which may represent a complete voltaic circuit, the direction of its forces
being determined by supposing for a moment the zinc _b_ and the platina _c_
as representing plates of those metals acting upon water, _d, e_, and other
substances, but having their energy exalted so as to effect several
decompositions by the use of a battery at _a_ (989.). This supposition may
be allowed, because the action in the battery will only consist of
repetitions of what would take place between _b_ and _c_, if they really
constituted but a single pair. The zinc _b_, and the oxygen _d_, by their
mutual affinity, tend to unite; but as the oxygen is already in association
with the hydrogen _e_, and has its inherent chemical or electric powers
neutralized for the time by those of the latter, the hydrogen _e_ must
leave the oxygen _d_, and advance in the direction of the arrow head, or
else the zinc _b_ cannot move in the same direction to unite to the oxygen
_d_, nor the oxygen _d_ move in the contrary direction to unite to the zinc
_b_, the relation of the _similar_ forces of _b_ and _c_, in contrary
directions, to the _opposite_ forces of _d_ being the preventive. As the
hydrogen _e_ advances, it, on coming against the platina _c, f_, which
forms a part of the circuit, communicates its electric or chemical forces
through it to the next electrolyte in the circuit, fused chloride of lead,
_g, h_, where the chlorine must move in conformity with the direction of
the oxygen at _d_, for it has to compensate the forces disturbed in its
part of the circuit by the superior influence of those between the oxygen
and zinc at _d, b_, aided as they are by those of the battery _a_; and for
a similar reason the lead must move in the direction pointed out by the
arrow head, that it may be in right relation to the first moving body of
its own class, namely, the zinc _b_. If copper intervene in the circuit
from _i_ to _k_, it acts as the platina did before; and if another
electrolyte, as the iodide of tin, occur at _l, m_, then the iodine _l_,
being an _anion_, must move in conformity with the exciting _anion_,
namely, the oxygen _d_, and the _cation_ tin _m_ move in correspondence
with the other _cations b, e_, and _h_, that the chemical forces may be in
equilibrium as to their direction and quantity throughout the circuit.
Should it so happen that the anions in their circulation can combine with
the metals at the _anodes_ of the respective electrolytes, as would be the
case at the platina _f_ and the copper _k_, then those bodies becoming
parts of electrolytes, under the influence of the current, immediately
travel; but considering their relation to the zinc _b_, it is evidently
impossible that they can travel in any other direction than what will
accord with its course, and therefore can never tend to pass otherwise than
_from_ the anode and _to_ the cathode.
964. In such a circle as that delineated, therefore, all the known _anions_
may be grouped within, and all the _cations_ without. If any number of them
enter as _ions_ into the constitution of _electrolytes_, and, forming one
circuit, are simultaneously subject to one common current, the anions must
move in accordance with each other in one direction, and the cations in the
other. Nay, more than that, equivalent portions of these bodies must so
advance in opposite directions: for the advance of every 32.5 parts of the
zinc _b_ must be accompanied by a motion in the opposite direction of 8
parts of oxygen at _d_, of 36 parts of chlorine at _g_, of 126 parts of
iodine at _l_; and in the same direction by electro-chemical equivalents of
hydrogen, lead, copper and tin, at _e, h, k_. and _m_.
965. If the present paper be accepted as a correct expression of facts, it
will still only prove a confirmation of certain general views put forth by
Sir Humphry Davy in his Bakerian Lecture for 1806[A], and revised and
re-stated by him in another Bakerian Lecture, on electrical and chemical
changes, for the year 1826[B]. His general statement is, that "_chemical
and electrical attractions were produced by the same cause, acting in one
case on particles, in the other on masses, of matter; and that the same
property, under different modifications, was the cause of all the phenomena
exhibited by different voltaic combinations_[C]." This statement I believe
to be true; but in admitting and supporting it, I must guard myself from
being supposed to assent to all that is associated with it in the two
papers referred to, or as admitting the experiments which are there quoted
as decided proofs of the truth of the principle. Had I thought them so,
there would have been no occasion for this investigation. It may be
supposed by some that I ought to go through these papers, distinguishing
what I admit from what I reject, and giving good experimental or
philosophical reasons for the judgment in both cases. But then I should be
equally bound to review, for the same purpose, all that has been written
both for and against the necessity of metallic contact,--for and against
the origin of voltaic electricity in chemical action,--a duty which I may
not undertake in the present paper[D].
[A] Philosophical Transactions, 1807.
[B] Ibid. 1826, p. 383.
[C] Ibid. 1826, p. 389.
[D] I at one time intended to introduce here, in the form of a note, a
table of reference to the papers of the different philosophers who
have referred the origin of the electricity in the voltaic pile to
contact, or to chemical action, or to both; but on the publication of
the first volume of M. Becquerel's highly important and valuable
Traité de l'Electricité et du Magnétisme, I thought it far better to
refer to that work for these references, and the views held by the
authors quoted. See pages 86, 91, 104, 110, 112, 117, 118, 120, 151,
152, 224, 227, 228, 232, 233, 252, 255, 257, 258, 290, &c.--July 3rd,
1834.
¶ ii. _On the Intensity necessary for Electrolyzation._
966. It became requisite, for the comprehension of many of the conditions
attending voltaic action, to determine positively, if possible, whether
electrolytes could resist the action of an electric current when beneath a
certain intensity? whether the intensity at which the current ceased to act
would be the same for all bodies? and also whether the electrolytes thus
resisting decomposition would conduct the electric current as a metal does,
after they ceased to conduct as electrolytes, or would act as perfect
insulators?
967. It was evident from the experiments described (904. 906.) that
different bodies were decomposed with very different facilities, and
apparently that they required for their decomposition currents of different
intensities, resisting some, but giving way to others. But it was needful,
by very careful and express experiments, to determine whether a current
could really pass through, and yet not decompose an electrolyte (910.).
968. An arrangement (fig. 84.) was made, in which two glass vessels
contained the same dilute sulphuric acid, sp. gr. 1.25. The plate _z_ was
amalgamated zinc, in connexion, by a platina wire _a_, with the platina
plate _e_; _b_ was a platina wire connecting the two platina plates PP';
_c_ was a platina wire connected with the platina plate P". On the plate
_e_ was placed a piece of paper moistened in solution of iodide of
potassium: the wire _c_ was so curved that its end could be made to rest at
pleasure on this paper, and show, by the evolution of iodine there, whether
a current was passing; or, being placed in the dotted position, it formed a
direct communication with the platina plate _e_, and the electricity could
pass without causing decomposition. The object was to produce a current by
the action of the acid on the amalgamated zinc in the first vessel A; to
pass it through the acid in the second vessel B by platina electrodes, that
its power of decomposing water might, if existing, be observed; and to
verify the existence of the current at pleasure, by decomposition at _e_,
without involving the continual obstruction to the current which would
arise from making the decomposition there constant. The experiment, being
arranged, was examined and the existence of a current ascertained by the
decomposition at _e_; the whole was then left with the end of the wire _c_
resting on the plate _e_, so as to form a constant metallic communication
there.
969. After several hours, the end of the wire _c_ was replaced on the
test-paper at _e_: decomposition occurred, and _the proof_ of a passing
current was therefore complete. The current was very feeble compared to
what it had been at the beginning of the experiment, because of a peculiar
state acquired by the metal surfaces in the second vessel, which caused
them to oppose the passing current by a force which they possess under
these circumstances (1040.). Still it was proved, by the decomposition,
that this state of the plates in the second vessel was not able entirely to
stop the current determined in the first, and that was all that was needful
to be ascertained in the present inquiry.
