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cluded by the platinum plates and (d) owing to the difference of the
levels of the water in the two burettes, the hydrogen is under
greater hydrostatic pressure than is the oxygen.

The chemical action is usually explained by saying that the
water is decomposed into its component gases hydrogen and
oxygen, and this is correct but it is not the primary reaction which
takes place. The sulphuric acid is first separated into H 2 and SO 4,
the hydrogen being released and the SO 4 then attacking the water,

so that the oxygen is
released as the result of a secondary reaction.

220. Faraday's Terminology. The decomposition of chemical
compounds by the electric current was investigated by Faraday
to whom is due the terminology now employed. As we have
already seen, the liquid which undergoes decomposition is called
the electrolyte and the process itself is electrolysis. The vessel in
which electrolysis takes place is called an electrolytic cell. The
plates or wires which dip into the liquid and by which the current
is brought in and taken out are termed collectively the electrodes;
that by which the current enters is the anode; that by which it
leaves is the cathode. The part molecules into which the substance
being decomposed is split are, in allusion to their movement
through the liquid, called ions (wanderers) ; those which appear at
the anode are anions; those released at the cathode are cathions
or kations.

221. Substances Subject to Electrolysis. In order that a
substance may be electrolyzed it must fulfill the following condi-


tions; it must be a compound substance; it must be a conductor;
it must be in a liquid state, either as the result of fusion or of
solution. Mercury and the fused metals are conducting liquids
but being elementary bodies can not be decomposed. All other
conducting liquids undergo electrolysis.

222. Electrolysis of a Fused Compound. The electrolysis of
lead chloride may be taken as an example of the decomposition
of a fused compound. The salt is kept in a molten state in a small
porcelain crucible placed over a bunsen burner. The electrodes
of iron dip into the fused mass. When a current passes, chlorine
is liberated at the anode, as may be shown by the bleaching effect
upon a piece of litmus paper held just above, and lead is released
at the cathode.

223. Electrolysis of a Base. In many cases of electrolysis the
primary reactions are obscured by the secondary. In the elec-
trolysis of water (Par. 219), it is really the sulphuric acid that is
electrolyzed, the decomposition of the water being the result of
secondary reactions. Similar results follow the electrolysis of the
strong bases. For example, a solution of potassium hydroxide
electrolyzes as follows:

2KOH = K 2 +H 2 0+0

the oxygen appearing

at the anode and the potassium being released at the cathode, but
as soon as this metal is released it attacks the water, thus

K+2H 2 0=2KOH+H 2

*SALJ so that the net

result is the same as when sulpflunc acid is electrolyzed, that is,
the water is decomposed. If, however, the cathode be of mercury,
the potassium amalgamates with it and by distilling off the mer-
cury from the amalgam the potassium may be separated and col-
lected. In a somewhat similar manner to this, Davy discovered
in October, 1807, first potassium and rapidly thereafter sodium
and other alkaline and alkaline-earth metals.

224. Electrolysis of a Metallic Salt. When a metallic salt in
solution is electrolyzed, the metal appears at the cathode, the acid
radicle at the anode, but, as mentioned above, this primary re-
action is frequently obscured by secondary reactions.

In the electrolysis of an alkali oxy-salt, these secondary reactions


occur at both anode and cathode. For example, if sodium sulphate
be electrolyzed the sodium is released at the cathode but imme-
diately reacts with the water releasing hydrogen. The SO 4 is
released at the anode and, as described above, reacts with the
water releasing oxygen. The net result therefore is simply the
electrolysis of the water.

If a solution of copper sulphate be electrolyzed the copper is
deposited upon the cathode and the S0 4 is released at the anode
where one of two effects may be produced according as the anode
is or is not attacked by the S0 4 . If the anode be of platinum, the
SO 4 attacks the water, forming sulphuric acid and releasing
oxygen. If, however, the anode be of copper, the S0 4 attacks it,
producing copper sulphate which goes into solution. As fast as
copper is deposited upon the cathode, an equal amount is dissolved
from the anode; the electrolyte therefore remains of constant
strength. This is true for other metals than copper. If a salt -of a
metal be electrolyzed between electrodes of that metal, the anode
wastes away, the cathode increases and the electrolyte remains of
constant concentration.

