Dionysius Lardner.

Hand-book of natural philosophy and astronomy (Volume 2) online

. (page 38 of 45)
Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 38 of 45)
Font size
QR-code for this ebook

metals relatively to that of distilled mercury taken at 100 :

Metals. Conducting Power.

Mercury - 100

Iron ... . 6OO to 700








500 to SOO

200 to 900






2045. Current passing through a compound circuit of uni-
form intensity. The current which passes through a compound

s 3


circuit is found to have an uniform intensity throughout its
entire course. In passing through a length of wire, which is
a bad conductor, its intensity is neither greater nor less than
upon one which is a good conductor, and its intensity on pieces
of unequal section and unequal length is in like manner exactly
the same.

2046. Equivalent simple circuit. A simple circuit composed
of a wire of any proposed metal and of any proposed thickness
can always be assigned upon which the current would have
the same intensity as it has on any given compound circuit ;
for by increasing the length of such circuit the intensity of the
current may be indefinitely diminished, and by diminishing its
length the intensity may be indefinitely increased. A length
may therefore be always found which will give the current any
required intensity.

The length of such a standard wire which would give the
current of a simple circuit the same intensity as that of a com-
pound circuit, is called the reduced length of the compound

2047. Ratio of intensities in tico compound circuits. It is
evident, therefore, that the intensities of the currents on two
compound circuits are in the inverse ratio of their reduced
lengths, for the wires composing such reduced lengths are sup-
posed to be of the same material and to have the same thickness.

2048. Intensity of the current on a given conductor varies
ivith the thermo-electric energy of the source. In all that has
been stated above, we have assumed that the source of thermo-
electric agency remains the same, and that the changes of in-
tensity of the current are altogether due to the greater or less
facility with which it is allowed to pass along the conducting
wires from one pole of the thermo-electric source to the other.
But it is evident, that with the same conducting circuit, whether
it be simple or compound, the intensity of the current will vary
either with the degree of disturbance of the thermal equilibrium
of the system or with the thermo-electric energy of the sub-
stance composing the system.

In the case already explained, the ends of the cylinders A and
B have been maintained at the fixed temperatures of 32 and
212. If they had been maintained at any other fixed tempera-
tures, like phenomena would have been manifested, with this
difference only, that with the same circuit the intensity of the


current would be different, since it would be increased if the
difference of the temperature of the extremities were increased,
and would be diminished if that difference were diminished.

In like manner, if, instead of bismuth, antimony, zinc, or any
other metal were used, the same circuit and the same tempera-
tures of the ends c and D or E and F would exhibit a current
of different intensity, such difference being due to the different
degree of thermo-electric agency with which the different
metals are endowed.

The relative thermo-electric agency of different sources of
these currents, whether it be due to a greater or less disturb-
ance of the thermal equilibrium, or to the peculiar properties of
the substance whose temperature is deranged, or, in fine, to both
of these causes combined, is in all cases proportional to the
intensity of the current which it produces in a wire of given
material, length, and thickness, or in general to the intensity of
the current it transmits through a given circuit.

The relative thermo-electric energy of two systems may be
ascertained by placing them as at A and B, fig. 656., and con-
necting them by simple circuits of similar wire with the differ-
ential reometer. Let the lengths of the wires composing the
two circuits be so adjusted, that the currents passing upon them
shall have the same intensity. The thermo-electric energy of
the two systems will then be in the direct ratio of the lengths
of the circuits.

2049. Thermo-electric piles. The intensity of a thermo-
electric current may be augmented indefinitely by combining
together a number of similar thermo-electric elements, in a
manner similar to that adopted in the formation of a common
voltaic battery. It is only necessary, in making such arrange-
ment, to dispose the elements so that the several partial cur-
rents shall all flow in the same direction.