970. This apparatus was examined from time to time, and an electric current
always found circulating through it, until twelve days had elapsed, during
which the water in the second vessel had been constantly subject to its
action. Notwithstanding this lengthened period, not the slightest
appearance of a bubble upon either of the plates in that vessel occurred.
From the results of the experiment, I conclude that a current _had_ passed,
but of so low an intensity as to fall beneath that degree at which the
elements of water, unaided by any secondary force resulting from the
capability of combination with the matter of the electrodes, or of the
liquid surrounding them, separated from each other.
971. It may be supposed, that the oxygen and hydrogen had been evolved in
such small quantities as to have entirely dissolved in the water, and
finally to have escaped at the surface, or to have reunited into water.
That the hydrogen can be so dissolved was shown in the first vessel; for
after several days minute bubbles of gas gradually appeared upon a glass
rod, inserted to retain the zinc and platina apart, and also upon the
platina plate itself, and these were hydrogen. They resulted principally in
this way:--notwithstanding the amalgamation of the zinc, the acid exerted a
little direct action upon it, so that a small stream of hydrogen bubbles
was continually rising from its surface; a little of this hydrogen
gradually dissolved in the dilute acid, and was in part set free against
the surfaces of the rod and the plate, according to the well-known action
of such solid bodies in solutions of gases (623. &c.).
972. But if the gases had been evolved in the second vessel by the
decomposition of water, and had tended to dissolve, still there would have
been every reason to expect that a few bubbles should have appeared on the
electrodes, especially on the negative one, if it were only because of its
action as a nucleus on the solution supposed to be formed; but none
appeared even after twelve days.
973. When a few drops only of nitric acid were added to the vessel A, fig.
84, then the results were altogether different. In less than five minutes
bubbles of gas appeared on the plates P' and P" in the second vessel. To
prove that this was the effect of the electric current (which by trial at
_c_ was found at the same time to be passing,) the connexion at _c_ was
broken, the plates P'P" cleared from bubbles and left in the acid of the
vessel B, for fifteen minutes: during that time no bubbles appeared upon
them; but on restoring the communication at _c_, a minute did not elapse
before gas appeared in bubbles upon the plates. The proof, therefore, is
most full and complete, that the current excited by dilute sulphuric acid
with a little nitric acid in vessel A, has intensity enough to overcome the
chemical affinity exerted between the oxygen and hydrogen of the water in
the vessel B, whilst that excited by dilute sulphuric acid alone has _not_
sufficient intensity.
974. On using a strong solution of caustic potassa in the vessel A, to
excite the current, it was found by the decomposing effects at _e_, that
the current passed. But it had not intensity enough to decompose the water
in the vessel B; for though left for fourteen days, during the whole of
which time the current was found to be passing, still not the slightest
appearance of gas appeared on the plates P'P", nor any other signs of the
water having suffered decomposition.
975. Sulphate of soda in solution was then experimented with, for the
purpose of ascertaining with respect to it, whether a certain electrolytic
intensity was also required for its decomposition in this state, in analogy
with the result established with regard to water (974). The apparatus was
arranged as in fig. 85; P and Z are the platina and zinc plates dipping
into a solution of common salt; _a_ and _b_ are platina plates connected by
wires of platina (except in the galvanometer _g_) with P and Z; _c_ is a
connecting wire of platina, the ends of which can be made to rest either on
the plates _a, b_, or on the papers moistened in solutions which are placed
upon them; so that the passage of the current without decomposition, or
with one or two decompositions, was under ready command, as far as
arrangement was concerned. In order to change the _anodes_ and _cathodes_
at the places of decomposition, the form of apparatus fig. 86, was
occasionally adopted. Here only one platina plate, _c_, was used; both
pieces of paper on which decomposition was to be effected were placed upon
it, the wires from P and Z resting upon these pieces of paper, or upon the
plate _c_, according as the current with or without decomposition of the
solutions was required.
976. On placing solution of iodide of potassium in paper at one of the
decomposing localities, and solution of sulphate of soda at the other, so
that the electric current should pass through both at once, the solution of
iodide was slowly decomposed, yielding iodine at the _anode_ and alkali at
the _cathode_; but the solution of sulphate of soda exhibited no signs of
decomposition, neither acid nor alkali being evolved from it. On placing
the wires so that the iodide alone was subject to the action of the current
(900.), it was quickly and powerfully decomposed; but on arranging them so
that the sulphate of soda alone was subject to action, it still refused to
yield up its elements. Finally, the apparatus was so arranged under a wet
bell-glass, that it could be left for twelve hours, the current passing
during the whole time through a solution of sulphate of soda, retained in
its place by only two thicknesses of bibulous litmus and turmeric paper. At
the end of that time it was ascertained by the decomposition of iodide of
potassium at the second place of action, that the current was passing and
had passed for the twelve hours, and yet no trace of acid or alkali from
the sulphate of soda appeared.
977. From these experiments it may, I think, be concluded, that a solution
of sulphate of soda can conduct a current of electricity, which is unable
to decompose the neutral salt present; that this salt in the state of
solution, like water, requires a certain electrolytic intensity for its
decomposition; and that the necessary intensity is much higher for this
substance than for the iodide of potassium in a similar state of solution.
978. I then experimented on bodies rendered decomposable by fusion, and
first on _chloride of lead_. The current was excited by dilute sulphuric
acid without any nitric acid between zinc and platina plates, fig. 87, and
was then made to traverse a little chloride of lead fused upon glass at
_a_, a paper moistened in solution of iodide of potassium at _b_, and a
galvanometer at _g_. The metallic terminations at _a_ and _b_ were of
platina. Being thus arranged, the decomposition at _b_ and the deflection
at _g_ showed that an electric current was passing, but there was no
appearance of decomposition at _a_, not even after a _metallic_
communication at _b_ was established. The experiment was repeated several
times, and I am led to conclude that in this case the current has not
intensity sufficient to cause the decomposition of the chloride of lead;
and further, that, like water (974.), fused chloride of lead can conduct an
electric current having an intensity below that required to effect
decomposition.
979. _Chloride of silver_ was then placed at _a_, fig. 87, instead of
chloride of lead. There was a very ready decomposition of the solution of
iodide of potassium at _b_, and when metallic contact was made there, very
considerable deflection of the galvanometer needle at _g_. Platina also
appeared to be dissolved at the anode of the fused chloride at _a_, and
there was every appearance of a decomposition having been effected there.
980. A further proof of decomposition was obtained in the following manner.
The platina wires in the fused chloride at _a_ were brought very near
together (metallic contact having been established at _b_), and left so;
the deflection at the galvanometer indicated the passage of a current,
feeble in its force, but constant. After a minute or two, however, the
needle would suddenly be violently affected, and indicate a current as
strong as if metallic contact had taken place at _a_. This I actually found
to be the case, for the silver reduced by the action of the current
crystallized in long delicate spiculæ, and these at last completed the
metallic communication; and at the same time that they transmitted a more
powerful current than the fused chloride, they proved that electro-chemical
decomposition of that chloride had been going on. Hence it appears, that
the current excited by dilute sulphuric acid between zinc and platina, has
an intensity above that required to electrolyze the fused chloride of
silver when placed between platina electrodes, although it has not
intensity enough to decompose chloride of lead under the same
circumstances.
981. A drop of _water_ placed at _a_ instead of the fused chlorides, showed
as in the former case (970.), that it could conduct a current unable to
decompose it, for decomposition of the solution of iodide at _b_ occurred
after some time. But its conducting power was much below that of the fused
chloride of lead (978.).
