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acetate of that metal, which passes into solution in the water.
The contents of the flask are gradually converted into a so-
lution of the acetate of zinc, and the metallic lead, the process
being very slow, is crystallised in a variety of beautiful forms
upon the divergent brass wire.

2101. Davy's method of preserving the copper sheathing of
ships. The method proposed by Sir H. Davy to preserve
from corrosion the copper sheathing of ships, depends on the
long-continued action of feeble currents. The copper is united
with a mass of zinc, iron, or some more oxydable metal, so as
to form a voltaic combination. The sea-water being a weak
solution of salt, a feeble permanent current is established
between the more and less oxydable metals, passing through the
water from the latter to the former, and causing its slow de-
composition. The oxygen combines with the protecting metal,
and the hydrogen disengaged on the copper, decomposes the
salts held in solution in the sea water, attracting their oxyde
constituents, such as lime, magnesia, &c., which are deposited
upon the copper in a rough crust. Upon the coating thus
formed collect marine vegetation, shells, and other substances.
Thus, while the copper sheathing is preserved from corrosion,
there arises the counteracting circumstance of an appendage to
the hull of the ship, which impedes its sailing qualities.

2102. Chemical effects produced in voltaic batteries. The
fluids interposed between the solid elements of all forms of
voltaic arrangements may be regarded as electrolytes, the solid
elements in contact with them being the electrodes. In all
arrangements in which one acid solution is interposed between


two solids, one of which is more oxydable than the other, the
primary chemical effect is the decomposition of the water of the
solution, the oxygen being disengaged upon the more and the
hydrogen upon the less oxydable metal. The partisans of the
chemical origin of the current contend, that in this case the
oxygen gives up its negative electricity to the more oxydable,
and the hydrogen its positive to the less oxydable metal, so that
if those metals be connected by a metallic wire outside the
system, a current will be established on the wire, directed from
the less to the more oxydable metal, which is found to be in
effect what actually takes place.

The secondary effects are very various and complicated, and
are in accordance with the principles of electrolytic action
already explained. The first is the production of an oxyde of
the more oxydable metal. If the liquid element be an acid
solution, this oxyde will, in general, enter into combination
with the acid, forming a salt which will be the next secondary
effect. The hydrogen evolved against the less oxydable metal,
may either be liberated in the gaseous state, or attract an equi-
valent of oxygen from one of the substances dissolved in the
liquid element, thereby reproducing water, and decomposing
the substances thus attacked.

2103. Effect of amalgamating the zinc. The advantage
which attends the amalgamation of the zinc (1862.), in batteries
in which that metal is used, is, that it is thus protected from
the direct action of the acid by which the metal would be con-
sumed, without contributing to the supply of the current.
Zinc being a metal eminently susceptible of oxydation, would
be affected by the acid even in the weakest solutions, and a
portion of the metal more or less according to the strength of
the solution would be oxydated, a corresponding quantity of
the acid being decomposed. This would produce the twofold
inconvenience of the ineffectual consumption of both the metal
and the acid. By the process of amalgamation the surface of
the zinc is coated with a thin stratum of amalgam, which is
proof against the acid, and which on the other hand increases
the susceptibility of the metal to combine with the oxygen
disengaged from the water by the current.

2104. Effects in Smee's battery. The preceding obser-
vations are more or less applicable to all the single fluid ar-
rangements described in (1858.) to (1863.), as well as to Smee's


system (1870.). It has been found that the intensity of the
current in the system of Smee, has been greatly augmented by
platinisation of the surface of the metal used as the negative
pole of the system. The process by which this effect is pro-
duced is as follows. The surface of the metal being scraped
quite clean, it is immersed in a solution of the double chloride
of potassium and platinum, and being connected with the ne-
gative pole of a battery, of which the positive pole is connected
with the solution by a plate of platinum immersed in it, elec-
trolysation takes place, platinum being deposited on the ne-
gative electrode as a secondary result. The positive platinum
electrode is meanwhile attacked by the chlorine, and dissolved
so as to maintain the solution in the proper state of saturation.

