potassa, PP was a plate of platina dipping into both solutions, and ZZ
two plates of amalgamated zinc connected with a delicate galvanometer. When
these were plunged at the same time into the two vessels, there was
generally a first feeble effect, and that in favour of the alkali, i.e. the
electric current tended to pass through the vessels in the direction of the
arrow, being the reverse direction of that which the acid in A would have
produced alone: but the effect instantly ceased, and the action of the
plates in the vessels was so equal, that, being contrary because of the
contrary position of the plates, no permanent current resulted.
936. Occasionally a zinc plate was substituted for the plate PP, and
platina plates for the plates ZZ; but this caused no difference in the
results: nor did a further change of the middle plate to copper produce any
alteration.
937. As the opposition of electro-motive pairs of plates produces results
other than those due to the mere difference of their independent actions
(1011. 1045.), I devised another form of apparatus, in which the action of
acid and alkali might be more directly compared. A cylindrical glass cup,
about two inches deep within, an inch in internal diameter, and at least a
quarter of an inch in thickness, was cut down the middle into halves, fig.
81. A broad brass ring, larger in diameter than the cup, was supplied with
a screw at one side; so that when the two halves of the cup were within the
ring, and the screw was made to press tightly against the glass, the cup
held any fluid put into it. Bibulous paper of different degrees of
permeability was then cut into pieces of such a size as to be easily
introduced between the loosened halves of the cup, and served when the
latter were tightened again to form a porous division down the middle of
the cup, sufficient to keep any two fluids on opposite sides of the paper
from mingling, except very slowly, and yet allowing them to act freely as
one _electrolyte_. The two spaces thus produced I will call the cells A and
B, fig. 82. This instrument I have found of most general application in the
investigation of the relation of fluids and metals amongst themselves and
to each other. By combining its use with that of the galvanometer, it is
easy to ascertain the relation of one metal with two fluids, or of two
metals with one fluid, or of two metals and two fluids upon each other.
938. Dilute sulphuric acid, sp. gr. 1.25, was put into the cell A, and a
strong solution of caustic potassa into the cell B; they mingled slowly
through the paper, and at last a thick crust of sulphate of potassa formed
on the side of the paper next to the alkali. A plate of clean platina was
put into each cell and connected with a delicate galvanometer, but no
electric current could be observed. Hence the _contact_ of acid with one
platina plate, and alkali with the other, was unable to produce a current;
nor was the combination of the acid with the alkali more effectual (925.).
939. When one of the platina plates was removed and a zinc plate
substituted, either amalgamated or not, a strong electric current was
produced. But, whether the zinc were in the acid whilst the platina was in
the alkali, or whether the reverse order were chosen, the electric current
was always from the zinc through the electrolyte to the platina, and back
through the galvanometer to the zinc, the current seeming to be strongest
when the zinc was in the alkali and the platina in the acid.
940. In these experiments, therefore, the acid seems to have no power over
the alkali, but to be rather inferior to it in force. Hence there is no
reason to suppose that the combination of the oxide formed with the acid
around it has any direct influence in producing the electricity evolved,
the whole of which appears to be due to the oxidation of the metal (919.).
941. The alkali, in fact, is superior to the acid in bringing a metal into
what is called the positive state; for if plates of the same metal, as
zinc, tin, lead, or copper, be used both in the acid or alkali, the
electric current is from the alkali across the cell to the acid, and back
through the galvanometer to the alkali, as Sir Humphry Davy formerly stated
[A]. This current is so powerful, that if amalgamated zinc, or tin, or lead
be used, the metal in the acid evolves hydrogen the moment it is placed in
communication with that in the alkali, not from any direct action of the
acid upon it, for if the contact be broken the action ceases, but because
it is powerfully negative with regard to the metal in the alkali.
[A] Elements of Chemical Philosophy, p. 149; or Philosophical
Transactions, 1826, p. 403.
942. The superiority of alkali is further proved by this, that if zinc and
tin be used, or tin and lead, whichsoever metal is put into the alkali
becomes positive, that in the acid being negative. Whichsoever is in the
alkali is oxidized, whilst that in the acid remains in the metallic state,
as far as the electric current is concerned.
943. When sulphuretted solutions are used (930.) in illustration of the
assertion, that it is the chemical action of the metal and one of the
_ions_ of the associated electrolyte that produces all the electricity of
the voltaic circuit, the proofs are still the same. Thus, as Sir Humphry
Davy[A] has shown, if iron and copper be plunged into dilute acid, the
current is from the iron through the liquid to the copper; in solution of
potassa it is in the same direction, but in solution of sulphuret of
potassa it is reversed. In the two first cases it is oxygen which combines
with the iron, in the latter sulphur which combines with the copper, that
produces the electric current; but both of these are _ions_, existing as
such in the electrolyte, which is at the same moment suffering
decomposition; and, what is more, both of these are _anions_, for they
leave the electrolytes at their _anodes_, and act just as chlorine, iodine,
or any other _anion_ would act which might have been previously chosen as
that which should be used to throw the voltaic circle into activity.