982. Fused _nitre_ at _a_ conducted much better than water: I was unable to
decide with certainty whether it was electrolyzed, but I incline to think
not, for there was no discoloration against the platina at the _cathode_.
If sulpho-nitric acid had been used in the exciting vessel, both the nitre
and the chloride of lead would have suffered decomposition like the water
(906.).
983. The results thus obtained of conduction without decomposition, and the
necessity of a certain electrolytic intensity for the separation of the
_ions_ of different electrolytes, are immediately connected with the
experiments and results given in § 10. of the Fourth Series of these
Researches (418. 423. 444. 419.). But it will require a more exact
knowledge of the nature of intensity, both as regards the first origin of
the electric current, and also the manner in which it may be reduced, or
lowered by the intervention of longer or shorter portions of bad
conductors, whether decomposable or not, before their relation can be
minutely and fully understood.
984. In the case of water, the experiments I have as yet made, appear to
show, that, when the electric current is reduced in intensity below the
point required for decomposition, then the degree of conduction is the same
whether sulphuric acid, or any other of the many bodies which can affect
its transferring power as an electrolyte, are present or not. Or, in other
words, that the necessary electrolytic intensity for water is the same
whether it be pure, or rendered a better conductor by the addition of these
substances; and that for currents of less intensity than this, the water,
whether pure or acidulated, has equal conducting power. An apparatus, fig.
84, was arranged with dilute sulphuric acid in the vessel A, and pure
distilled water in the vessel B. By the decomposition at _c_, it appeared
as if water was a _better_ conductor than dilute sulphuric acid for a
current of such low intensity as to cause no decomposition. I am inclined,
however, to attribute this apparent superiority of water to variations in
that peculiar condition of the platina electrodes which is referred to
further on in this Series (1040.), and which is assumed, as far as I can
judge, to a greater degree in dilute sulphuric acid than in pure water. The
power therefore, of acids, alkalies, salts, and other bodies in solution,
to increase conducting power, appears to hold good only in those cases
where the electrolyte subject to the current suffers decomposition, and
loses all influence when the current transmitted has too low an intensity
to affect chemical change. It is probable that the ordinary conducting
power of an electrolyte in the solid state (419.) is the same as that which
it possesses in the fluid state for currents, the tension of which is
beneath the due electrolytic intensity.
985. Currents of electricity, produced by less than eight or ten series of
voltaic elements, can be reduced to that intensity at which water can
conduct them without suffering decomposition, by causing them to pass
through three or four vessels in which water shall be successively
interposed between platina surfaces. The principles of interference upon
which this effect depends, will be described hereafter (1009. 1018.), but
the effect may be useful in obtaining currents of standard intensity, and
is probably applicable to batteries of any number of pairs of plates.
986. As there appears every reason to expect that all electrolytes will be
found subject to the law which requires an electric current of a certain
intensity for their decomposition, but that they will differ from each
other in the degree of intensity required, it will be desirable hereafter
to arrange them in a table, in the order of their electrolytic intensities.
Investigations on this point must, however, be very much extended, and
include many more bodies than have been here mentioned before such a table
can be constructed. It will be especially needful in such experiments, to
describe the nature of the electrodes used, or, if possible, to select such
as, like platina or plumbago in certain cases, shall have no power of
assisting the separation of the _ions_ to be evolved (913).
987. Of the two modes in which bodies can transmit the electric forces,
namely, that which is so characteristically exhibited by the metals, and
usually called conduction, and that in which it is accompanied by
decomposition, the first appears common to all bodies, although it occurs
with almost infinite degrees of difference; the second is at present
distinctive of the electrolytes. It is, however, just possible that it may
hereafter be extended to the metals; for their power of conducting without
decomposition may, perhaps justly, be ascribed to their requiring a very
high electrolytic intensity for their decomposition.
987-1/2. The establishment of the principle that a certain electrolytic
intensity is necessary before decomposition can be effected, is of great
importance to all those considerations which arise regarding the probable
effects of weak currents, such for instance as those produced by natural
thermo-electricity, or natural voltaic arrangements in the earth. For to
produce an effect of decomposition or of combination, a current must not
only exist, but have a certain intensity before it can overcome the
quiescent affinities opposed to it, otherwise it will be conducted,
producing no permanent chemical effects. On the other hand, the principles
are also now evident by which an opposing action can be so weakened by the
juxtaposition of bodies not having quite affinity enough to cause direct
action between them (913.), that a very weak current shall be able to raise
the sum of actions sufficiently high, and cause chemical changes to occur.
988. In concluding this division _on the intensity necessary for
electrolyzation_, I cannot resist pointing out the following remarkable
conclusion in relation to intensity generally. It would appear that when a
voltaic current is produced, having a certain intensity, dependent upon the
strength of the chemical affinities by which that current is excited
(916.), it can decompose a particular electrolyte without relation to the
quantity of electricity passed, the _intensity_ deciding whether the
electrolyte shall give way or not. If that conclusion be confirmed, then we
may arrange circumstances so that the _same quantity_ of electricity may
pass in the _same time_, in at the _same surface_, into the _same
decomposing body in the same state_, and yet, differing in intensity, will
_decompose in one case and in the other not_:--for taking a source of too
low an intensity to decompose, and ascertaining the quantity passed in a
given time, it is easy to take another source having a sufficient
intensity, and reducing the quantity of electricity from it by the
intervention of bad conductors to the same proportion as the former
current, and then all the conditions will be fulfilled which are required
to produce the result described.
¶ iii. _On associated Voltaic Circles, or the Voltaic Battery._
989. Passing from the consideration of single circles (875. &c.) to their
association in the voltaic battery, it is a very evident consequence, that
if matters are so arranged that two sets of affinities, in place of being
opposed to each other as in figg. 73. 76. (880. 891.), are made to act in
conformity, then, instead of either interfering with the other, it will
rather assist it. This is simply the case of two voltaic pairs of metals
arranged so as to form one circuit. In such arrangements the activity of
the whole is known to be increased, and when ten, or a hundred, or any
larger number of such alternations are placed in conformable association
with each other, the power of the whole becomes proportionally exalted, and
we obtain that magnificent instrument of philosophic research, the _voltaic
battery_.
990. But it is evident from the principles of definite action already laid
down, that the _quantity_ of electricity in the current cannot be increased
with the increase of the _quantity of metal_ oxidized and dissolved at each
new place of chemical action. A single pair of zinc and platina plates
throws as much electricity into the form of a current, by the oxidation of
32.5 grains of the zinc (868.) as would be circulated by the same
alteration of a thousand times that quantity, or nearly five pounds of
metal oxidized at the surface of the zinc plates of a thousand pairs placed
in regular battery order. For it is evident, that the electricity which
passes across the acid from the zinc to the platina in the first cell, and
which has been associated with, or even evolved by, the decomposition of a
definite portion of water in that cell, cannot pass from the zinc to the
platina across the acid in the second cell, without the decomposition of
the same quantity of water there, and the oxidation of the same quantity of
zinc by it (924. 949.). The same result recurs in every other cell; the
electro-chemical equivalent of water must be decomposed in each, before the
current can pass through it; for the quantity of electricity passed and the
quantity of electrolyte decomposed, _must_ be the equivalents of each
other. The action in each cell, therefore, is not to increase the quantity
set in motion in any one cell, but to aid in urging forward that quantity,
the passing of which is consistent with the oxidation of its own zinc; and
in this way it exalts that peculiar property of the current which we
endeavour to express by the term _intensity_, without increasing the
_quantity_ beyond that which is proportionate to the quantity of zinc
oxidized in any single cell of the series.