Such an arrangement is represented in fig. 657., where the
two metals (bismuth and copper, for example) composing each


Fig. 657.
B 4


thermo-electric pair are distinguished by the thin and thick
bars. If the points of junction marked 1, 3, 5, &c. be raised to
212, while the points 2, 4, 6, &c. are kept at 32, a current will
flow from each of the points 1, 3, 5, &c. towards the points 2, 4, 6,
&c. respectively, and these currents severally overlaying each
other, exactly as in the voltaic batteries, will form a current
having the sum of their intensities.

2050. Thermo-electric pile of Nobili and Melloni Various

expedients have been suggested for the practical construction
of such thermo-electric piles, one of the most efficient of which
is that of MM. Nobili and Melloni. This pile is composed of a
series of thin plates of bismuth and antimony, bent at their ex-
tremities, so that when soldered together they have the form
and arrangement indicated in Jig. 658. The

\ -rr ^ S p aces Between the successive plates are filled

' by pieces of pasteboard, by which the corn-

Fig. 658. bination acquires sufficient solidity, and the
plates are retained in their position without being pressed into
contact with each other. The pile thus formed is mounted in
a frame as represented in^/fy. 659., and its poles are connected
with two pieces of metal by which the
current may be transmitted to any conduc-
tors destined to receive it. It will be per-
ceived that all the points of junction of the
plates of bismuth and antimony which are
presented at the same side of the frame are
Fig. 659. alternate in their order, the 1st, 3rd, 5th, &c.
being on one side, and the 2nd, 4th, 6th, &c. on the other. If,
then, one side be exposed to any source of heat or cold from
which the other is removed, a corresponding difference of tem-
perature will be produced at the alternate joints of the metal, and
a current of proportionate intensity will flow between the poles
p and n upon any conductor by which they may be connected.

It is necessary, in the practical construction of this apparatus,
that the metallic plates composing it should be all of the same
length, so that when combined the ends of the system where
the metallic joints are collected should form an even and plain
surface, which it is usual to coat with lampblack, so as to
augment its absorbing power, and at the same time to render
it more even and uniform.

This was the form of thermo-electric pile used by M. Melloni


in the series of exprimental researches adverted to in 1564,
and the manner in which it was applied is exhibited in all its
details in^. 451., where jaw are the poles of the system, and
p the reometer through which the current is transmitted.



2051. Decomposing power of a voltaic current. When a vol-
taic current of sufficient intensity is made to pass through cer-
tain bodies consisting of constituents chemically combined, it is
found that decomposition is produced attended by peculiar cir-
cumstances and conditions. The compound is resolved into
two constituents, which appear to be transported in contrary
directions, one with and the other against the course of the
current. The former is disengaged at the place where the
current leaves, and the other at the place where it enters the

All compounds are not resolvable into their constituents by
this agency, and those which are are not equally so ; some being
resolved by a very feeble current, while others yield only to one
of extreme intensity.

2052. Electrolytes and electrolysis. Bodies which are capa-
ble of being decomposed by an electric current have been called
ELECTROLYTES, and decomposition thus produced has been de-

2053. Liquids alone susceptible of electrolysis. To render
electrolysis practicable, the molecules of the electrolyte must
have a perfect freedom of motion amongst each other. The
electrolyte must therefore be liquid. It may be reduced to this
state either by solution or fusion.

2054. Faraday's electro-chemical nomenclature. It has
been usual to apply the term poles either to the terminal ele-
ments of the pile, or to the extremities of the wire or other
conductor by which the current passes from one end and enters
the other. These are not always identical with the points at
which the current enters and leaves an electrolyte. The same

s 5


current may pass successively through several electrolytes, and
each will have its point of entrance and exit ; but it is not
considered that the same current shall have more than two
poles. These and other considerations induced Dr. Faraday to
propose a nomenclature for the exposition of the phenomena
of electrolysis, which has to some extent obtained acceptation.

2055. Positive and negative electrodes. He proposed to
call the points at which the current enters and departs from
the electrolyte, ELECTRODES, from the Greek word 6(5<ie (odos),
a path or way. He proposed further to distinguish the points
of entrance and departure by the terms ANODE and KATHODE,
from the Greek words avotios (anodos), the way up, and KadoSoc
(kathodes), the way down.

