Voltaic Electricity—The Voltaic Battery—Intensity—Quantity—Static
Electricity, and Electricity in Motion—Luminous Effects—Mr.
Grove on the Electric Arc and Light—Decomposition of Water—Formation
of Crystals by Voltaic Electricity—Photo-galvanic
Engraving—Conduction—Heat of Voltaic Electricity—Electric Fish.
VOLTAIC or Dynamic electricity is elicited by the force of chemical
action. It is connected with some of the most brilliant periods of
British science, from the splendid discoveries to which it led Sir
Humphry Davy and Dr. Faraday.
In 1790, while Galvani, Professor of Anatomy in Bologna, was making
experiments on electricity, he was surprised to see convulsive motions
in the limbs of a dead frog accidentally lying near the machine during
an electrical discharge. Though a similar action had been noticed long
before his time, he was so much struck with this singular phenomenon,
that he examined all the circumstances carefully, and at length found
that convulsions take place when the nerve and muscle of a frog are
connected by a metallic conductor. This excited the attention of all
Europe; and it was not long before Volta, Professor at Pavia, showed
that the mere contact of different bodies is sufficient to disturb
electrical equilibrium, and that a current of electricity flows in one
direction through a circuit of three conducting substances. From this he
was led, by acute reasoning and experiment, to the construction of the
Voltaic pile, which, in its early form, consisted of alternate discs of
zinc and copper, separated by pieces of wet cloth, the extremities being
connected by wires. This simple apparatus, perhaps the most wonderful
instrument that has been invented by the ingenuity of man, by divesting
electricity of its sudden and uncontrollable violence, and giving in a
continued stream a greater quantity at a diminished intensity, has
exhibited that force under a new and manageable form, possessing powers
the most astonishing and unexpected. The expression current has no
relation to a fluid, which is now considered to be as inconsistent with
the phenomena of dynamic as with static electricity. It was shown by
Grotthus that the transmission of Voltaic electricity through liquids
consists of a series of chemical affinities acting in definite
directions; and Mr. Grove, from an examination of its action on the
various kinds of matter, has come to the same conclusion. Indeed it is
now the generally received opinion that a current of electricity is
merely a continuous transmission of chemical affinity from particle to
particle of the substance through which it is passing, and consequently
that it is a continuous transmission of force. As the Voltaic battery
has become one of the most important engines of physical research, some
account of its present condition may not be out of place.
The disturbance of electric equilibrium, and a development of
electricity, invariably accompany the chemical action of a fluid on
metallic substances, and the electricity is most plentiful when that
action occasions oxidation. Metals vary in the quantity of electricity
afforded by their combination with oxygen. But the greatest abundance is
developed by the oxidation of zinc by weak sulphuric acid. And, in
conformity with the law that one kind of electricity cannot be evolved
without an equal quantity of the other being brought into activity, it
is found that the acid is positively, and the zinc negatively electric.
It has not yet been ascertained why equilibrium is not restored by the
contact of these two substances, which are both conductors, and in
opposite electrical states. However, the electrical and chemical changes
are so connected, that, unless equilibrium be restored, the action of
the acid will go on languidly, or stop as soon as a certain quantity of
electricity is accumulated in it. Equilibrium, nevertheless, will be
restored, and the action of the acid will be continuous, if a plate of
copper be placed in contact with the zinc, both being immersed in the
fluid; for the copper, not being acted upon by the acid, will serve as a
conductor to convey the positive electricity from the acid to the zinc,
and will at every instant restore the equilibrium, and then the
oxidation of the zinc will go on rapidly. Thus three substances are
concerned in forming a Voltaic circuit, but it is indispensable that one
of them should be a fluid. The electricity so obtained will be very
feeble in overcoming resistances offered by imperfect conductors
interposed in the circuit, or by very long wires, but it may be
augmented by increasing the number of plates. In the common Voltaic
battery, the electricity which the fluid has acquired from the first
plate of zinc exposed to its action is taken up by the copper plate
belonging to the second pair, and transferred to the second zinc plate,
with which it is connected. The second plate of zinc, possessing equal
powers, and acting in conformity with the first, having thus acquired a
larger portion of electricity than its natural share, communicates a
larger quantity to the fluid in the second cell. This increased quantity
