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ELECTRO-CHEMISTRY. 411

properties, and only recover them when dismissed at the elec-
trodes by which they have been respectively attracted.

If the direction of the current be reversed, so that it shall
enter at E and issue from A, the constituents of the salt will
be transported back to the opposite ends of the series, the acid
which had been deposited in A, will be transferred successively
through the cups B, c, D, and the intermediate siphons to the
cup E, and the alkali in the contrary direction from E through
D, c, B, and the siphons to A. This will be manifested by the
changes of colour of the infusions. The liquid in A which had
been reddened by the acid, will first recover its original colour,
and then become green according as the ratio of the acid to
the alkali in it is diminished ; and in like manner the infusion
in E, which had been rendered green by the alkali, will gra-
dually recover its primitive colour, and then become red as the
proportion of the acid to the alkali in it is augmented.

During these processes no change of colour will be observed
in the intermediate cups B and D.

The intermediate cups B and D being filled with various che-
mical solutions for which the constituents of the salt had strong
affinities, and with which under any ordinary circumstances
they would immediately enter into combination, these consti-
tuents nevertheless invariably passed through the intermediate
vessels without producing any discoverable effect upon their
contents. Thus, sulphuric acid passed in this manner through
solutions of ammonia, lime, potash, and soda, without affecting
them. In like manner hydrochloric and nitric acids passed
through concentrated alkaline menstrua without any chemical
effect. In a word, acids and alkalis having the strongest mu-
tual affinities, were thus reciprocally made to pass each through
the other without manifesting any tendency to combination.

2084. Exception in the case of producing insoluble com-
pounds. Strontia and baryta passed in the same way through
muriatic and nitric acids, and reciprocally these acids passed
with equal facility through solutions of strontia and baryta.
But an exception was encountered when it was attempted to
transmit strontia or baryta through a solution of sulphuric acid,
or vice versa. In this case the alkali was arrested in transitu
by the acid, or the acid by the alkali, and the salt resulting
from their combination was precipitated in the intermediate
cup.

x 2



412 VOLTAIC ELECTRICITY.

The exception therefore generalised included those cases in
which bodies were attempted to be transmitted through men-
strua for which they have an affinity, and with which they
would form an insoluble compound.

2085. This transfer denied by Faraday. This transmission
of chemical substances through solutions with which they have
affinities by the voltaic current, those affinities being rendered
dormant by the influence of the current which appeared to be
established by the researches of Davy, published in 1807, and
since that period received by the whole scientific world as an
established principle, has lately been affirmed by Dr. Faraday
to be founded in error. According to Faraday no such transfer
of the constituents of a body decomposed by the current can or
does take place. He maintains that in all cases of electro-
lysation it is an absolutely indispensable condition that there
be a continuous and unbroken series of particles of the elec-
trolyte between the two electrodes at which its constituents are
disengaged. Thus, when water is decomposed, there must be
a continuous line of water between the positive electrode at
which the oxygen is developed and the negative electrode at
which the hydrogen is disengaged. In like manner, when the
sulphate of soda or any other salt is decomposed, there must be
a continuous line of particles of the salt between the positive
electrode at which the acid appears and the negative electrode
at which the alkali is deposited.

Dr. Faraday affirms, that in Davy's celebrated experiments,
in which the acid and alkaline constituents of the salt appear
to be drawn through intermediate cups containing pure water or
solutions of substances foreign to the salt, the decomposition
and apparent transfer of the constituents of the salt could not
have commenced until, by capillary attraction, a portion of the
salt had passed over through the siphons, so that a continuous
line of saline particles was established between the electrodes.
Dr. Faraday admits such a transfer of the constituents as may
be explained by the series of decompositions and recompo-
sitions involved in the hypothesis of Grotthus.