2056. Only partially accepted. Dr. Faraday also gave the
name IONS to the two constituents into which an electrolyte is
resolved by the current, from the Greek word lav (Ion), going
or passing, their characteristic property being the tendency to
pass to the one or the other electrode. That which passes to
the positive electrode, and which therefore moves against the
current, he called the ANION ; and that which passes to the
negative electrode and therefore moves with the current, he
called the KATHION. These terms have not, however, ob-
tained acceptation. Neither have the terms "Anode" and
" Kathode," positive and negative electrode, or positive and
negative pole, being almost universally preferred.

The constituent of an electrolyte which moves with the cur-
rent is distinguished as the positive element, and that which
moves against it as the negative element. These terms are de-
rived from the hypothesis that the constituent which appears
at the positive electrode, and which moves, or seems to move
towards it after decomposition, is attracted by it as a particle
negatively electrified would be ; while that which appears at the
negative electrode is attracted to it as would be a particle
positively electrified.

2057. Composition of water. To render intelligible the
process of electrolysis, let us take the example of water, the
first substance upon which the decomposing power of the pile
was observed. Water is a binary compound, whose simple
constituents are the gases called oxygen and hydrogen. Nine
grains weight of water consist of eight grains of oxygen and
one grain of hydrogen.


The specific gravity of oxygen being sixteen times that of
hydrogen (779), it follows that the volumes of these gases
which compose water are in the ratio of two to one, so that a
quantity of water which contains as much oxygen as in the
gaseous state would have the volume of a cubic inch, contains
as much hydrogen as would, under the same pressure, have the
volume of two cubic inches.

The combination of these gases, so as to convert them
into water, is determined by passing the electric spark taken
from a common machine through a mixture of them. If eight
parts by weight of oxygen and one of hydrogen, or, what is
the same, one part by measure of oxygen and two of hydrogen,
be introduced into the same receiver, on passing through them
the electric spark an explosion will take place ; the gases will
disappear, and the receiver will be filled first with steam, which
being condensed, will be presented in the form of water. The
weight of water contained in the receiver will be equal pre-
cisely to the sum of the weight of the two gases.

These being premised, the phenomena attending the electro-
lysis of water may be easily understood.

2058. Electrolysis of water. Let a glass tube, closed at one
end, be filled with water slightly acidulated, and stopping the
open end, let it be inverted and immersed in similarly acidu-
lated water contained in any open vessel. The column in the
tube will be sustained there by the atmospheric pressure as the
mercurial column is sustained in a barometric tube ; but in this
case the tube will remain completely filled, no vacant space ap-
pearing at the top, the height of the column being considerably
less than that which would balance the atmospheric pressure.
Let two platinum wires be connected with the poles of a vol-
taic pile, and let their extremities, being immersed in the vessel
containing the tube, be bent so as to be presented upwards in
the tube without touching each other. Immediately small
bubbles of gas will be observed to issue from the points of the
wires, and to rise through the water and collect in the top of
the tube, and this will continue until the entire tube is filled
with gas, by the pressure of which the water will be expelled
from it. If the tube be now removed from the vessel, and the
gas be transferred to a receiver, so arranged that the electric
spark may be transmitted through it, on such transmission the
gas will be reconverted into water.
s 6


The gases, therefore, evolved at the points of the wire, which
in this case are the electrodes, are the constituents of water ;
and since they cannot combine to form water, except in the
definite ratio of 1 to 2 by measure, they must have been
evolved in that exact portion at the electrodes.

2059. Explanation of this phenomenon by the electro-che-
mical hypothesis. This phenomenon is explained by the sup-
position that the voltaic current exercises forces directed upon
each molecule of the water, by which the molecules of oxygen
are impelled or attracted towards the positive electrode, and
therefore against the current, and the molecules of hydrogen
towards the negative electrode, and therefore with the current,
The electro-chemical hypothesis is adopted by different parties
in different senses.