is again transferred to the next pair of plates; and thus every
succeeding alternation is productive of a further increase in the
quantity of the electricity developed. This action, however, would stop
unless a vent were given to the accumulated electricity, by establishing
a communication between the positive and negative poles of the battery
by means of wires attached to the extreme plate at each end. When the
wires are brought into contact, the Voltaic circuit is completed, the
electricities meet and neutralize each other, producing the shock and
other electrical phenomena; and then the electric current continues to
flow uninterruptedly in the circuit, as long as the chemical action
lasts. The stream of positive electricity flows from the zinc to the
copper. The construction and power of the Voltaic battery have been much
improved of late years, but the most valuable improvement is the
constant battery of Professor Daniell. In all batteries of the ordinary
construction, the power, however energetic at first, rapidly diminishes,
and ultimately becomes very feeble. Professor Daniell found that this
diminution of power is occasioned by the adhesion of the evolved
hydrogen to the surface of the copper, and by the precipitation of the
sulphate formed by the action of the acid on the zinc. He prevents the
latter by interposing between the copper and the zinc, in the cell
containing the liquid, a membrane which, without impeding the electric
current, prevents the transfer of the salt; and the former, by placing
between the copper and the membrane solution of sulphate of copper,
which being reduced by the hydrogen prevents the adhesion of this gas to
the metallic surface. Each element of the battery consists of a hollow
cylinder of copper, in the axis of which is placed a cylindrical rod of
zinc; between the zinc and the copper a membranous bag is placed, which
divides the cell into two portions, the inner of which is filled with
dilute acid, and the one nearer the copper is supplied with crystals of
the sulphate of that metal. The battery consists of several of these
elementary cells connected together by metallic wires, the zinc rod of
one with the copper cylinder of that next to it. The zinc rods are
amalgamated, so that local action, which, in ordinary cases, is so
destructive of the zinc, does not take place, and no chemical action is
manifested unless the circuit be completed. The rods are easily
detached, and others substituted for them when worn out. This battery,
which possesses considerable power, and is constant in its effects for a
very long time, is greatly superior to all former arrangements, either
as an instrument of research, or for exhibiting the ordinary phenomena
of Voltaic electricity.
A battery charged with water alone, instead of acid, is constant in its
action, but the quantity of electricity it develops is comparatively
very small. Mr. Cross, of Broomfield in Somersetshire, kept a battery of
this kind in full force during twelve months. M. Becquerel had invented
an instrument for comparing the intensities of the different kinds of
electricity by means of weights; but, as it is impossible to make the
comparison with Voltaic electricity produced by the ordinary batteries,
on account of the perpetual variation to which the intensity of the
current is liable, he has constructed a battery which affords a
continued stream of electricity of uniform power, but it is also of very
feeble force. The current is produced by the chemical combination of an
acid with an alkali.
Metallic contact is not necessary for the production of Voltaic
electricity, which is entirely due to chemical action. The intensity of
the Voltaic electricity is in proportion to the intensity of the
affinities concerned in its production, and the quantity produced is in
proportion to the quantity of matter which has been chemically active
during its evolution. Dr. Faraday considers this definite production to
be one of the strongest proofs that electricity is of chemical origin.
Galvanic or Voltaic electricity is manifested by two continuous forces
or currents passing in opposite directions through the circuit: the zinc
is the positive end or pole of the battery, and the copper the negative.
Voltaic electricity is distinguished by two marked characters. Its
intensity increases with the number of plates, its quantity with the
extent of their surfaces. The most intense concentration of force is
displayed by a numerous series of large plates: light and heat are
copiously evolved, and chemical decomposition is accomplished with
extraordinary energy; whereas the electricity from one pair of plates,
whatever their size may be, is so feeble that it gives no sign either of
attraction or repulsion. Common or static electricity is of greater
intensity and has a greater power of overcoming resistance than Voltaic
electricity, but it acts upon a smaller quantity of matter. However, by
diminishing the size of the plates, and increasing their number, the
intensity of a battery may be increased till it becomes equal to that of
the electrical machine.