2086. Apparent transfer explained by him on Grotthus 1
hypothesis. It is also admitted by Dr. Faraday, that when
pure water intervenes between the metallic conductors pro-
ceeding from the pile and the electrolyte, decomposition may
ensue, but he considers that in this case the true electrodes



ELECTRO-CHEMISTRY. 413

are not the extremities of the metallic conductors, but the
points where the pure water ends and the electrolyte begins,
and that accordingly in such cases the constituents of the elec-
trolyte will be disengaged, not at the surfaces of the metallic
conductors, but at the common surfaces of the water and the
electrolyte. As an example of this he produces the following
experiment. Let a solution of the sulphate of magnesia be
covered with pure water, care being taken to avoid all ad-
mixture of the water with the saline solution. Let a plate of
platinum proceeding from the negative pole of a battery be
immersed in the water at some distance from the surface of the
solution on which the water rests, and at the same time let the
solution be put in metallic communication with the positive
pole of the battery. The decomposition of the sulphate will
speedily commence, but the magnesia, instead of being deposited
on the platinum plate immersed in the water, will appear at
the common surface of the water and the solution. The water,
therefore, and not the platinum, is in this case the negative
electrode.

2087. Faraday thinks that conduction and decomposition
are closely related. Dr. Faraday maintains that the con-
nection between conduction and decomposition, so far as relates
to liquids which are not metallic, is so constant that decom-
position may be regarded as the chief means by which the
electric current is transmitted through liquid compounds. Ne-
vertheless, he admits, that when the intensity of a current is
too feeble to effect decomposition, a quantity of electricity is
transmitted sufficient to affect the reoscope.

In accordance with those principles, Faraday affirms that
water which conducts the electric current in its liquid state,
ceases to do so when it is congealed, and then it also resists de-
composition, and in fine ceases to be an electrolyte. He holds
that the same is true of all electrolytes.

2088. Maintains that non-metallic liquids only conduct when
capable of decomposition by the current. The connection
between decomposition and conduction is further manifested,
according to Dr. Faraday, by the fact that liquids which do not
admit of electro-chemical decomposition, do not give passage
to the voltaic current. In short, that electrolytes are the only
liquid non-metallic conductors.

2089. Faraday's doctrine, not universally accepted Pouil-



414 VOLTAIC ELECTRICITY,

let's observations. These views of Dr. Faraday have not yet
obtained general acceptation ; nor have the discoveries of Davy
of the transfer and decantation of the constituents of electro-
lytes through solutions foreign to them, been yet admitted to be
overthrown. Peschel and other German authorities in full
possession of Faraday's views and the results of his experi-
mental researches, still continue to reproduce Davy's expe-
riments, and to refer to their results and consequences as
established facts. Pouillet, writing in 1847, and also in pos-
session of Faraday's researches, which he largely quotes, main-
tains nevertheless the transport of the constituents under
conditions more extraordinary still, and more incompatible with
Faraday's doctrine than any imagined by Davy. In electro-
chemical decomposition he says, " There is at once separation
and transport. Numberless attempts have been made to seize
the molecule of water which is decomposed, or to arrest en
route the atoms of the constituent gases before their arrival at
the electrodes, but without success. For example, if two cups
of water, one containing the positive and the other the ne-
gative wire of a battery, be connected by any conductor, singular
phenomena will be observed. If the intermediate conductor
be metallic, decomposition will take place independently in
both cups" (as already described), " but if the intermediate
conductor be the human body, as when a person dips a finger
of one hand into the water in one cup, and a finger of the
other hand into the other, the decomposition will sometimes
proceed as in the case of a metallic connection ; but more gene-
rally oxygen will be disengaged at the wire which enters the
positive cup, and hydrogen at the wire which enters the ne-
gative cup, no gases appearing at the fingers immersed in the
one and the other. It would thus appear that one or other of
the constituent gases must pass through the body of the ope-
rator in order to arrive at the pole at which it is disengaged.
And even when the two cups are connected by a piece of ice,
the decomposition proceeds in the same manner, one or other
gas appearing to pass through the ice, since they are disen-
gaged at the poles in the separate cups in the same manner."*
2090. Davy's experiments repeated and confirmed by Bec-
querel. The experiments of Davy, in which the transfer of

* Pouillet, Elements de Physique. Ed. 1847, vol. i. p. 598.