991. To prove this, I arranged ten pairs of amalgamated zinc and platina
plates with dilute sulphuric acid in the form of a battery. On completing
the circuit, all the pairs acted and evolved gas at the surfaces of the
platina. This was collected and found to be alike in quantity for each
plate; and the quantity of hydrogen evolved at any one platina plate was in
the same proportion to the quantity of metal dissolved from any one zinc
plate, as was given in the experiment with a single pair (864. &c.). It was
therefore certain, that, just as much electricity and no more had passed
through the series of ten pair of plates as had passed through, or would
have been put into motion by, any single pair, notwithstanding that ten
times the quantity of zinc had been consumed.
992. This truth has been proved also long ago in another way, by the action
of the evolved current on a magnetic needle; the deflecting power of one
pair of plates in a battery being equal to the deflecting power of the
whole, provided the wires used be sufficiently large to carry the current
of the single pair freely; but the _cause_ of this equality of action could
not be understood whilst the definite action and evolution of electricity
(783. 869.) remained unknown.
993. The superior decomposing power of a battery over a single pair of
plates is rendered evident in two ways. Electrolytes held together by an
affinity so strong as to resist the action of the current from a single
pair, yield up their elements to the current excited by many pairs; and
that body which is decomposed by the action of one or of few pairs of
metals, &c., is resolved into its _ions_ the more readily as it is acted
upon by electricity urged forward by many alternations.
994. Both these effects are, I think, easily understood. Whatever
_intensity_ may be, (and that must of course depend upon the nature of
electricity, whether it consist of a fluid or fluids, or of vibrations of
an ether, or any other kind or condition of matter,) there seems to be no
difficulty in comprehending that the _degree_ of intensity at which a
current of electricity is evolved by a first voltaic element, shall be
increased when that current is subjected to the action of a second voltaic
element, acting in conformity and possessing equal powers with the first:
and as the decompositions are merely opposed actions, but exactly of the
same kind as those which generate the current (917.), it seems to be a
natural consequence, that the affinity which can resist the force of a
single decomposing action may be unable to oppose the energies of many
decomposing actions, operating conjointly, as in the voltaic battery.
995. That a body which can give way to a current of feeble intensity,
should give way more freely to one of stronger force, and yet involve no
contradiction to the law of definite electrolytic action, is perfectly
consistent. All the facts and also the theory I have ventured to put forth,
tend to show that the act of decomposition opposes a certain force to the
passage of the electric current; and, that this obstruction should be
overcome more or less readily, in proportion to the greater or less
intensity of the decomposing current, is in perfect consistency with all
our notions of the electric agent.
996. I have elsewhere (947.) distinguished the chemical action of zinc and
dilute sulphuric acid into two portions; that which, acting effectually on
the zinc, evolves hydrogen at once upon its surface, and that which,
producing an arrangement of the chemical forces throughout the electrolyte
present, (in this case water,) tends to take oxygen from it, but cannot do
so unless the electric current consequent thereon can have free passage,
and the hydrogen be delivered elsewhere than against the zinc. The electric
current depends altogether upon the second of these; but when the current
can pass, by favouring the electrolytic action it tends to diminish the
former and increase the latter portion.
997. It is evident, therefore, that when ordinary zinc is used in a voltaic
arrangement, there is an enormous waste of that power which it is the
object to throw into the form of an electric current; a consequence which
is put in its strongest point of view when it is considered that three
ounces and a half of zinc, properly oxidized, can circulate enough
electricity to decompose nearly one ounce of water, and cause the evolution
of about 2100 cubic inches of hydrogen gas. This loss of power not only
takes place during the time the electrodes of the battery are in
communication, being then proportionate to the quantity of hydrogen evolved
against the surface of any one of the zinc plates, but includes also _all_
the chemical action which goes on when the extremities of the pile are not
in communication.
998. This loss is far greater with ordinary zinc than with the pure metal,
as M. De la Rive has shown[A]. The cause is, that when ordinary zinc is
acted upon by dilute sulphuric acid, portions of copper, lead, cadmium, or
other metals which it may contain, are set free upon its surface; and
these, being in contact with the zinc, form small but very active voltaic
circles, which cause great destruction of the zinc and evolution of
hydrogen, apparently upon the zinc surface, but really upon the surface of
these incidental metals. In the same proportion as they serve to discharge
or convey the electricity back to the zinc, do they diminish its power of
producing an electric current which shall extend to a greater distance
across the acid, and be discharged only through the copper or platina plate
which is associated with it for the purpose of forming a voltaic apparatus.
[A] Quarterly Journal of Science, 1831, p. 388; or Bibliothèque
Universelle, 1830, p. 391.
999. All these evils are removed by the employment of an amalgam of zinc in
the manner recommended by Mr. Kemp[A], or the use of the amalgamated zinc
plates of Mr. Sturgeon (863.), who has himself suggested and objected to
their application in galvanic batteries; for he says, "Were it not on
account of the brittleness and other inconveniences occasioned by the
incorporation of the mercury with the zinc, amalgamation of the zinc
surfaces in galvanic batteries would become an important improvement; for
the metal would last much longer, and remain bright for a considerable
time, even for several successive hours; essential considerations in the
employment of this apparatus[B]."
[A] Jameson's Edinburgh Journal, October 1828.
[B] Recent Experimental Researches, p. 42, &c. Mr. Sturgeon is of
course unaware of the definite production of electricity by chemical
action, and is in fact quoting the experiment as the strongest
argument _against_ the chemical theory of galvanism.
1000. Zinc so prepared, even though impure, does not sensibly decompose the
water of dilute sulphuric acid, but still has such affinity for the oxygen,
that the moment a metal which, like copper or platina, has little or no
affinity, touches it in the acid, action ensues, and a powerful and
abundant electric current is produced. It is probable that the mercury acts
by bringing the surface, in consequence of its fluidity, into one uniform
condition, and preventing those differences in character between one spot
and another which are necessary for the formation of the minute voltaic
circuits referred to (998.). If any difference does exist at the first
moment, with regard to the proportion of zinc and mercury, at one spot on
the _surface_, as compared with another, that spot having the least mercury
is first acted on, and, by solution of the zinc, is soon placed in the same
condition as the other parts, and the whole plate rendered superficially
uniform. One part cannot, therefore, act as a discharger to another; and
hence _all_ the chemical power upon the water at its surface is in that
equable condition (949.), which, though it tends to produce an electric
current through the liquid to another plate of metal which can act as a
discharger (950.), presents no irregularities by which any one part, having
weaker affinities for oxygen, can act as a discharger to another. Two
excellent and important consequences follow upon this state of the metal.
The first is, that _the full equivalent_ of electricity is obtained for the
oxidation of a certain quantity of zinc; the second, that a battery
constructed with the zinc so prepared, and charged with dilute sulphuric
acid, is active only whilst the electrodes are connected, and ceases to act
or be acted upon by the acid the instant the communication is broken.
1001. I have had a small battery of ten pairs of plates thus constructed,
and am convinced that arrangements of this kind will be very important,
especially in the development and illustration of the philosophical
principles of the instrument. The metals I have used are amalgamated zinc
and platina, connected together by being soldered to platina wires, the
whole apparatus having the form of the couronne des tasses. The liquid used
was dilute sulphuric acid of sp. gr. 1.25. No action took place upon the
metals except when the electrodes were in communication, and then the
action upon the zinc was only in proportion to the decomposition in the
experimental cell; for when the current was retarded there, it was retarded
also in the battery, and no waste of the powers of the metal was incurred.