The action of Voltaic electricity differs in some respects materially
from that of the ordinary kind. When a quantity of common electricity is
accumulated, the restoration of equilibrium is attended by an
instantaneous violent explosion, accompanied by the development of
light, heat, and sound. The concentrated power of the electricity forces
its way through every obstacle, disrupting and destroying the cohesion
of the particles of the bodies through which it passes, and occasionally
increasing its destructive effects by the conversion of fluids into
steam from the intensity of the momentary heat, as when trees are torn
to pieces by a stroke of lightning. Even the vivid light which marks the
path of the electricity is probably owing in part to the sudden
compression of the air and the rapidity of its passage. But the instant
equilibrium is restored by this energetic action the whole is at an end.
On the contrary, when an accumulation takes place in a Voltaic battery,
equilibrium is restored the moment the circuit is completed. But so far
is the electric stream from being exhausted, that it continues to flow
silently and invisibly in an uninterrupted current supplied by a
perpetual reproduction. And, although its action on bodies is neither so
sudden nor so intense as that of common electricity, yet it acquires
such power from constant accumulation and continued action, that it
ultimately surpasses the energy of the other. The two kinds of
electricity differ in no circumstance more than in the development of
heat. Instead of a momentary evolution, the circulation of the Voltaic
electricity is accompanied by a continued development of heat, lasting
as long as the circuit is complete, without producing either light or
sound. Its intensity from a very powerful battery is greater than that
of any heat that can be obtained by artificial means, so that it fuses
substances which resist the action of the most powerful furnaces. The
temperature of every part of a Voltaic battery itself is raised during
its activity. With the greater number of metals Mr. Grove found that the
positive terminal or pole is hotter than the negative.
According to Mr. Joule, the quantity of heat generated in a unit of time
is proportional to the strength of the current, and when a galvanic
current is employed in chemical analysis, the heat in the entire circuit
generated in a unit of time is equal to the work expended in producing
it, minus that employed in the analysis. In fact, a current of
electricity cannot pass through a homogeneous conductor without
generating heat in overcoming resistance, an effect proved by Mr. Joule
to be proportional to the square of the force of the current, and the
same in whatever direction the current may be flowing. Any other thermal
action that can take place must depend upon the heterogeneousness of the
circuit, and must be reversible with the current. For example, if a
semicircle of bismuth be joined to a semicircle of antimony, an electric
current in passing through it produces cold where it passes from the
bismuth to the antimony by absorption, and heat where it passes from the
antimony to the bismuth.
The transit of the electricity from pole to pole is accompanied by
light, and in consequence of the continuous current sparks occur every
time the contact of the wires is either broken or renewed; but
considerable intensity is requisite to enable the electricity to force
its way through atmospheric air or gas. Both its length and colour are
affected by the density of the medium through which it passes. If the
medium be gradually rarefied the discharge increases from a spark to a
luminous glow, differing in colour in different gases, but white in air.
When very much attenuated a discharge may be made to pass across 6 or 7
feet of space, while in air of the ordinary density it will not pass
through an inch. In rarefied gas it resembles the Aurora by its
continuous flashes. When the battery is powerful the luminous effects
are very brilliant.
The most splendid artificial light known is produced by fixing pencils
of charcoal at the extremities of the wires, and bringing them into
contact. This light is the more remarkable as it is independent of
combustion, since the charcoal suffers no apparent change, and,
likewise, because it is equally vivid in such gases as do not contain
oxygen. It depends upon the molecular arrangement of the charcoal; for
Mr. Grove observes that “carbon in a transparent crystalline state, as
diamond, is as perfect a non-conductor as we know, while in an opaque
amorphous state, as graphite or charcoal, it is one of the best
conductors: thus in one state it transmits light and stops electricity,
in the other it transmits electricity and stops light. It is a
circumstance worthy of remark, that the arrangement of molecules which
renders a solid body capable of transmitting light is most unfavourable
to the transmission of electricity, transparent solids being very
imperfect conductors of electricity; so all gases readily transmit
light, but are amongst the worst conductors of electricity, if indeed
they can be said to conduct it at all. The fact that the molecular
structure or arrangement of a body influences, indeed I may say
determines, its conducting power, is by no means explained by the theory
of a fluid; but if electricity be only a transmission of force or
motion, the influence of the molecular state is just what would be
expected.”