Mr. Smee, after trying plates of iron and silver, substituted
for them plates of platinum, the surface of which was plati-
nised by this process, and an improved action ensued. M. Bou-
quillon substituted for them a plate of copper, upon the surface
of which a rough coating of copper was first deposited by a
similar process, and upon this a coating of silver, over which a
coating of platinum was deposited. By this means the plate is
stated to have acquired in the highest degree the power of li-
berating the hydrogen, and stimulating the current.

2105. In Wheatstone s battery. In the system of Wheat-
stone (1871.), in which a single fluid, a saturated solution of
sulphate of copper, is used as the exciting fluid, the water of
the solution is electrolysed. The oxygen combines with the
zinc of the amalgam, and forms as a secondary result an oxyde.
The hydrogen disengaged upon the copper acts upon the
sulphate, whose constituents are sulphuric acid and the pro-
toxyde of copper. It attracts the oxygen from the protoxyde,
with which it combines, forming water. The metallic copper
and the acid are both liberated, the latter entering into com-
bination with the oxyde of zinc, and forming the sulphate of
that oxyde. For each equivalent of zinc oxydated, there is
therefore an equivalent of metallic copper, and an equivalent of
the sulphate of the oxyde of zinc produced.

2106. In the two fluid batteries. In batteries in which two
fluids separated by a porous diaphragm are interposed between
the solid elements, chemical effects somewhat more complicated
are exhibited, and indeed authorities are not in accordance as
to the principles on which the phenomena are explicable.


2107. Grove's battery. In the case of Grove's battery
(1865.), the primary phenomenon is the decomposition of the
water of the two solutions which is effected through the cylinder
of porous porcelain, the oxygen being disengaged upon the
zinc, and the hydrogen upon the platinum. The secondary
effects are 1. The oxydation of the zinc. 2. The combination
of the oxyde with the acid, producing sulphate of zinc, which
is deposited in the acid solution. 3. The combination of the
hydrogen with one equivalent of the oxygen of the nitric acid,
by which water is reproduced, and the nitric reduced to nitrous
acid, which is evolved in the gaseous state.

2108. Bunsen's battery. The same phenomena are mani-
fested in the battery of Bunsen (1866.), which differs from that
of Grove only in the substitution of the carbon for the platinum

2109. DanieVs battery The theory of the chemical phe-
nomena developed in the operation of Daniel's constant battery
(1867.), has not been clearly determined. During the per-
formance of the apparatus metallic copper is deposited on the
copper cylinder cc, Jig. 541., which contains the solution of the
sulphate of copper. The solution itself would become gradually
less concentrated, but it is maintained in a state of saturation by
the continual dissolution of the salt, which rests upon the wire
grating G, and the sulphate of the oxyde of zinc accumulates in
the cylinder pp. The fluid in this vessel pp, which is at the
beginning a weak solution of sulphuric acid, is gradually con-
verted into a solution of the sulphate of zinc, which is stated by
Pfaff, Paggendorf, and others to be nearly as effective as an
exciting fluid as the original acid solution. The electromotive
virtue of the zinc in this case is therefore but little, and that of
the copper not at all impaired by the continued action of the

The chemical changes which are effected in this battery may
be explained as follows. A double electrolysis may be ima-
gined as a primary effect, that of the water contained in the
two solutions, the electrolytic action being transmitted through
the intermediate porous cylinder and that of the sulphate, the
acid constituent being transmitted to the zinc through the
porous cylinder, and through the solution contained in it in the
same manner as the constituents of the electrolyte in Davy's
experiments (2082.), were transmitted through the capillary


siphons and the intermediate solutions. The secondary phe-
nomena would be as follows : The hydrogen of the decom-
posed water developed on the copper in a nascent state, would
attract the oxygen of the oxyde of copper also developed on the
copper in a nascent state by the decomposition of the salt.
Water would thus be reproduced in a quantity equal to that
lost by decomposition, and the metallic copper of the oxyde
would be deposited on the copper cylinder. The solution of
the sulphate being reduced below saturation by the amount of
the salt thus decomposed, an equal quantity would be received
by dissolution from the salt in G, which would restore it to
the state of saturation. Meanwhile the acid constituent of the
salt would be transferred to the zinc, where it would combine
with the oxyde of that metal formed by its combination with
the oxygen of the decomposed water developed upon it, and
the sulphate of the oxyde would be formed, which would be
dissolved in the solution, leaving the amalgamated zinc free to
the further operation of the current.