1002. In consequence of this circumstance, the acid in the cells remained
active for a very much longer time than usual. In fact, time did not tend
to lower it in any sensible degree: for whilst the metal was preserved to
be acted upon at the proper moment, the acid also was preserved almost at
its first strength. Hence a constancy of action far beyond what can be
obtained by the use of common zinc.
1003. Another excellent consequence was the renewal, during the interval of
rest, between two experiments of the first and most efficient state. When
an amalgamated zinc and a platina plate, immersed in dilute sulphuric acid,
are first connected, the current is very powerful, but instantly sinks very
much in force, and in some cases actually falls to only an eighth or a
tenth of that first produced (1036.). This is due to the acid which is in
contact with the zinc becoming neutralized by the oxide formed; the
continued quick oxidation of the metal being thus prevented. With ordinary
zinc, the evolution of gas at its surface tends to mingle all the liquid
together, and thus bring fresh acid against the metal, by which the oxide
formed there can be removed. With the amalgamated zinc battery, at every
cessation of the current, the saline solution against the zinc is gradually
diffused amongst the rest of the liquid; and upon the renewal of contact at
the electrodes, the zinc plates are found most favourably circumstanced for
the production of a ready and powerful current.
1004. It might at first be imagined that amalgamated zinc would be much
inferior in force to common zinc, because, of the lowering of its energy,
which the mercury might be supposed to occasion over the whole of its
surface; but this is not the case. When the electric currents of two pairs
of platina and zinc plates were opposed, the difference being that one of
the zincs was amalgamated and the other not, the current from the
amalgamated zinc was most powerful, although no gas was evolved against it,
and much was evolved at the surface of the unamalgamated metal. Again, as
Davy has shown[A], if amalgamated and unamalgamated zinc be put in contact,
and dipped into dilute sulphuric acid, or other exciting fluids, the former
is positive to the latter, i.e. the current passes from the amalgamated
zinc, through the fluid, to the unprepared zinc. This he accounts for by
supposing that "there is not any inherent and specific property in each
metal which gives it the electrical character, but that it depends upon its
peculiar state--on that form of aggregation which fits it for chemical
change."
[A] Philosophical Transactions, 1826, p. 405.
1005. The superiority of the amalgamated zinc is not, however, due to any
such cause, but is a very simple consequence of the state of the fluid in
contact with it; for as the unprepared zinc acts directly and alone upon
the fluid, whilst that which is amalgamated does not, the former (by the
oxide it produces) quickly neutralizes the acid in contact with its
surface, so that the progress of oxidation is retarded, whilst at the
surface of the amalgamated zinc, any oxide formed is instantly removed by
the free acid present, and the clean metallic surface is always ready to
act with full energy upon the water. Hence its superiority (1037.).
1006. The progress of improvement in the voltaic battery and its
applications, is evidently in the contrary direction at present to what it
was a few years ago; for in place of increasing the number of plates, the
strength of acid, and the extent altogether of the instrument, the change
is rather towards its first state of simplicity, but with a far more
intimate knowledge and application of the principles which govern its force
and action. Effects of decomposition can now be obtained with ten pairs of
plates (417.), which required five hundred or a thousand pairs for their
production in the first instance. The capability of decomposing fused
chlorides, iodides, and other compounds, according to the law before
established (380. &c.), and the opportunity of collecting certain of the
products, without any loss, by the use of apparatus of the nature of those
already described (789. 814. &c.), render it probable that the voltaic
battery may become a useful and even economical manufacturing instrument;
for theory evidently indicates that an equivalent of a rare substance may
be obtained at the expense of three or four equivalents of a very common
body, namely, zinc: and practice seems thus far to justify the expectation.
In this point of view I think it very likely that plates of platina or
silver may be used instead of plates of copper with advantage, and that
then the evil arising occasionally from solution of the copper, and its
precipitation on the zinc, (by which the electromotive power of the zinc is
so much injured,) will be avoided (1047.).
¶ iv. _On the Resistance of an Electrolyte to Electrolytic Action, and on
Interpositions._
1007. I have already illustrated, in the simplest possible form of
experiment (891. 910.), the resistance established at the place of
decomposition to the force active at the exciting place. I purpose
examining the effects of this resistance more generally; but it is rather
with reference to their practical interference with the action and
phenomena of the voltaic battery, than with any intention at this time to
offer a strict and philosophical account of their nature. Their general and
principal cause is the resistance of the chemical affinities to be
overcome; but there are numerous other circumstances which have a joint
influence with these forces (1034. 1040. &c.), each of which would require
a minute examination before a correct account of the whole could be given.
1008. As it will be convenient to describe the experiments in a form
different to that in which they were made, both forms shall first be
explained. Plates of platina, copper, zinc, and other metals, about three
quarters of an inch wide and three inches long, were associated together in
pairs by means of platina wires to which they were soldered, fig. 88, the
plates of one pair being either alike or different, as might be required.
These were arranged in glasses, fig. 89, so as to form Volta's crown of
cups. The acid or fluid in the cups never covered the whole of any plate;
and occasionally small glass rods were put into the cups, between the
plates, to prevent their contact. Single plates were used to terminate the
series and complete the connexion with a galvanometer, or with a
decomposing apparatus (899. 968. &c.), or both. Now if fig. 90 be examined
and compared with fig. 91, the latter may be admitted as representing the
former in its simplest condition; for the cups i, ii, and iii of the
former, with their contents, are represented by the cells i, ii, and iii of
the latter, and the metal plates Z and P of the former by the similar
plates represented Z and P in the latter. The only difference, in fact,
between the apparatus, fig. 90, and the trough represented fig. 91, is that
twice the quantity of surface of contact between the metal and acid is
allowed in the first to what would occur in the second.
1009. When the extreme plates of the arrangement just described, fig. 90,
are connected metallically through the galvanometer _g_, then the whole
represents a battery consisting of two pairs of zinc and platina plates
urging a current forward, which has, however, to decompose water unassisted
by any direct chemical affinity before it can be transmitted across the
cell iii, and therefore before it can circulate. This decomposition of
water, which is opposed to the passage of the current, may, as a matter of
convenience, be considered as taking place either against the surfaces of
the two platina plates which constitute the electrodes in the cell in, or
against the two surfaces of that platina plate which separates the cells ii
and iii, fig. 91, from each other. It is evident that if that plate were
away, the battery would consist of two pairs of plates and two cells,
arranged in the most favourable position for the production of a current.
The platina plate therefore, which being introduced as at _x_, has oxygen
evolved at one surface and hydrogen at the other (that is, if the
decomposing current passes), may be considered as the cause of any
obstruction arising from the decomposition of water by the electrolytic
action of the current; and I have usually called it the interposed plate.
1010. In order to simplify the conditions, dilute sulphuric acid was first
used in all the cells, and platina for the interposed plates; for then the
initial intensity of the current which tends to be formed is constant,
being due to the power which zinc has of decomposing water; and the
opposing force of decomposition is also constant, the elements of the water
being unassisted in their separation at the interposed plates by any
affinity or secondary action at the electrodes (744.), arising either from
the nature of the plate itself or the surrounding fluid.
1011. When only one voltaic pair of zinc and platina plates was used, the
current of electricity was entirely stopped to all practical purposes by
interposing one platina plate, fig. 92, i.e. by requiring of the current
that it should decompose water, and evolve both its elements, before it
should pass. This consequence is in perfect accordance with the views
before given (910. 917. 973.). For as the whole result depends upon the
opposition of forces at the places of electric excitement and
electro-decomposition, and as water is the substance to be decomposed at
both before the current can move, it is not to be expected that the zinc
should have such powerful attraction for the oxygen, as not only to be able
to take it from its associated hydrogen, but leave such a surplus of force
as, passing to the second place of decomposition, should be there able to
effect a second separation of the elements of water. Such an effect would
require that the force of attraction between zinc and oxygen should under
the circumstances be _at least_ twice as great as the force of attraction
between the oxygen and hydrogen.