According to some, each molecule of oxygen is invested with
an atmosphere of negative, and each molecule of hydrogen with
an atmosphere of positive electricity, which are respectively
inseparable from them. When these gases are in their free
and uncombined state, these fluids are neutralized by equal
doses of the opposite fluids received from some external source,
since otherwise they would have all the properties of electrified
bodies, which they are not observed to have. But when they
enter into combination, the molecule of oxygen dismisses the
dose of positive electricity, and the molecule of hydrogen the
dose of negative electricity which previously neutralized their
proper fluids ; and these latter fluids then exercising their
mutual attraction, cause the two gaseous molecules to coalesce
and to form a molecule of water.

When decomposition takes place, a series of opposite effects
are educed. The molecule of oxygen after decomposition is
charged with its natural negative, and the molecule of hydrogen
with its natural positive flyid, and these molecules must borrow
from the decomposing agent or some other source the doses of
the opposite fluids which are necessary to neutralize them. In
the present case, the molecule of oxygen is reduced to its na-
tural state by the positive fluid it receives at the positive
electrode, and the molecule of hydrogen by the negative fluid it
receives at the negative electrode.

The electro-chemical hypothesis is, however, differently un-
derstood and differently stated by different scientific authorities.



It is considered by some that the decomposing forces in the
case of the voltaic current are the attractions and repulsions
which the two opposite fluids developed at the electrodes ex-
ercise upon the atmospheres of electric fluid, which are assumed
in this theory to surround and to be inseparable from the mo-
lecules of oxygen and hydrogen which compose each molecule
of water, the resultants of these attractions and repulsions being
two forces, one acting on the oxygen and directed towards the
positive electrode, and the other acting on the hydrogen and
directed towards the negative electrode. Others, with Dr. Fa-
raday, deny the existence of these attractions, and regard the
electrodes as mere paths by which the current enters and
leaves the electrolyte, and that the effect of the cuiTent in
passing through the electrolyte is to propel the molecules of
oxygen and hydrogen in contrary directions, the latter in the
direction of the current, and the former in the contrary di-
rection ; and that this, combined with the series of decom-
positions and recompositions imagined by Grotthus, which we
shall presently explain, supplies the most satisfactory exposition
of the phenomena.

Our limits, however, compel us to dismiss these speculations,
and confine our observations rather to the facts developed by
experimental research, using, nevertheless, the language derived
from the theory for the purposes of explanation.

2060. Method of electrolysis which separates the constituents.
The process of electrolysis may be so conducted that the
constituent gases shall be developed and collected in separate

The apparatus represented in fig. 660., contrived by Mit-
scherlich, is very convenient for the exhibition
of this and other electrolytic phenomena.
Two glass tubes o and h, about half an inch
in diameter, and 6 or 8 inches in length, are
closed at the top and open at the bottom,
having two short lateral tubes projecting
from them, which are stopped by corks,
through which pass two platinum wires which
terminate within the tubes in a small brush
of fine platinum wire, which may with ad-
vantage be surrounded at the ends with
Fig. 660. spongy platinum. The tubes o, h being uni-


formly cylindrical and conveniently graduated, are filled with
acidulated water, and immersed in a cistern of similarly aci-
dulated water g.

If the external extremities of the platinum wires be con-
nected by means of binding screws a and b, or by mercurial
cups with wires which proceed from the poles of a voltaic
arrangement, their internal extremities will become electrodes,
and electrolysis will commence. Oxygen gas will be evolved
from the positive, and hydrogen from the negative electrode,
and these gases will collect in the two tubes, the oxygen in the
tube o containing the positive, and the hydrogen in the tube h
containing the negative electrode. The graduated scales will
indicate the relative measures of the two gases evolved, and it
will be observed that throughout the process the quantity of
gas in the tube h is double the quantity in the tube o. If the
gases be removed from the tubes to other receivers and sub-
mitted to chemical tests, one will be found to be oxygen and
the other hydrogen.