ELECTRO-CHEMISTRY. 415

the constituents of an electrolyte through water and through
solutions for which these constituents have affinities, was
demonstrated, have been repeated by Becquerel, who has ob-
tained the same results. The capillary siphons used by Bec-
querel were glass tubes filled with moistened clay. He also
found that the case in which the constituent transferred would
form an insoluble compound with the matter forming the inter-
mediate solution, forms an exception to this principle of transfer ;
but he observed that this only happens when the intensity of
the current is insufficient to decompose the compound thus
formed in the intermediate solution.*

2091. The electrodes proved to exercise different electrolytic
powers by Pouillet. The question whether the decomposing
agency resides altogether at one or at the other electrode, or is
shared between them, has been recently investigated by M.
Pouillet.

Let three tubes of glass having the form of the letter U,
, fig. 662., be prepared, each of the
vertical arms being about five
inches long, and half an inch in
diameter. Let the curved part
of the tubes connecting the legs
have a diameter of about the
twentieth of an inch when the so-
lutions used are good conductors,
but the same diameter as the tubes
themselves when the conducting
power is more imperfect. In this

latter case, however, the results are less exact and satisfactory.
Let platinum wire E and E' proceeding from the poles of a
voltaic battery be plunged in the first and last tubes, and let
the intermediate tubes be connected by similar wires 11' and
i" i'". Let acidulated water be poured into the tube E i, and
the solutions on which the relative effects of the two electrodes
are to be examined, into the other tubes 1 1" and i'" E'. After
the electrolysis has been continued for a certain time, the quan-
tity of the solution decomposed in each leg may be ascertained
by submitting the contents of each leg to analysis. The quan-
tity remaining undecomposed being thus ascertained and sub-

* Becquerel, Traite de Physique, vol. ii. p. 330. Ed. 1844.
T 4




416 VOLTAIC ELECTRICITY.

traded from the original quantity, the remainder will be the
quantity decomposed, since the fluids are prevented from inter-
mixing to any sensible extent by the smallness of the con-
necting tube, and by being nearly at the same level during the
process. It may be assumed that the decomposing agencies of
the two electrodes will be proportional to the quantities of the
solutions decomposed in the legs in which they are respectively
immersed.

2092. Case in which the negative electrode alone acts. The
current being first transmitted through a voltameter to indi-
cate the actual quantity of electricity transmitted, the tubes EI,
i' i" and i'" E' were filled, the first with a solution of the chlo-
ride of gold, the next with the chloride of copper, and the third
with the chloride of zinc. After the lapse of a certain interval
the contents of the tubes were severally examined, and it was
found that the solutions in legs in which the positive electrodes
were immersed had suffered no decomposition. The quantities
of the chlorides contained in them respectively were undimi-
nished, while the chloride in each of the legs containing the
negative electrodes was diminished by exactly the quantity
corresponding to the metal deposited in the negative wire, and
the chlorine transferred to the positive leg.

It was therefore inferred that in these cases the entire decom-
posing agency must be ascribed to the negative electrode.

The same results were obtained for the other metallic
chlorides.

2093. Cases in which the electrodes act unequally. The
alkaline chlorides showed somewhat different properties. In
the case of the chloride of magnesium the agency of the ne-
gative was found to be greater than that of the positive elec-
trode, but it was not exclusively efficacious. In the cases of
the chlorides of potassium, sodium, barium, &c., the agency
was also shared by the true electrodes, but the agency of the
positive electrode was found to be greater than the negative in
the ratio of about three to one.

2094. Liquid electrodes Series of electrolytes in immediate

contact In general, the electrodes by which the current

enters and departs from an electrolyte, are solid and most fre-
quently metallic conductors. In an experiment already cited
(2086.), Faraday has shown that water may become an elec-
trode, and Pouillet in some recent experiments has succeeded



ELECTRO-CHEMISTRY.