This interpretation of the phenomena cannot be admitted by
those who, with Dr. Faraday, reject the principle of the transfer
of an electrolyte through a menstruum foreign to it, unless
indeed it be assumed that some small portions of the sulphate
must pass through the porous cylinder, and mix with the acid

However this may be, we are not aware that any other
satisfactory explanation has been proposed for the phenomena
developed in this battery.



2110. Origin oj this art. The decomposing power of the
voltaic current applied to solutions of the salts and oxydes of
metals has supplied various processes to the industrial arts,
which inventors, improvers, and manufacturers have denomi-
nated galvano-plastic, electro- plastic, galvano-type, electrotype,
and electro plating and gilding. These processes and their re-


suits may be comprehended under the more general denomi-

2111. The metallic constituent deposited on the negative
electrode. If a current of sufficient intensity be transmitted
through a solution of a salt or oxyde, having a metallic base,
it will be understood, from what has been already explained,
that while the oxygen or acid is developed at the positive elec-
trode, the metal will be evolved either by the primary or second-
ary action of the current at the negative electrode, and being
in the nascent state, will have a tendency to combine with it, if
there be an affinity, or to adhere to it by mere cohesion, if not.

2112. Any body may be used as the negative electrode. The
bodies used as electrodes must be superficially conductors, since
otherwise the current could not pass between them; but subject
to this condition, they may have any material form or mag-
nitude which is compatible with their immersion in the solution.
If the body be metallic, its surface has necessarily the conduct-
ing property. If it be formed of a material which is a non-
conductor, or an imperfect conductor, the power of conduction
may be imparted to its surface by coating it with finely pow-
dered black lead and other similar expedients. This process is
called metallising the surface.

2113. Use of a soluble positive electrode. By the continuance
of the process of decomposition the solution would be rendered
gradually weaker, and the deposition of the metal would go on
more slowly. This inconvenience is remedied by using, as the
positive electrode, a plate of the same metal, which is deposited
on the negative electrode. The acid or oxygen liberated in the
decomposition, in this case, enters into combination with the
metal of the positive electrode, and produces as much salt or
oxyde, as is decomposed at the other electrode, which salt or
oxyde being dissolved as fast as it is formed, maintains the
solution at a nearly uniform degree of strength.

2114. Conditions which affect the state of the metal depo-
sited. The state of the metal disengaged at the negative
electrode depends on the intensity of the current, the strength
of the solution, its acidity, and its temperature, and the regula-
tion of these conditions in each particular case, will require
much practical skill on the part of the operator, since few
general rules can be given for his direction.

In the case, for example, of a solution of one of the salts of


copper, a feeble current will deposit on the electrode a coating
of copper so malleable that it may be cut with a knife. With
a more intense current the metal will become harder. As the
intensity of the current is gradually augmented, it becomes
successively brittle, granulous, crystalline, rough, pulverulent,
and in fine loses all cohesion, practice alone will enable the
operator to observe the conditions necessary to give the coating
deposited on the electrode the desired quality.

2115. The deposit to be of uniform thickness. It is in all
cases desirable, and in many indispensable, that the metallic
coating deposited on the electrode shall have an uniform thick-
ness. To insure this, conditions should be established which
will render the action of the current on every part of the sur-
face of the electrode uniform, so that the same quantity of
metal may be deposited in the same time. Many precautions
are necessary to attain this object. Both electrodes should be
connected at several points with the conductors, which go to the
poles of the battery, and they should be presented to each other
so that the intermediate spaces should be as nearly as possible
equal, since the intensities of the currents between point and
point vary with the distance. The deposition of the metal is
also much influenced by the form of the body. It is in general
more freely made on the salient and projecting parts, than in
those which are sunk.