The metals, which in the above are said to be released at the
cathode, are really deposited upon the cathode in a compact and
tightly adhering layer. This is the basis of the important proc-
esses of electroplating and electrotyping to be described later.
Electrolysis has many other important applications, such as the
electrolytic refining of copper, the manufacture of chlorine, of the
alkaline hydroxides, of aluminum, etching on metal, photo-en-
graving, etc.

225. Electro-Chemical Classification of the Elements. The
elements have been classed according to their behaviour under
electrolysis. Those which move in the direction of the current and
are released at the cathode are called electro-positive, this name
being given because they move to the negative plate. Those which
move against the current and appear at the anode or positive plate
are called electro-negative. Hydrogen and the metals are electro-
positive; the non-metals are electro-negative. It will be noted
that in its electro-chemical behaviour hydrogen conforms to its
purely chemical behaviour and arranges itself with the metals.
The above classification, which is also extended to compound ions,
is not absolute; an element in certain compounds being electro-
positive, while in others it may be electro-negative.


226. Faraday's First Law. It was stated above (Par. 215) that
when an electric current is flowing there is no material substance
in movement but there is a transfer of energy which manifests
itself in the production of heat, of magnetic effects, and of chemical
decomposition. It is a known fact that the same amount of chem-
ical action always produces the same amount of energy and, con-
versely, the same expenditure of energy in the production of
chemical decomposition always brings about the same amount.
The truth of this was recognized by Faraday, the first to investi-
gate the laws of electrolysis, and was formulated by him to the
effect that the amount of chemical action produced in an electrolytic
cell is proportional to the quantity of electricity which flows through
the cell. The amount of chemical action produced by the passage
of an electric current may therefore be taken as a measure of the
quantity transferred.

227. Voltameters. An electrolytic cell so made that the
chemical action produced by the current can be accurately
measured, and hence the current determined, is called a voltameter.
Voltameters are arranged so that the metal (usually silver or
copper), deposited upon the cathode may be weighed, or the
amount of gas released may be measured and its weight calculated.
The latter class, the gas voltameters, may collect the gases sepa-
rately, as shown in Fig. 107, or may gather these gases in a common
burette thereby obtaining a greater volume for measurement.

We shall shortly see that there is another instrument, a volt-
meter, used for quite a different purpose, the measurement of
electro-motive force. It is unfortunate that these names are so
much alike and the beginner must be on his guard not to confound
the two.

228. The Coulomb and the Ampere. To define a current of
water, it is not sufficient to state the amount of water which will
flow past a certain point but we must also state the rate at which
it flows past. So also with the electric current; we must know both
the quantity and the rate at which this quantity is delivered.

The practical unit of electrical quantity, the coulomb, is defined as
that quantity of electricity which flowing through a gas volta-
meter liberates .00001035+ of a gram of hydrogen.

Now, a very feeble current must flow a long time to accom-
plish the same amount of chemical work as a current of greater


strength; on the other hand, the greater the current, the greater
the amount of work done in a given time. We can therefore com-
pare currents by comparing the amount of chemical work done in
a given time. The practical unit of current, the ampere, is defined
as that unvarying current which flowing through a gas voltameter
liberates .00001035+ of a gram of hydrogen per second. Why this
particular weight was selected will be explained later (Pars. 231,
232, 450). From the foregoing, it is seen that a current of one
ampere delivers one coulomb per second, or that if Q be the number
of coulombs, / be the current in amperes, and t be the time in
seconds, then


This may also be written I=Q/t, whence we see that the cur-
rent in amperes is equal to the rate at which coulombs are de-
livered, or the number of coulombs per second.

The unit of quantity, the coulomb, must not be confused with
the electro-static unit of quantity as defined in Par. 56. The
coulombjs equal to very nearly 3 XlO 9 or three billion of the elec-
trostatic units.