Professor Wheatstone, by fixing metallic points at the extremities of
the wires or poles, has found that the appearance of the spectrum of the
voltaic arc or vivid flame that is seen between the terminals of a
battery, depends, as in static electricity, upon the metal from whence
it is taken. The spectrum of that from mercury consists of seven
definite rays, separated from each other by dark intervals; these
visible rays are two orange lines close together, a bright green line,
two blueish-green lines near each other, a very bright purple line, and,
lastly, a blue line. It is the same when it passes through carbonic acid
gas, oxygen gas, air, or vacuum. The light from zinc, cadmium, tin,
bismuth, and lead, in a melted state, gives similar results; but the
number, position, and colour of the lines vary so much in each case, and
the appearances are so different, that the metals may easily be
distinguished from one another by this mode of investigation. The
electric spark is considered by M. Angström to be the overlapping of two
spectra, one of which belongs to the metal, and the other to the gas
through which the spark passes, and that the bright lines vary with the
gas as well as with the metal. In an oxygen spectrum the greatest number
of bright lines occur in the blue and violet, in nitrogen in the green
and yellow, and in hydrogen in the red. These effects must necessarily
be connected with the chemical and thermal properties of the gases.
Mr. Grove considers that the colour of the voltaic arc, or flame, which
appears between the poles of a very powerful battery, depends upon the
substance of the metal from whence it proceeds and on the medium through
which it passes. The spark from zinc is blue, from silver it is green,
from iron it is red and scintillating—precisely the colours afforded by
these metals in their ordinary combustion. But the colour varies also
with the medium through which the light passes, for when the medium is
changed a change takes place in the colour, showing an affection of the
intervening matter. A portion of the metal terminals or poles is
actually transmitted with every electrical or Voltaic discharge, whence
Mr. Grove concludes that the electrical discharge arises, at least in
part, from an actual repulsion and severance of the electrified matter
itself, which flies off at the points of least resistance. He observes
that “the phenomena attending the electric spark or Voltaic arc tends to
modify considerably our previous idea of the nature of the electric
force as a producer of ignition and combustion. The Voltaic arc is
perhaps, strictly speaking, neither ignition nor combustion. It is not
simply ignition; because the matter of the terminals is not merely
brought to a state of incandescence, but is physically separated, and
partially transferred from one terminal to another, much of it being
dissipated in a vaporous state. It is not combustion; for the phenomena
will take place independently of atmospheric air, oxygen gas, or any of
the bodies usually called supporters of combustion; combustion being in
fact chemical union attended with heat and light. In the Voltaic arc we
may have no chemical union, for if the experiment be performed in an
exhausted receiver, or in nitrogen, the substance forming the terminals
is condensed and precipitated upon the interior of the vessel, in,
chemically speaking, an unaltered state. Thus, to take a very striking
example, if the Voltaic discharge be taken between zinc terminals in an
exhausted receiver, a fine black powder of zinc is deposited on the
sides of the receiver; this can be collected, and takes fire readily in
air by being touched with a match, or ignited wire, instantly burning
into white oxide of zinc. To an ordinary observer the zinc would appear
to be burned twice—first in the receiver, where the phenomenon presents
all the appearance of combustion, and, secondly, in the real combustion
in air. With iron the experiment is equally instructive. Iron is
volatilized by the Voltaic arc in nitrogen, or in an exhausted receiver;
and when a scarcely perceptible film has lined the receiver, if it be
washed with an acid, it then gives, with ferrocyanide of potassium, the
Prussian-blue precipitate. In this case we readily distil iron, a metal
by ordinary means _fusible_ only at a very high temperature.”
Another strong evidence that the Voltaic discharge consists of the
material itself of which the terminals are composed, is the peculiar
rotation which is observed in the light when iron is employed, the
magnetic character of this metal causing its particles to rotate by the
influence of the Voltaic current. In short, Mr. Grove concludes that,
although it would be hasty to assert that the electrical disruptive
discharge can in no case take place without the terminals being
affected, yet he had met with no instance of such a result, provided the
discharge had been sufficiently prolonged, and the terminals in such a
state as could be expected to render manifest slight changes![15]
Some years ago Mr. Grove discovered that the electrical discharge
possesses certain phases or fits of an alternate character, forming
rings of alternate oxidation and deoxidation on metallic surfaces. A
highly polished silver plate in an air-pump was connected with the pole
of a powerful inductive battery, while a fine metallic wire, or even a
common sewing needle, was fixed at the other pole, and so arranged as to
be perpendicular to the silver plate, and very near, but not touching
it. By means of this apparatus the electrical discharge could be sent
through any kind of rarefied media. In some of the experiments a series
of concentric coloured rings of oxide alternating with rings of polished
or unoxidated silver were formed on the plate under the point of the
needle or wire. When the plate was previously coated with a film of
oxide, the oxide was removed in concentric spaces by the discharge, and
increased on the alternate ones, showing an alternate positive and
negative electricity, or electricity of an opposite character in the
same discharge.