1012. When two pairs of zinc and platina exciting plates were used, the
current was also practically stopped by one interposed platina plate, fig.
93. There was a very feeble effect of a current at first, but it ceased
almost immediately. It will be referred to, with many other similar
effects, hereafter (1017.).
1013. Three pairs of zinc and platina plates, fig. 94, were able to produce
a current which could pass an interposed platina plate, and effect the
electrolyzation of water in cell iv. The current was evident, both by the
continued deflection of the galvanometer, and the production of bubbles of
oxygen and hydrogen at the electrodes in cell iv. Hence the accumulated
surplus force of three plates of zinc, which are active in decomposing
water, is more than equal, when added together, to the force with which
oxygen and hydrogen are combined in water, and is sufficient to cause the
separation of these elements from each other.
1014. The three pairs of zinc and platina plates were now opposed by two
intervening platina plates, fig. 95. In this case the current was stopped.
1015. Four pairs of zinc and platina plates were also neutralized by two
interposed platina plates, fig. 96.
1016. Five pairs of zinc and platina, with two interposed platina plates,
fig. 97, gave a feeble current; there was permanent deflection at the
galvanometer, and decomposition in the cells vi and vii. But the current
was very feeble; very much less than when all the intermediate plates were
removed and the two extreme ones only retained: for when they were placed
six inches asunder in one cell, they gave a powerful current. Hence five
exciting pairs, with two interposed obstructing plates, do not give a
current at all comparable to that of a single unobstructed pair.
1017. I have already said that a _very feeble current_ passed when the
series included one interposed platina and two pairs of zinc and platina
plates (1012.). A similarly feeble current passed in every case, and even
when only one exciting pair and four intervening platina plates were used,
fig. 98, a current passed which could be detected at _x_, both by chemical
action on the solution of iodide of potassium, and by the galvanometer.
This current I believe to be due to electricity reduced in intensity below
the point requisite for the decomposition of water (970. 984.); for water
can conduct electricity of such low intensity by the same kind of power
which it possesses in common with metals and charcoal, though it cannot
conduct electricity of higher intensity without suffering decomposition,
and then opposing a new force consequent thereon. With an electric current
of, or under this intensity, it is probable that increasing the number of
interposed platina plates would not involve an increased difficulty of
conduction.
1018. In order to obtain an idea of the additional interfering power of
each added platina plate, six voltaic pairs and four intervening platinas
were arranged as in fig. 99; a very feeble current then passed (985.
1017.). When one of the platinas was removed so that three intervened, a
current somewhat stronger passed. With two intervening platinas a still
stronger current passed; and with only one intervening platina a very fair
current was obtained. But the effect of the successive plates, taken in the
order of their interposition, was very different, as might be expected; for
the first retarded the current more powerfully than the second, and the
second more than the third.
1019. In these experiments both amalgamated and unamalgamated zinc were
used, but the results generally were the same.
1020. The effects of retardation just described were altered altogether
when changes were made in the _nature of the liquid_ used between the
plates, either in what may be called the _exciting_ or the _retarding_
cells. Thus, retaining the exciting force the same, by still using pure
dilute sulphuric acid for that purpose, if a little nitric acid were added
to the liquid in the _retarding_ cells, then the transmission of the
current was very much facilitated. For instance, in the experiment with one
pair of exciting plates and one intervening plate (1011.), fig. 92, when a
few drops of nitric acid were added to the contents of cell ii, then the
current of electricity passed with considerable strength (though it soon
fell from other causes (1036; 1040.),) and the same increased effect was
produced by the nitric acid when many interposed plates were used.
1021. This seems to be a consequence of the diminution of the difficulty of
decomposing water when its hydrogen, instead of being absolutely expelled,
as in the former cases, is transferred to the oxygen of the nitric acid,
producing a secondary result at the _cathode_ (752.); for in accordance
with the chemical views of the electric current and its action already
advanced (913.), the water, instead of opposing a resistance to
decomposition equal to the full amount of the force of mutual attraction
between its oxygen and hydrogen, has that force counteracted in part, and
therefore diminished by the attraction of the hydrogen at the _cathode_ for
the oxygen of the nitric acid which surrounds it, and with which it
ultimately combines instead of being evolved in its free state.
1022. When a little nitric acid was put into the exciting cells, then again
the circumstances favouring the transmission of the current were
strengthened, for the _intensity_ of the current itself was increased by
the addition (906.). When therefore a little nitric acid was added to both
the _exciting_ and the _retarding_ cells, the current of electricity passed
with very considerable freedom.
1023. When dilute muriatic acid was used, it produced and transmitted a
current more easily than pure dilute sulphuric acid, but not so readily as
dilute nitric acid. As muriatic acid appears to be decomposed more freely
than water (765.), and as the affinity of zinc for chlorine is very
powerful, it might be expected to produce a current more intense than that
from the use of dilute sulphuric acid; and also to transmit it more freely
by undergoing decomposition at a lower intensity (912.).
1024. In relation to the effect of these interpositions, it is necessary to
state that they do not appear to be at all dependent upon the size of the
electrodes, or their distance from each other in the acid, except that when
a current _can pass_, changes in these facilitate or retard its passage.
For on repeating the experiment with one intervening and one pair of
exciting plates (1011.), fig. 92, and in place of the interposed plate P
using sometimes a mere wire, and sometimes very large plates (1008.), and
also changing the terminal exciting plates Z and P, so that they were
sometimes wires only and at others of great size, still the results were
the same as those already obtained.
1025. In illustration of the effect of distance, an experiment like that
described with two exciting pairs and one intervening plate (1012.), fig.
93, was arranged so that the distance between the plates in the third cell
could be increased to six or eight inches, or diminished to the thickness
of a piece of intervening bibulous paper. Still the result was the same in
both cases, the effect not being sensibly greater, when the plates were
merely separated by the paper, than when a great way apart; so that the
principal opposition to the current in this case does not depend upon the
_quantity_ of intervening electrolytic conductor, but on the _relation of
its elements to the intensity of the current_, or to the chemical nature of
the electrodes and the surrounding fluids.
1026. When the acid was sulphuric acid, _increasing its strength_ in any of
the cells, caused no change in the effects; it did not produce a more
intense current in the exciting cells (908.), or cause the current produced
to traverse the decomposing cells more freely. But if to very weak
sulphuric acid a few drops of nitric acid were added, then either one or
other of those effects could be produced; and, as might be expected in a
case like this, where the exciting or conducting action bore a _direct_
reference to the acid itself, increasing the strength of this (the nitric
acid), also increased its powers.
1027. The _nature of the interposed plate_ was now varied to show its
relation to the phenomena either of excitation or retardation, and
amalgamated zinc was first substituted for platina. On employing one
voltaic pair and one interposed zinc plate, fig. 100, there was as powerful
a current, apparently, as if the interposed zinc plate was away. Hydrogen
was evolved against P in cell ii, and against the side of the second zinc
in cell i; but no gas appeared against the side of the zinc in cell ii, nor
against the zinc in cell i.
1028. On interposing two amalgamated zinc plates, fig. 101, instead of one,
there was still a powerful current, but interference had taken place. On
using three intermediate zinc plates, fig. 102, there was still further
retardation, though a good current of electricity passed.