With practical experience in the Laboratory, the student will
soon form a conception of the ampere which at first must be to
him more or less of an abstraction. The current employed in the
16 candle power 110 volt incandescent lamp is about one-half

In solving ordinary problems given for practice, it is sufficiently
accurate to take the amount of hydrogen released by one coulomb
as .00001 (one one-hundred thousandth) of a gram.

229. Equality of Current at Every Cross-Section of a Circuit.

At every cross-section of a circuit through which a current is
flowing, the current is the same. This is a simple principle but
often confuses the beginner who has a tendency to suppose that a
current may start out of a certain strength but may be used up
and dwindle away as it progresses around the circuit. The current
may be compared to water which completely fills a pipe bent into
the shape of a ring. No water can move at any point unless
exactly the same amount moves at every other cross-section of the

A corollary following directly from the above is that the amount
of chemical action at every cross-section of a circuit is the same.



This may be shown experimentally as follows. In Fig. 108, A
represents a battery of Daniell cells connected one after the other,
or in series (three are represented in the diagram but as many as
may be necessary are used), and B, C, D, and E represent copper
voltameters. When the key K is closed, completing the circuit,
a current flows through the battery, through B, then divides, part
going through C and the rest through D, then reunites, passes
through E and back to the negative pole of the battery. Before

Fig. 108.

closing the key, the cathodes of the voltameters and of each of the
Daniell cells are carefully weighed. After the current has flowed
for a while, the key is opened, stopping the current, and the
cathodes are removed, dried, and carefully reweighed. They are
all found to have increased in weight, the increase being exactly
the same in all except C and D and in these their joint increase
being equal to the increase in each of the other cathodes. It is to
be especially noted that the amount of chemical action is also the
same in every one of the battery cells in series.

230. Faraday's Second Law. Suppose we arrange a similar
experiment with a number of voltameters in series but each con-
taining different compounds. Suppose one to be a gas voltameter,
one to contain a solution of silver nitrate, one of copper sulphate,
one of cuprous chloride and one of tin tetra-chloride. If now the
key be closed, the same current will traverse them all. After the
current has flowed for a while, open the key, remove and weigh
the cathodes and measure and calculate the weight of the hydrogen
evolved in the gas voltameter. If we take the weight of this
hydrogen as unity we will find that 107.9 parts of silver, 31.8 parts
of copper in the copper sulphate solution, 63.6 parts in the cuprous
chloride solution and 29.8 parts of tin have been deposited. But


these numbers, 107.9, 31.8, 63.6, and 29.8 are the equivalent
weights of the corresponding elements in the respective compounds.
(The equivalent weight of an element or of a radicle is defined as
that weight of it which combines with or displaces or is chemically
equal to one part by weight of hydrogen. It may be obtained by
dividing the atomic weight of the element, or the molecular weight
of the radicle, by the valency which it has in the compound under
consideration.) The foregoing results are expressed in Faraday's
second law which is to the effect that the weights of the ions of
different substances liberated by the same quantity of electricity
are to each other as the equivalent weights of these ions.

231. Electro -Chemical Equivalent. The electro-chemical equiv-
alent of an element is the weight in grams of that element
liberated by one coulomb. By definition (Par. 228) one coulomb
liberates .00001035+ of a gram of hydrogen, which is therefore
its electro-chemical equivalent. The electro-chemical equivalent
of any other element is obtained by multiplying its equivalent
weight by this electro-chemical equivalent of hydrogen. For
example, for silver it is 107.93 X. 00001035+ =.001118, and for
copper it is .000328.

To liberate one gram of hydrogen (about four-tenths of a cubic
foot at ordinary temperature) requires 1/.00001035+, or, in
round numbers, 96,540 coulombs. This would require a current
of one ampere to flow for nearly 27 hours. This quantity of elec-
tricity, 96,540 coulombs, will release one gram-equivalent of any
ion, as for example 8 grams of oxygen, 107.93 grams of silver, etc.