417



in generalising this result, and has shown not only that the
current may be transmitted to and received from an electrolyte
by liquid conductors, but that a series of different electrolytes
may become mutual electrodes, the current passing immediately
from one to the other without any intermediate conductor, solid
or liquid, and that each of them shall be electrolysed. Thus
suppose that the series of electrolytes are expressed by

a a' bb' cc' d<I

the current as indicated by the arrows entering A, and de-
parting from D, and being supposed to have sufficient intensity
to effect the electrolysis of all the solutions. Let the electro-
negative constituents be expressed by a, b, c, d, and the electro-
positive by a', b 1 , c', d'. It is evident that the points at which
any two succeeding solutions touch, will be at the same time
the negative electrode of the first, and the positive electrode of
the second, and that,' consequently, the positive constituent of
the first and the negative constituent of the second will be dis-
engaged at this point, and being in the nascent state will be
under the most favourable conditions to combine in virtue of
their affinities, and so to form new compounds as secondary
effects. Thus, the common surface of A and B will be the ne-
gative electrode of A, and the positive electrode of B, because it
is at this surface that the current departs from A and enters B,
and accordingly the electro-positive constituent a' of A, and the
electro-negative constituent b of B, will be developed at this
common surface, and if they have affinity, will enter into
combination.

2095. Experimental illustration of this. These principles
may be experimentally illustrated and verified by placing the
electrolytic solutions in U-shaped
tubes T, T', T", as represented
\nfig. 663. Let two electrolytic
solutions A and B be introduced
into the first tube T, so carefully
as to prevent them from inter-
mixing, and let their common
surface be at o. In like manner
let the solutions B and c be in-
troduced into the tube T', and
the solutions c and D into the

T 5




418 VOLTAIC ELECTRICITY.

tube T", their common surfaces being at o' and o". Let the
legs of the tubes T and T', which contain the solution B, be
connected by a glass siphon containing the same solution, and
the legs of the tubes T' and T", containing the solution c, be
similarly connected. Let the positive wire of a battery be im-
mersed in A, and the negative wire in D, the current being
sufficiently intense to electrolyse all the solutions. *

In this case o will be the positive electrode of B, and the
negative electrode of A, o' the positive electrode of c, and the
negative electrode of B, and o" the positive electrode of D, and
the negative electrode of C.

If A be pure water, B the chloride of zinc, the water being
decomposed, oxygen will be disengaged at the positive wire,
and hydrogen at the common surface o. The chloride being
also decomposed, the chlorine, its electro-negative constituent,
will be disengaged at o, where it will enter into combination
with the hydrogen, and form hydrochloric acid, the presence of
which may be ascertained by the usual tests. The oxyde of
zinc, the electro-positive constituent of B, will be disengaged at
o', and will form a compound with the electro-negative con-
stituent of c, and so on.

2096. Electrolysis of the alkalis and earths. The decom-
posing power of the voltaic current had not long been known
before it became, in the hands of Sir H. Davy and his suc-
cessors, the means of resolving the alkalis and earths, before that
time considered as simple bodies, into their constituents. This
class of bodies was shown to be oxydised metals. When sub-
mitted to such conditions as enabled a strong voltaic current to
pass through them, oxygen was liberated at the positive elec-
trode, and the metallic base appeared at the negative electrode.

2097. The series of new metals. A new series of metals
was thus discovered, which received names derived from those
of the alkalis and earths of which they formed the bases. Thus,
the metallic base of potash was called POTASSIUM, that of soda,
SODIUM, that of lime, CALCIUM, that of silica, SILICIUM, and
so on.

* This is not the experimental arrangement adopted by M. Pouillet. It
lias occurred to me, as a method of exhibiting his principle under a more
general form and somewhat more clearly and satisfactorily than his apparatus,
in which the siphons s, s' have no place.



ELECTRO-CHEMISTRY. 419

In many cases it is difficult to maintain those metals in their
simple state, owing to their strong affinity for oxygen. Thus
potassium, if exposed to the atmosphere at common tempe-
ratures, enters directly into combination with the air, and
burns. When it is desired to collect and preserve it in the
metallic state it is decomposed by the current in contact with
mercury, with which it enters into combination, forming an
amalgam. It is afterwards separated by distillation from the
mercury, and preserved in the metallic state under the oil of
naphtha, in a glass tube hermetically closed, the air being
previously expelled.