1029. Considering the retardation as due to the inaction of the amalgamated
zinc upon the dilute acid, in consequence of the slight though general
effect of diminished chemical power produced by the mercury on the surface,
and viewing this inaction as the circumstance which rendered it necessary
that each plate should have its tendency to decompose water assisted
slightly by the electric current, it was expected that plates of the metal
in the unamalgamated state would probably not require such assistance, and
would offer no sensible impediment to the passing of the current. This
expectation was fully realized in the use of two and three interposed
unamalgamated plates. The electric current passed through them as freely as
if there had been no such plates in the way. They offered no obstacle,
because they could decompose water without the current; and the latter had
only to give direction to a part of the forces, which would have been
active whether it had passed or not.
1030. Interposed plates of copper were then employed. These seemed at first
to occasion no obstruction, but after a few minutes the current almost
entirely ceased. This effect appears due to the surfaces taking up that
peculiar condition (1010.) by which they tend to produce a reverse current;
for when one or more of the plates were turned round, which could easily be
effected with the couronne des tasses form of experiment, fig. 90, then the
current was powerfully renewed for a few moments, and then again ceased.
Plates of platina and copper, arranged as a voltaic pile with dilute
sulphuric acid, could not form a voltaic trough competent to act for more
than a _few_ minutes, because of this peculiar counteracting effect.
1031. All these effects of retardation, exhibited by decomposition against
surfaces for which the evolved elements have more or less affinity, or are
altogether deficient in attraction, show generally, though beautifully, the
chemical relations and source of the current, and also the balanced state
of the affinities at the places of excitation and decomposition. In this
way they add to the mass of evidence in favour of the identity of the two;
for they demonstrate, as it were, the antagonism of the _chemical powers_
at the electromotive part with the _chemical powers_ at the interposed
parts; they show that the first are _producing_ electric effects, and the
second _opposing_ them; they bring the two into direct relation; they prove
that either can determine the other, thus making what appears to be cause
and effect convertible, and thereby demonstrating that both chemical and
electrical action are merely two exhibitions of one single agent or power
(916. &c.).
1032. It is quite evident, that as water and other electrolytes can conduct
electricity without suffering decomposition (986.), when the electricity is
of sufficiently low intensity, it may not be asserted as absolutely true in
all cases, that whenever electricity passes through an electrolyte, it
produces a definite effect of decomposition. But the quantity of
electricity which can pass in a given time through an electrolyte without
causing decomposition, is so small as to bear no comparison to that
required in a case of very moderate decomposition, and with electricity
above the intensity required for electrolyzation, I have found no sensible
departure as yet from the law of _definite electrolytic action_ developed
in the preceding series of these Researches (783. &c.).
1033. I cannot dismiss this division of the present Paper without making a
reference to the important experiments of M. Aug. De la Rive on the effects
of interposed plates[A]. As I have had occasion to consider such plates
merely as giving rise to new decompositions, and in that way only causing
obstruction to the passage of the electric current, I was freed from the
necessity of considering the peculiar effects described by that
philosopher. I was the more willing to avoid for the present touching upon
these, as I must at the same time have entered into the views of Sir
Humphry Davy upon the same subject[B] and also those of Marianini[C] and
Hitter[D], which are connected with it.
[A] Annales de Chimie, tom. xxviii. p 190; and Mémoires de Génève.
[B] Philosophical Transactions, 1826, p. 413.
[C] Annales de Chimie, tom. xxxiii. pp. 117, 119, &c.
[D] Journal de Physique, tom. lvii. pp. 319, 350.
¶ v. _General Remarks on the active Voltaic Battery._
1034. When the ordinary voltaic battery is brought into action, its very
activity produces certain effects, which re-act upon it, and cause serious
deterioration of its power. These render it an exceedingly inconstant
instrument as to the _quantity_ of effect which it is capable of producing.
They are already, in part, known and understood; but as their importance,
and that of certain other coincident results, will be more evident by
reference to the principles and experiments already stated and described, I
have thought it would be useful, in this investigation of the voltaic pile,
to notice them briefly here.
1035. When the battery is in action, it causes such substances to be formed
and arranged in contact with the plates as very much weaken its power, or
even tend to produce a counter current. They are considered by Sir Humphry
Davy as sufficient to account for the phenomena of Ritter's secondary
piles, and also for the effects observed by M.A. De la Rive with interposed
platina plates[A].
[A] Philosophical Transactions, 1826, p. 113.
1036. I have already referred to this consequence (1003.), as capable, in
some cases, of lowering the force of the current to one-eighth or one-tenth
of what it was at the first moment, and have met with instances in which
its interference was very great. In an experiment in which one voltaic pair
and one interposed platina plate were used with dilute sulphuric acid in
the cells fig. 103, the wires of communication were so arranged, that the
end of that marked 3 could be placed at pleasure upon paper moistened in
the solution of iodide of potassium at _x_, or directly upon the platina
plate there. If, after an interval during which the circuit had not been
complete, the wire 3 were placed upon the paper, there was evidence of a
current, decomposition ensued, and the galvanometer was affected. If the
wire 3 were made to touch the metal of _p_, a comparatively strong sudden
current was produced, affecting the galvanometer, but lasting only for a
moment; the effect at the galvanometer ceased, and if the wire 3 were
placed on the paper at _x_, no signs of decomposition occurred. On raising
the wire 3, and breaking the circuit altogether for a while, the apparatus
resumed its first power, requiring, however, from five to ten minutes for
this purpose; and then, as before, on making contact between 3 and _p_,
there was again a momentary current, and immediately all the effects
apparently ceased.
1037. This effect I was ultimately able to refer to the state of the film
of fluid in contact with the zinc plate in cell i. The acid of that film is
instantly neutralized by the oxide formed; the oxidation of the zinc
cannot, of course, go on with the same facility as before; and the chemical
action being thus interrupted, the voltaic action diminishes with it. The
time of the rest was required for the diffusion of the liquid, and its
replacement by other acid. From the serious influence of this cause in
experiments with single pairs of plates of different metals, in which I was
at one time engaged, and the extreme care required to avoid it, I cannot
help feeling a strong suspicion that it interferes more frequently and
extensively than experimenters are aware of, and therefore direct their
attention to it.
1038. In considering the effect in delicate experiments of this source of
irregularity of action, in the voltaic apparatus, it must be remembered
that it is only that very small portion of matter which is directly in
contact with the oxidizable metal which has to be considered with reference
to the change of its nature; and this portion is not very readily displaced
from its position upon the surface of the metal (582. 605.), especially if
that metal be rough and irregular. In illustration of this effect, I will
quote a remarkable experiment. A burnished platina plate (569.) was put
into hot strong sulphuric acid for an instant only: it was then put into
distilled water, moved about in it, taken out, and wiped dry: it was put
into a second portion of distilled water, moved about in it, and again
wiped: it was put into a third portion of distilled water, in which it was
moved about for nearly eight seconds; it was then, without wiping, put into
a fourth portion of distilled water, where it was allowed to remain five
minutes. The two latter portions of water were then tested for sulphuric
acid; the third gave no sensible appearance of that substance, but the
fourth gave indications which were not merely evident, but abundant for the
circumstances under which it had been introduced. The result sufficiently
shows with what difficulty that portion of the substance which is in
_contact_ with the metal leaves it; and as the contact of the fluid formed
against the plate in the voltaic circuit must be as intimate and as perfect
as possible, it is easy to see how quickly and greatly it must vary from
the general fluid in the cells, and how influential in diminishing the
force of the battery this effect must be.