From the foregoing, it is seen that to find the weight of any ion
released by a given current in a given time, we determine the num-
ber of coulombs and multiply this number by the electro-chemical
equivalent of the ion.

232. Definition of the Ampere in Terms of Silver. For prac-
tical purposes, because of the difficulty of handling and weighing
a gas, it is desirable to have the ampere defined in terms of some
solid element instead of hydrogen. Silver is found to be the most
suitable and copper the next. In the preceding paragraph we have
seen that the electro-chemical equivalent of silver, the weight
deposited by one coulomb, or one ampere flowing for one second,
is .001118 gram. The International Congress of Electricians
of 1893 in the resolutions already referred to (Par. 212), accord-


ingly defined the ampere as that unit "which is represented suf-
ficiently well for practical use by the unvarying current which
when passed through a solution of nitrate of silver in water, in
accordance with the accompanying specification, deposits silver
at the rate of .001118 gramme per second."

233. Applications of Electrolysis, Refining of Copper. Copper
as it comes from the smelter may contain impurities of two kinds,
first, objectionable substances such as arsenic, antimony, etc.,
which injure its ductility and its electrical properties and, second,
small amounts of gold and silver which it is desirable to recover
if possible. The impure copper is cast into slabs which are used
as the anodes in large electrolytic tanks, the electrolyte being a
solution of copper sulphate and the cathodes being thin sheets of
pure copper. As the current passes, the anode is eaten away, the
pure copper being deposited upon the cathode and the impurities
settling as a slime to the bottom of the tank whence they are
removed from time to time and treated according to their value.
If the impure copper contains much gold or silver, the anodes may
be enclosed in canvas bags which permit the free passage of the
solution but catch the slime which falls. The copper is refined at
the rate of about seven pounds per hour per horse-power expended.

234. Electroplating. The object to be plated is immersed in
the electrolyte and serves as the cathode. In gold and silver
plating, the anode is a plate of the desired metal and the electrolyte
is a double cyanide of potassium and this metal. The deposits
from these cyanides are smoother and more compact than those
from other salts. There must be a certain relation between the
current and the area of the surface to be plated. If the current be
too great, the deposit is granular or coarsely crystalline and may
not adhere. Portions of the surface which are not to be plated
may be covered with a coating of wax or varnish.

235. Electrotyping. The process of electrotyping is employed
to obtain exact reproductions of wood cuts, engraved plates,
forms of set type, etc. The need for such reproductions is readily
understood. If impressions be taken direct from a wood cut it
rapidly wears away and frequently gives out when about 5000
have been struck off. By electrotyping, a reproduction of the cut
can be made in copper and this reproduction can be used many
thousand times and as many others may be made as desired, the


original cut not suffering in the slightest. Again, a great many
million postage stamps are printed annually by the Government
and not only must they be struck off several hundred in a sheet
but several presses must be running at the same time. If each
plate had to be engraved separately the cost would be tremendous
and no two stamps on a sheet would be exactly alike. However,
the engraver prepares a plate for a single stamp and hundreds of
reproductions can be made and these reproductions can then be
united in one large plate. Finally, when type have been set for a
printed page they are withdrawn from the printer's stock. Should
this run low, he must either purchase more or distribute those
which have been set up, thus undoing the work. However, by
electrotyping he can reproduce the entire page in one piece and the
type then become available for other use.

The process consists in pressing the cut or type to be reproduced
into a sheet of wax or other plastic material, thus making a mould.
The interior of this mould is then dusted with very finely powdered
graphite or bronze by which the surface is made a conductor, and
using this as the cathode a thin layer of copper is deposited upon
it. This thin layer is then backed by pouring into it melted type
metal and the resulting plate is fastened to a wooden block.