2116. Means to prevent absorption of the solution by the
electrode. If the body on which the metallic deposit is made
be one Avhich is liable to absorb the solution, a coating of some
substance must be previously given to it which shall be im-
pervious to the solution.

2117. Non-conducting coating used where partial deposit is
required. When a part only of a metallic or other conducting
body is desired to be coated with the metallic deposit, all the
parts immersed not intended to be so coated are protected by
a coating of wax, tallow, or other non-conductor.

2118. Application of these principles to gilding, silvering, fyc.
The most extensive and useful application of these principles
in the arts is the processes of gilding and silvering articles made
of the baser metals. The article to be coated with gold being
previously made perfectly clean, is connected with the negative
pole of the battery, while a plate of gold is connected with its
positive pole. Both are then immersed in a bath consisting of


a solution of the chloride of gold and cyanide of potassium, in
proportions which vary with different gilders. Practice varies
also as to the temperature and strength of the solution. The
chloride is decomposed, the metallic base being deposited as a
coating on the article connected with the negative pole, and the
chloride combining with a corresponding portion of the gold
connected with the positive pole, and reproducing the chloride
which is dissolved in the bath as fast as it is decomposed, thus
maintaining the strength of the solution.

A coating of silver, copper, cobalt, nickel, and other metals is
deposited by similar processes.

2119. Cases in which the coating is inadhesive. When the
article on which the coating is deposited is metallic, the coating
will in some cases adhere with great tenacity. In others, the
result is less satisfactory ; as, for example, where gold is de-
posited on iron or steel. In such cases the difficulty may be
surmounted by first coating the article with a metal which
will adhere to it, and then depositing upon this the definitive

2120. Application to gilding, silvering, or bronzing objects of
art. The extreme tenuity with which a metallic coating may
be deposited by such processes, supplies the means of imparting
to various objects of art the external appearance and qualities
of any proposed metal, without impairing in the slightest de-
gree their most delicate forms and lineaments. The most ex-
quisitely moulded statuette in plaster may thus acquire all the
appearance of having been executed in gold, silver, copper, or
bronze, without losing any of the artistic details on which its
beauty depends.

2121. Production of metallic moulds of articles. If it be
desired to produce a metallic mould of any object, it is generally
necessary to mould it in separate pieces, which being afterwards
combined, a mould of the whole is obtained. That part in-
tended to be moulded is first rubbed with sweet oil, blacklead,
or some other lubricant, which will prevent the metal deposited
from adhering to it, without separating the mould from the
surface, in so sensible a degree as to prevent the perfect cor-
respondence of the mould with the original. All that part not
intended to be moulded is invested with wax or other material,
to intercept the solution. The object being then immersed, and
the electrolysis established, the metal will be deposited on the


exposed surface. When it has attained a sufficient thickness
the object is withdrawn from the solution, and the metallic
deposit detached. It will be found to exhibit, with the utmost
possible precision, an impression of the original. The same
process being repeated for each part of the object, and the par-
tial moulds thus obtained being combined, a metallic mould of
the whole will be produced.

2122. Production of objects in solid metal. To reproduce
any object in metal it is only necessary to fill the mould of it,
obtained by the process above explained, with the solution of
the metal of which it is desired to form the object, the surface
of the mould being previously prepared, so as to prevent ad-
hesion. The solution is then put in connection with the posi-
tive pole of the pile, while the mould is put in connection with
the negative pole. The metal is deposited on the mould, and
when it has attained the necessary thickness the mould is de-
tached, and the object is obtained.