[A] Elements of Chemical Philosophy, p. 148.
944. The following experiments complete the series of proofs of the origin
of the electricity in the voltaic pile. A fluid amalgam of potassium,
containing not more than a hundredth of that metal, was put into pure
water, and connected, through the galvanometer with a plate of platina in
the same water. There was immediately an electric current from the amalgam
through the electrolyte to the platina. This must have been due to the
oxidation only of the metal, for there was neither acid nor alkali to
combine with, or in any way act on, the body produced.
945. Again, a plate of clean lead and a plate of platina were put into
_pure_ water. There was immediately a powerful current produced from the
lead through the fluid to the platina: it was even intense enough to
decompose solution of the iodide of potassium when introduced into the
circuit in the form of apparatus already described (880.), fig. 73. Here no
action of acid or alkali on the oxide formed from the lead could supply the
electricity: it was due solely to the oxidation of the metal.
* * * * *
946. There is no point in electrical science which seems to me of more
importance than the state of the metals and the electrolytic conductor in a
simple voltaic circuit _before and at_ the moment when metallic contact is
first completed. If clearly understood, I feel no doubt it would supply us
with a direct key to the laws under which the great variety of voltaic
excitements, direct and incidental, occur, and open out new fields of
research for our investigation[A].
[A] In connexion with this part of the subject refer now to Series XI.
1164, Series XII. 1343-1358, and Series XIII. 1621. &c.--_Dec. 1838._
947. We seem to have the power of deciding to a certain extent in numerous
cases of chemical affinity, (as of zinc with the oxygen of water, &c. &c.)
which of _two modes of action of the attractive power_ shall be exerted
(996.). In the one mode we can transfer the power onwards, and make it
produce elsewhere its equivalent of action (867. 917.); in the other, it is
not transferred, but exerted wholly at the spot. The first is the case of
volta-electric excitation, the other ordinary chemical affinity: but both
are chemical actions and due to one force or principle.
948. The general circumstances of the former mode occur in all instances of
voltaic currents, but may be considered as in their perfect condition, and
then free from those of the second mode, in some only of the cases; as in
those of plates of zinc and platina in solution of potassa, or of
amalgamated zinc and platina in dilute sulphuric acid.
949. Assuming it sufficiently proved, by the preceding experiments and
considerations, that the electro-motive action depends, when zinc, platina,
and dilute sulphuric acid are used, upon the mutual affinity of the metal
zinc and the oxygen of the water (921. 924.), it would appear that the
metal, when alone, has not power enough, under the circumstances, to take
the oxygen and expel the hydrogen from the water; for, in fact, no such
action takes place. But it would also appear that it has power so far to
act, by its attraction for the oxygen of the particles in contact with it,
as to place the similar forces already active between these and the other
particles of oxygen and the particles of hydrogen in the water, in a
peculiar state of tension or polarity, and probably also at the same time
to throw those of its own particles which are in contact with the water
into a similar but opposed state. Whilst this state is retained, no further
change occurs; but when it is relieved, by completion of the circuit, in
which case the forces determined in opposite directions, with respect to
the zinc and the electrolyte, are found exactly competent to neutralize
each other, then a series of decompositions and recompositions takes place
amongst the particles of oxygen and hydrogen constituting the water,
between the place of contact with the platina and the place where the zinc
is active; these intervening particles being evidently in close dependence
upon and relation to each other. The zinc forms a direct compound with
those particles of oxygen which were, previously, in divided relation to
both it and the hydrogen: the oxide is removed by the acid, and a fresh
surface of zinc is presented to the water, to renew and repeat the action.
950. Practically, the state of tension is best relieved by dipping a metal
which has less attraction for oxygen than the zinc, into the dilute acid,
and making it also touch the zinc. The force of chemical affinity, which
has been influenced or polarized in the particles of the water by the
dominant attraction of the zinc for the oxygen, is then transferred, in a
most extraordinary manner, through the two metals, so as to re-enter upon
the circuit in the electrolytic conductor, which, unlike the metals in that
respect, cannot convey or transfer it without suffering decomposition; or
rather, probably, it is exactly balanced and neutralized by the force which
at the same moment completes the combination of the zinc with the oxygen of
the water. The forces, in fact, of the two particles which are acting
towards each other, and which are therefore in opposite directions, are the
origin of the two opposite forces, or directions of force, in the current.