2098. Schcenbein's experiments on the passivity of iron.
Among the effects of the voltaic current which have been not
satisfactorily or not at all explained, are those by which iron,
under certain conditions, is enabled to resist oxydation even
when exposed to agents of the greatest power ; such, for ex-
ample, as nitric acid. The most remarkable researches on this
subject are those of Schoenbein. In his experiments, the wires
proceeding from the poles of the battery were immersed in two
mercurial cups, which we shall call P and N. A bath of water B,
acidulated with about 8 per cent, of sulphuric acid, was then
connected with the cup N by a platinum wire. A piece of iron
wire was placed with one extremity in P, and the other in the
bath B. No oxydation was manifested at the end immersed in
the bath, and no hydrogen was evolved at the platinum wire.
In fine, no electrolysis took place.

Several circumstances were found to restore to the iron its
oxydable property, and to establish the electrolysis of the liquid
in the bath, but only for a short interval of a few seconds.
These circumstances were : 1. The contact for a moment of
the platinum and iron wires in the bath. 2. The momentary
suspension of the current by breaking the contact at any point
of the circuit. 3. The contact of any oxydable metal, such as
zinc, tin, copper, or silver, with the iron in the bath. 4. The
momentary diversion of a portion of the current, by connecting
the cups P and N by a copper wire, without breaking the con-
nections of the original circuit. 5. By agitating the end of
the iron wire in the bath.

If in connecting P and B by the iron wire the wire be first
immersed in B, oxydation will take place for some seconds after
the other acid is immersed in P.
T 6



420 VOLTAIC ELECTRICITY.

The intensity of the current diverted by connecting the cups
p and N by a copper wire, can be varied at pleasure by varying
the length and section of the connecting wire (2063.). When
such a derived current is established, several curious and inte-
resting phenomena are observed. When the derived current
has great intensity, no effect is produced upon the iron. Upon
gradually diminishing the intensity of the derived current, the
iron becomes active, that is, susceptible of oxydation. With a
less intensity it again becomes passive, and the oxydation
ceases. As the derived current is gradually reduced to that
intensity at which the iron becomes permanently passive, there
are several successive periods during which it is alternately
active and passive, the intervals between these periods being
less and less. In the apparatus of Schosnbein the iron became
permanently active when the copper wire conducting the de-
rived current was half a line thick, and from 6 inches to
16 feet long.

These effects are reproduced with all the oxacids, but are
not manifested either with the hydracids or the Haloid salts.

2099. Other methods of rendering iron passive. Iron may
be rendered passive also by placing it as the positive electrode
in a solution of acetate of lead with a current of ordinary in-
tensity. The iron should be immersed in the solution for about
half a minute to a depth of about half an inch. A wire thus
treated being washed clean, acquires the permanently passive
property, even though the part immersed in the solution has
not been coated with the peroxyde of lead. And in this case
the conditions above stated under which it recovers moment-
arily its active character, become inoperative.

Iron thus galvanised acquires to a great degree the virtue of
platinum and the other highly negative metals, and for many
purposes may be substituted for them. Thus Schcenbein has
constructed voltaic batteries of passive iron and zinc.

The iron wire used for telegraphic purposes is rendered
passive by this process.

2100. Tree of Saturn. The well known experiment of the
TREE OF SATURX presents a remarkable example of the effect of
a feeble current of long continuance. A bundle of brass wires
is passed through a hole made longitudinally through the centre
of a bottle cork, and fitted tightly in it so as to diverge in a
sort of cone from the bottom of the cork. A plate of zinc is



ELECTRO-CHEMISTRY. 421

then tied round the wires at the point where they diverge from
the cork, so as to be in contact with all the wires. The wires
and cork are then introduced into a glass flask containing a
limpid solution of the acetate of lead, and the top of the cork
luted over to prevent the admission of air. The zinc and brass
thus immersed in the solution form a voltaic pair, and a current
passes through the solution from the zinc to the wire. The
water of the solution is slowly decomposed, the oxygen com-
bining with the zinc, and the hydrogen attracting the oxygen
from the oxyde of lead, and reproducing water, while the me-
tallic lead attaches itself to the wires. The acetic acid liberated
by the secondary decomposition of the acetate of lead, enters
into combination with the oxyde of zinc, and produces the



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