236. Reversibility of Cells. Should a simple zinc-carbon cell
be connected in closed circuit, a current will be produced and
while it is flowing the zinc will waste away and go into solution
as zinc sulphate, the electrolyte will grow weaker and hydrogen
will be evolved at the carbon plate. Suppose now the circuit to
be broken and that there be inserted in it a battery or an electrical
machine faced in the opposite direction to the original cell. If
this battery or machine produces a greater electro-motive force
than the ceH, a current will be set up opposite to the original cur-
rent and will flow through the cell in a reverse direction, that is,
the simple cell now becomes an electrolytic cell (Par. 220). The
zinc sulphate in solution will be decomposed, the zinc being rede-
posited upon the zinc plate (Par. 224), the electrolyte increasing
in strength and oxygen being released at the carbon plate, in other
words, if the current continues to flow for a sufficient length of
time the previous chemical action will be undone and, with the
exception of the loss of a small amount of water in the form of
hydrogen and oxygen, the cell will be restored to its primary con-
dition. Such a cell is said to be reversible. It is evident that a
primary cell in which the chemical action results in the escape in
the form of gas of a portion of the active material can not be en-
tirely reversible.

237. Storage Battery. A cell which is thus reversible and
which when exhausted is regenerated by passing through it from
an extraneous source of electrical energy a current opposite in
direction to the flow of discharge, is called a secondary cell, or an
accumulator, or, more commonly, a storage battery, although
strictly the word "battery," as already pointed out, should be
applied to a group of two or more cells. When such a battery
approaches exhaustion it is said to be discharged, and the operation
of restoring it is called charging. As commonly understood, a
storage battery is one whose primary condition is that of exhaus-


tion, that is, one which can not be used until it has first been
charged. Reflection will show that the charging current must
enter the battery by the same pole from which the discharging
current leaves, that is, by the positive pole. The academic dis-
tinction between the positive pole and the positive plate of voltaic
cells (Par. 193) is not observed in dealing with storage batteries
and the positive plate is that which carries the positive pole and is
that plate from which the current issues on discharge and by which
it enters on charge. In these storage batteries there is no elec-
tricity stored up. The charging current enters at the positive
pole, passes through the battery and leaves by the negative pole,
but in its passage it performs chemical work or builds up a certain
chemical potential which later produces electrical energy when the
proper connections are made.

238. Elements of a Secondary Cell. Experiments have been
conducted with many substances to determine their fitness for
the elements of a secondary cell but, with the exception of
the recently introduced nickel-iron-potassium hydroxide cell of
Edison (Par. 250), the great majority of storage batteries employ
positive plates of lead peroxide, Pb0 2 , negative plates of pure
lead, and an electrolyte of dilute sulphuric acid of a specific
gravity of about 1.20, or about one part of acid by bulk to five of
water. There are many objections to lead; it is very heavy, it is
soft, and the workmen in it frequently suffer from lead poisoning.
There must then be some peculiar qualities of lead which outweigh
these disadvantages. Upon examining its chemical properties we
are at once struck by the fact that it is the only commercial metal
whose sulphate is insoluble. When, therefore, the electrolyte
attacks the plates and produces lead sulphate, this salt does not
pass off into solution but remains at the precise spot where formed
and when the cell is charged the sulphate is reconverted into lead
without any change of position. Repeated charging and dis-
charging, therefore, does not materially alter the shape of the

239. Preparation of the Plates. The peroxide of lead of the
positive plate and the pure lead of the negative plate are designated
as the active material of the cell. Since the chemical action, the
source of the electrical energy developed, takes place only at
the surface of contact of the active material and the electrolyte,


the object held in view in preparing the plates is to give to this
active material the maximum amount of surface. This object is
attained in any one or combination of three ways.

(a) Mechanical. The plate may be deeply incised, or grooved,
or fluted, or thin tape-like ribbons of lead may be corrugated,
coiled up and inserted in apertures in the plate proper, or the active
material may be applied to the plate as a paste, or it may be
powdered and placed in perforated receptacles which are attached
to the plate.

(b) Chemical. The metal may be eaten by acids until it
becomes more or less spongy, or it may be cast mixed with a
granulated substance which is subsequently dissolved out leaving
the plate porous.

(c) Electrolytic. The plate may have attached to it or enclosed
in cavities in it a salt of the metal, which salt, as may be desired,

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