They are of necessity equivalent to each other. Being transferred forward
in contrary directions, they produce what is called the voltaic current:
and it seems to me impossible to resist the idea that it must be preceded
by a _state of tension_ in the fluid, and between the fluid and the zinc;
the _first consequence_ of the affinity of the zinc for the oxygen of the
water.
951. I have sought carefully for indications of a state of tension in the
electrolytic conductor; and conceiving that it might produce something like
structure, either before or during its discharge, I endeavoured to make
this evident by polarized light. A glass cell, seven inches long, one inch
and a half wide, and six inches deep, had two sets of platina electrodes
adapted to it, one set for the ends, and the other for the sides. Those for
the _sides_ were seven inches long by three inches high, and when in the
cell were separated by a little frame of wood covered with calico; so that
when made active by connexion with a battery upon any solution in the cell,
the bubbles of gas rising from them did not obscure the central parts of
the liquid.
952. A saturated solution of sulphate of soda was put into the cell, and
the electrodes connected with a battery of 150 pairs of 4-inch plates: the
current of electricity was conducted across the cell so freely, that the
discharge was as good as if a wire had been used. A ray of polarized light
was then transmitted through this solution, directly across the course of
the electric current, and examined by an analysing plate; but though it
penetrated seven inches of solution thus subject to the action of the
electricity, and though contact was sometimes made, sometimes broken, and
occasionally reversed during the observations, not the slightest trace of
action on the ray could be perceived.
953. The large electrodes were then removed, and others introduced which
fitted the _ends_ of the cell. In each a slit was cut, so as to allow the
light to pass. The course of the polarized ray was now parallel to the
current, or in the direction of its axis (517.); but still no effect, under
any circumstances of contact or disunion, could be perceived upon it.
954. A strong solution of nitrate of lead was employed instead of the
sulphate of soda, but no effects could be detected.
955. Thinking it possible that the discharge of the electric forces by the
successive decompositions and recompositions of the particles of the
electrolyte might neutralize and therefore destroy any effect which the
first state of tension could by possibility produce, I took a substance
which, being an excellent electrolyte when fluid, was a perfect insulator
when solid, namely, borate of lead, in the form of a glass plate, and
connecting the sides and the edges of this mass with the metallic plates,
sometimes in contact with the poles of a voltaic battery, and sometimes
even with the electric machine, for the advantage of the much higher
intensity then obtained, I passed a polarized ray across it in various
directions, as before, but could not obtain the slightest appearance of
action upon the light. Hence I conclude, that notwithstanding the new and
extraordinary state which must be assumed by an electrolyte, either during
decomposition (when a most enormous quantity of electricity must be
traversing it), or in the state of tension which is assumed as preceding
decomposition, and which might be supposed to be retained in the solid form
of the electrolyte, still it has no power of affecting a polarized ray of
light; for no kind of structure or tension can in this way be rendered
evident.
956. There is, however, one beautiful experimental proof of a state of
tension acquired by the metals and the electrolyte before the electric
current is produced, and _before contact_ of the different metals is made
(915.); in fact, at that moment when chemical forces only are efficient as
a cause of action. I took a voltaic apparatus, consisting of a single pair
of large plates, namely, a cylinder of amalgamated zinc, and a double
cylinder of copper. These were put into a jar containing dilute sulphuric
acid[A], and could at pleasure be placed in metallic communication by a
copper wire adjusted so as to dip at the extremities into two cups of
mercury connected with the two plates.
[A] When nitro-sulphuric acid is used, the spark is more powerful, but
local chemical action can then commence, and proceed without requiring
metallic contact.
957. Being thus arranged, there was no chemical action whilst the plates
were not connected. On _making_ the connexion a spark was obtained[A], and
the solution was immediately decomposed. On breaking it, the usual spark
was obtained, and the decomposition ceased. In this case it is evident that
the first spark must have occurred before metallic contact was made, for it
passed through an interval of air; and also that it must have tended to
pass before the electrolytic action began; for the latter could not take
place until the current passed, and the current could not pass before the
spark appeared. Hence I think there is sufficient proof, that as it is the
zinc and water which by their mutual action produce the electricity of this
apparatus, so these, by their first contact with each other, were placed in
a state of powerful tension (951.), which, though it could not produce the
actual decomposition of the water, was able to make a spark of electricity
pass between the zinc and a fit discharger as soon as the interval was
rendered sufficiently small. The experiment demonstrates the direct
production of the electric spark from pure chemical forces.
[A] It has been universally supposed that no spark is produced on
making the contact between a single pair of plates. I was led to
expect one from the considerations already advanced in this paper. The
wire of communication should be short; for with a long wire,
circumstances strongly affecting the spark are introduced.