In general, however, it is found more convenient to mould
the object to be reproduced in metal by the ordinary processes
in wax, plaster of paris, or fusible alloy. When they are made
in wax, plaster, or any non-conducting material, their inner sur-
faces must be rubbed with black-lead, to give them the con-
ducting power. When the deposit is made of the necessary
thickness, the mould is broken off or otherwise detached.

Statues, statuettes, and bas-reliefs in plaster can thus be re-
produced in metal with the greatest facility and precision, at an
expense not much exceeding that of the metal of which they
are formed.

2123. Reproduction of stereotypes and engraved plates.
A mould in plaster of paris or wax being taken from a wood
engraving and a stereotype plate, a stereotype may be obtained
from the mould by the processes above described. Stereotypes
by the ordinary processes of casting in type metal are, however,
produced at less expense.

Copper or steel engraved plates may be multiplied by like
methods. A mould is first taken, which exhibits the engraving
in relief. A metallic plate deposited upon this by the electro-
lytic process will reproduce the engraved plate.

2124. Metallising textile fabrics. The electro-metallurgic
processes have been extended by ingenious contrivances to
other substances besides metal. Thus a coating of metal may


be deposited on cloth, lace, or other woven fabric, by various
ingenious expedients, of which the following is an example :
On a plate of copper attach smoothly a cloth of linen, cotton,
or wool, and then connect the plate with a negative pole of a
voltaic battery, immerse it in a solution of the metal with
which it is to be coated, and connect a piece of the same metal
with the positive pole ; decomposition will then commence, and
the molecules of metal, as they are separated from the solution,
must pass through the cloth in advancing to the copper to
which the cloth is attached. In their passage through the
cloth they are more or less arrested by it. They insinuate
themselves into its pores, and, in fine, form a complete metallic
cloth. Lace is metallised in this way by first coating it with
plumbago, and then subjecting it to the electro-metallurgic

Quills, feathers, flowers, and other delicate fibrous substances
may be metallised in the same way. In the case of the most
delicate of these the article is first dipped into a solution of
phosphorus and sulphate of carbon, and is well wetted with the
liquid. It is then immersed in a solution of nitrate of silver.
Phosphorus has the property of reviving silver and gold from
their solutions. Consequently, the article is immediately
coated with a very attenuated fibre of the metal.

2125. Glyphography. If a thin stratum of wax or other
soft substance be spread upon a plate of metal, any subject or
design may be engraved upon the coating without more labour
than would be expended on a pencil drawing. When the en-
graving is thus made on the wax it is subjected to the electro-
type process, by which a sheet of copper or other metal is
deposited upon it. When this is detached it exhibits in relief
the engraving, from which impressions may be produced in the
same manner as from a wood engraving, to which it is altogether

2126. Reproduction of Daguerreotypes One of the most

remarkable and unexpected applications of the electrotype pro-
cess is to Daguerreotypes. The picture being taken upon the
plate by the usual process of Daguerreotype, a small part of
the back is cleaned with sand-paper, taking care not to allow
the face of the plate to be touched. A piece of wire is then
soldered to the part of the back thus prepared. The plate is
then immersed in a solution of copper, and connected with the


battery, the back being protected by a coating of wax. After a
deposit of sufficient depth has been made upon the face of the
plate, it is withdrawn from the solution, and the plate of copper
deposited being detached, exhibits the picture with an expres-
sion softer and finer than the original. By this process when
conducted with skill, several copies may be taken from the
same Daguerreotype.

If the electrotype copy thus obtained be passed through a
weak solution of the cyanide of gold and potassium in con-
nexion with a weak battery, a beautiful golden tint will be
imparted to the picture, which serves to protect it from being



2127. Common principle of all electric telegraphs. Of all
the applications of electric agency to the uses of life, that
which is transcendently the most admirable in its effects, and
the most important in its consequences, is the electric telegraph.
No force of habit, however long continued, no degree of fami-
liarity can efface J;he sense of wonder which the effects of this
most marvellous application of science excite.

The electric telegraph, whatever form it may assume, derives
its efficiency from the three following conditions :

Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 41 of 45)