1039. In the ordinary voltaic pile, the influence of this effect will occur
in all variety of degrees. The extremities of a trough of twenty pairs of
plates of Wollaston's construction were connected with the
volta-electrometer, fig. 66. (711.), of the Seventh Series of these
Researches, and after five minutes the number of bubbles of gas issuing
from the extremity of the tube, in consequence of the decomposition of the
water, noted. Without moving the plates, the acid between the copper and
zinc was agitated by the introduction of a feather. The bubbles were
immediately evolved more rapidly, above twice the number being produced in
the same portion of time as before. In this instance it is very evident
that agitation by a feather must have been a very imperfect mode of
restoring the acid in the cells against the plates towards its first equal
condition; and yet imperfect as the means were, they more than doubled the
power of the battery. The _first effect_ of a battery which is known to be
so superior to the degree of action which the battery can sustain, is
almost entirely due to the favourable condition of the acid in contact with
the plates.
1040. A _second_ cause of diminution in the force of the voltaic battery,
consequent upon its own action, is that extraordinary state of the surfaces
of the metals (969.) which was first described, I believe, by Ritter[A], to
which he refers the powers of his secondary piles, and which has been so
well experimented upon by Marianini, and also by A. De la Rive. If the
apparatus, fig. 103. (1096.), be left in action for an hour or two, with
the wire 3 in contact with the plate _p_, so as to allow a free passage for
the current, then, though the contact be broken for ten or twelve minutes,
still, upon its renewal, only a feeble current will pass, not at all equal
in force to what might be expected. Further, if P^{1} and P^{2} be
connected by a metal wire, a powerful momentary current will pass from
P^{2} to P^{1} through the acid, and therefore in the reverse direction to
that produced by the action of the zinc in the arrangement; and after this
has happened, the general current can pass through the whole of the system
as at first, but by its passage again restores the plates P^{2} and P^{1}
into the former opposing condition. This, generally, is the fact described
by Ritter, Marianini, and De la Rive. It has great opposing influence on
the action of a pile, especially if the latter consist of but a small
number of alternations, and has to pass its current through many
interpositions. It varies with the solution in which the interposed plates
are immersed, with the intensity of the current, the strength of the pile,
the time of action, and especially with accidental discharges of the plates
by inadvertent contacts or reversions of the plates during experiments, and
must be carefully watched in every endeavour to trace the source, strength,
and variations of the voltaic current. Its effect was avoided in the
experiments already described (1036. &c.), by making contact between the
plates P^{1} and P^{2} before the effect dependent upon the state of the
solution in contact with the zinc plate was observed, and by other
precautions.
[A] Journal de Physique, lvii. p. 349.
1041. When an apparatus like fig. 98. (1017.) with several platina plates
was used, being connected with a battery able to force a current through
them, the power which they acquired, of producing a reversed current, was
very considerable.
1042. _Weak and exhausted charges_ should never be used at the same time
with _strong and fresh ones_ in the different cells of a trough, or the
different troughs of a battery: the fluid in all the cells should be alike,
else the plates in the weaker cells, in place of assisting, retard the
passage of the electricity generated in, and transmitted across, the
stronger cells. Each zinc plate so circumstanced has to be assisted in
decomposing power before the whole current can pass between it and the
liquid. So, that, if in a battery of fifty pairs of plates, ten of the
cells contain a weaker charge than the others, it is as if ten decomposing
plates were opposed to the transit of the current of forty pairs of
generating plates (1031.). Hence a serious loss of force, and hence the
reason why, if the ten pairs of plates were removed, the remaining forty
pairs would be much more powerful than the whole fifty.
1043. Five similar troughs, of ten pairs of plates each, were prepared,
four of them with a good uniform charge of acid, and the fifth with the
partially neutralized acid of a used battery. Being arranged in right
order, and connected with a volta-electrometer (711.), the whole fifty
pairs of plates yielded 1.1 cubic inch of oxygen and hydrogen in one
minute: but on moving one of the connecting wires so that only the four
well-charged troughs should be included in the circuit, they produced with
the same volta-electrometer 8.4 cubical inches of gas in the same time.
Nearly seven-eighths of the power of the four troughs had been lost,
therefore, by their association with the fifth trough.
1044. The same battery of fifty pairs of plates, after being thus used, was
connected with a volta-electrometer (711.), so that by quickly shifting the
wires of communication, the current of the whole of the battery, or of any
portion of it, could be made to pass through the instrument for given
portions of time in succession. The whole of the battery evolved 0.9 of a
cubic inch of oxygen and hydrogen in half a minute; the forty plates
evolved 4.6 cubic inches in the same time; the whole then evolved 1 cubic
inch in the half-minute; the ten weakly charged evolved 0.4 of a cubic inch
in the time given: and finally the whole evolved 1.15 cubic inch in the
standard time. The order of the observations was that given: the results
sufficiently show the extremely injurious effect produced by the mixture of
strong and weak charges in the same battery[A].
[A] The gradual increase in the action of the whole fifty pairs of
plates was due to the elevation of temperature in the weakly charged
trough by the passage of the current, in consequence of which the
exciting energies of the fluid within were increased.
1045. In the same manner associations of _strong and weak_ pairs of plates
should be carefully avoided. A pair of copper and platina plates arranged
in _accordance_ with a pair of zinc and platina plates in dilute sulphuric
acid, were found to stop the action of the latter, or even of two pairs of
the latter, as effectually almost as an interposed plate of platina
(1011.), or as if the copper itself had been platina. It, in fact, became
an interposed decomposing plate, and therefore a retarding instead of an
assisting pair.
1046. The _reversal_, by accident or otherwise, of the plates in a battery
has an exceedingly injurious effect. It is not merely the counteraction of
the current which the reversed plates can produce, but their effect also in
retarding even as indifferent plates, and requiring decomposition to be
effected upon their surface, in _accordance_ with the course of the
current, before the latter can pass. They oppose the current, therefore, in
the first place, as interposed platina plates would do (1011-1018.); and to
this they add a force of opposition as counter-voltaic plates. I find that,
in a series of four pairs of zinc and platina plates in dilute sulphuric
acid, if one pair be reversed, it very nearly neutralizes the power of the
whole.
1047. There are many other causes of reaction, retardation, and
irregularity in the voltaic battery. Amongst them is the not unusual one of
precipitation of copper upon the zinc in the cells, the injurious effect of
which has before been adverted to (1006.). But their interest is not
perhaps sufficient to justify any increase of the length of this paper,
which is rather intended to be an investigation of the theory of the
voltaic pile than a particular account of its practical application[A].
[A] For further practical results relating to these points of the
philosophy of the voltaic battery, see Series X. § 17.
1163.--1160.--_Dec. 1838._
_Note_.--Many of the views and experiments in this Series of my
Experimental Researches will be seen at once to be corrections and
extensions of the theory of electro-chemical decomposition, given in the
Fifth and Seventh Series of these Researches. The expressions I would now
alter are those which concern the independence of the evolved elements in
relation to the poles or electrodes, and the reference of their evolution
to powers entirely internal (524. 537. 661.). The present paper fully shows
my present views; and I would refer to paragraphs 891. 904. 910. 917. 918.
947. 963. 1007. 1031. &c., as stating what they are. I hope this note will
be considered as sufficient in the way of correction at present; for I
would rather defer revising the whole theory of electro-chemical
decomposition until I can obtain clearer views of the way in which the
power under consideration can appear at one time as associated with
particles giving them their chemical attraction, and at another as free
electricity (493. 957.).--M.F.
_Royal Institution,
March 31st, 1834._