2061. How are the constituents transferred to the electrodes'?
In the apparatus^. 660., the tubes containing the electrodes
are represented as being near together. The process of elec-
trolysis, however, will equally ensue when the cistern g is a
trough of considerable length, the tubes o and h being at its
extremities. It appears, therefore, that a considerable extent of
liquid may intervene between the electrodes without arresting
the process of decomposition. The question then arises, where
does the decomposition take place ? At the positive electrode,
or at the negative electrode, or at what intermediate point ? If
it take place at the positive electrode, a constant current of
hydrogen must flow from that point through the liquid to the
negative electrode ; if at the negative electrode a like current
of oxygen must flow from that point to the positive electrode ;
and if at any intermediate point, two currents must flow in con-
trary directions from that point, one of oxygen to the positive,
and one of hydrogen to the negative electrode. But no trace
of the existence of any such currents has ever been found. In-
numerable expedients have been contrived to arrest the one or
the other gas in its progress to the electrode without success ;
and therefore the strongest physical evidence supports the po-
sition that neither of these constituent gases do actually exist
in the separate state at any part of the electrolyte, except at


the very, electrodes themselves at which they are respectively

If this be assumed, then it will follow that the molecules of
oxygen and hydrogen evolved at the two electrodes were not
previously the component parts of the same molecule of water.
The molecule of oxygen evolved at the positive electrode must
be supplied by a molecule of water contiguous to that electrode,
while the molecule of hydrogen simultaneously evolved at the
negative electrode must have been supplied by another mole-
cule of water contiguous to the latter electrode. What then
becomes of the molecule of hydrogen dismissed by the former,
and the molecule of oxygen dismissed by the latter ? Do they
coalesce and form a molecule of water ? But such a combi-
nation would again involve the supposition of currents of gas
passing through the electrolyte, of the existence of which no
trace has been observed.

2062. Solution of the hypothesis of Grotthus. The only
hypothesis which has been proposed presenting any satisfactory
explanation of the phenomena is that of Grotthus, in which
a series of decompositions and recompositions are supposed to
take place between the electrodes. Let OH, O'H', O"H", &c.,
represent a series of molecules of water ranged between the
positive electrode P and the negative electrode N.

p. . . OH. . . O'H' . . . O"H" . . . O'"H'" . . . o'"

"When OH is decomposed and o is detached in a separate
state at P, the positive fluid inseparable from H, according to
the electro-chemical hypothesis, being no longer neutralized by
an opposite fluid, attracts the negative fluid of o', and repels
the positive fluid of H', and decomposing the molecule of
water O'H', the molecule o' coalesces with H and forms a mo-
lecule of water. In like manner, H' decomposes O"H", and com-
bines with o" ; H" decomposes O^'H'", and combines with o"' ;
and H'" decomposes o"" B"" } and combines with o"" ; and, in
fine, H"" is disengaged at the negative electrode N. Thus, as
the series of decompositions and recompositions proceeds, the
molecules of oxygen are disengaged at the positive electrode P,
and those of hydrogen at the negative electrode N.

In this hypothesis it is further supposed, as already stated,
that the molecule of oxygen o, disengaged at the positive elec-
trode P, receives from that electrode a dose of positive electri-


city, which being equal in quantity to its own proper negative
electricity, neutralizes it ; and, in like manner, the molecule of
hydrogen a"", disengaged at the negative electrode N, receives
from it a corresponding dose of negative electricity which neu-
tralizes its own positive electricity. It is thus that the two
gases, when liberated at the electrodes, are in their natural and
unelectrified state.

2063. Effect of acid and salt on the electrolysis of water.
In the electrolysis of water as described above, the acid held in
solution undergoes no change. It produces, nevertheless, an
important influence on the development of the phenomena. If
the electrodes be immersed in pure water, decomposition will
only be produced when the current is one of extraordinary in-
tensity. But if a quantity of sulphuric acid even so inconsi-
derable as one per cent, be present, a current of much less
intensity will effect the electrolysis ; and by increasing the pro-

Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 38 of 45)