958. There are a few circumstances connected with the production of this
spark by a single pair of plates, which should be known, to ensure success
to the experiment[B]. When the amalgamated surfaces of contact are quite
clean and dry, the spark, on making contact, is quite as brilliant as on
breaking it, if not even more so. When a film of oxide or dirt was present
at either mercurial surface, then the first spark was often feeble, and
often failed, the breaking spark, however, continuing very constant and
bright. When a little water was put over the mercury, the spark was greatly
diminished in brilliancy, but very regular both on making and breaking
contact. When the contact was made between clean platina, the spark was
also very small, but regular both ways. The true electric spark is, in
fact, very small, and when surfaces of mercury are used, it is the
combustion of the metal which produces the greater part of the light. The
circumstances connected with the burning of the mercury are most favourable
on breaking contact; for the act of separation exposes clean surfaces of
metal, whereas, on making contact, a thin film of oxide, or soiling matter,
often interferes. Hence the origin of the general opinion that it is only
when the contact is broken that the spark passes.
[B] See in relation to precautions respecting a spark, 1074.--_Dec.
1838._
959. With reference to the other set of cases, namely, those of local
action (947.) in which chemical affinity being exerted causes no
transference of the power to a distance where no electric current is
produced, it is evident that forces of the most intense kind must be
active, and in some way balanced in their activity, during such
combinations; these forces being directed so immediately and exclusively
towards each other, that no signs of the powerful electric current they can
produce become apparent, although the same final state of things is
obtained as if that current had passed. It was Berzelius, I believe, who
considered the heat and light evolved in cases of combustion as the
consequences of this mode of exertion of the electric powers of the
combining particles. But it will require a much more exact and extensive
knowledge of the nature of electricity, and the manner in which it is
associated with the atoms of matter, before we can understand accurately
the action of this power in thus causing their union, or comprehend the
nature of the great difference which it presents in the two modes of action
just distinguished. We may imagine, but such imaginations must for the time
be classed with the great mass of _doubtful knowledge_ (876.) which we
ought rather to strive to diminish than to increase; for the very extensive
contradictions of this knowledge by itself shows that but a small portion
of it can ultimately prove true[A].
[A] Refer to 1738, &c. Series XIV.--_Dec. 1838._
960. Of the two modes of action in which chemical affinity is exerted, it
is important to remark, that that which produces the electric current is as
_definite_ as that which causes ordinary chemical combination; so that in
examining the _production_ or _evolution_ of electricity in cases of
combination or decomposition, it will be necessary, not merely to observe
certain effects dependent upon a current of electricity, but also their
_quantity_: and though it may often happen that the forces concerned in any
particular case of chemical action may be partly exerted in one mode and
partly in the other, it is only those which are efficient in producing the
current that have any relation to voltaic action. Thus, in the combination
of oxygen and hydrogen to produce water, electric powers to a most enormous
amount are for the time active (861. 873.); but any mode of examining the
flame which they form during energetic combination, which has as yet been
devised, has given but the feeblest traces. These therefore may not,
cannot, be taken as evidences of the nature of the action; but are merely
incidental results, incomparably small in relation to the forces concerned,
and supplying no information of the way in which the particles are active
on each other, or in which their forces are finally arranged.
961. That such cases of chemical action produce no _current of
electricity_, is perfectly consistent with what we know of the voltaic
apparatus, in which it is essential that one of the combining elements
shall form part of, or be in direct relation with, an electrolytic
conductor (921. 923.). That such cases produce _no free electricity of
tension_, and that when they are converted into cases of voltaic action
they produce a current in which the opposite forces are so equal as to
neutralize each other, prove the equality of the forces in the opposed
acting particles of matter, and therefore the equality of electric power in
those quantities of matter which are called _electro-chemical equivalents_
(824). Hence another proof of the definite nature of electro-chemical
action (783. &c.), and that chemical affinity and electricity are forms of
the same power (917. &c.).
962. The direct reference of the effects produced by the voltaic pile at
the place of experimental decomposition to the chemical affinities active
at the place of excitation (891. 917.), gives a very simple and natural
view of the cause why the bodies (or _ions_) evolved pass in certain
directions; for it is only when they pass in those directions that their
forces can consist with and compensate (in direction at least) the superior
forces which are dominant at the place where the action of the whole is
determined. If, for instance, in a voltaic circuit, the activity of which
is determined, by the attraction of zinc for the oxygen of water, the zinc
move from right to left, then any other _cation_ included in the circuit,
being part of an electrolyte, or forming part of it at the moment, will
also move from right to left: and as the oxygen of the water, by its
natural affinity for the zinc, moves from left to right, so any other body
of the same class with it (i.e. any other _anion_), under its government
for the time, will move from left to right.