When the silver plate was polished the centre of the rings formed on it
was yellow-green surrounded by blue-green; then a ring of polished
silver, followed by a crimson ring with a slight orange tint on the
inner side and deep purple on the outer; lastly the indication of a
polished one. When the air-pump was filled with attenuated olefiant gas
the rings were precisely the same with those seen in thin plates; hence
the effect is the same as that produced by the interference of light. In
these experiments the luminous appearance extended from three quarters
of an inch to an inch round the point of the needle or wire.
When the silver plate was connected with the negative pole of the
battery a polished point appeared upon it opposite the needle,
surrounded by a dusky ill-defined areola of a brown colour tinged with
purple when viewed in one direction, and greenish-white when seen in
another.
In the present year Mr. Gassiot, Vice-President of the Royal Society,
has shown that the stratified character of the electric discharge is
remarkably developed in the Torricellian vacuum. Among the various
experiments made by that gentleman two may be selected as strongly
illustrative of this new and singular property of electrical light.
In a closed glass tube about an inch internal diameter and 38 inches
long, in which a vacuum had been made, two platinum wires were
hermetically sealed, 32 inches apart, and connected with the poles of an
inductive battery. The luminous appearance at the two poles was very
different when electricity passed through the wires. A glow surrounded
the negative pole, and in close approximation to the glow a well-defined
dark space appeared, while from the positive pole or wire the light
proceeded in a stream; but unless the charge be great or the tube short,
the stream will not extend to the black band, which is totally different
from the intervening space. When discharges of electricity were sent
through this vacuum tube a series of bands or stratifications were
formed which were concave towards the positive pole; and as in the
changes in making and breaking the circuit the electricity emanates from
the different terminals or wires, their concavities were in opposite
directions.
When instead of platinum wires narrow tinfoil coatings were placed round
the exterior of the glass tube and connected with the wires of the
battery, brilliant stratifications filled the interior of the tube
between the foil coatings, but no dark band appeared. At present Mr.
Gassiot is inclined to believe that the dark band is due to
interference; but that the stratifications arise from pulsations or
impulses of a force acting in a highly attenuated but resisting medium,
for even with the best air-pumps it is impossible to make a perfect
void; he is still occupied with experiments on this new subject, and no
doubt will obtain very remarkable results, of which none can be more
extraordinary than his discovery of the powerful influence of the magnet
on this electric light. The stratifications are formed in rapid
succession in the tube with platinum wires and are turned different
ways, but they can be separated at any part of the tube by the pole of a
magnet round which the whole stratifications have a tendency to revolve.
In the second experiment, where the tinfoil was used, the discharge was
divided in two by the pole of a magnet, and the two parts had a tendency
to rotate round the magnet in opposite directions.
Voltaic electricity is a powerful agent in chemical analysis. When
transmitted through conducting fluids, it separates them into their
constituent parts, which it conveys in an invisible state through a
considerable space or quantity of liquid to the poles, where they come
into evidence. Numerous instances might be given, but the decomposition
of water is perhaps the most simple and elegant. Suppose a glass tube
filled with water, and corked at both ends; if one of the wires of an
active Voltaic battery be made to pass through one cork, and the other
through the other cork, into the water, so that the extremities of the
two wires shall be opposite and about a quarter of an inch asunder,
chemical action will immediately take place, and gas will continue to
rise from the extremities of both wires till the water has vanished. If
an electric spark be then sent through the tube, the water will
reappear. By arranging the experiment so as to have the gas given out by
each wire separately, it is found that water consists of two volumes of
hydrogen and one of oxygen. The hydrogen is given out at the positive
wire of the battery, and the oxygen at the negative. The oxides are also
decomposed; the oxygen appears at the positive pole, and the metal at
the negative. The decomposition of the alkalies and earths by Sir
Humphry Davy formed a remarkable era in the history of science. Soda,
potash, lime, magnesia, and other substances heretofore considered to be
simple bodies incapable of decomposition, were resolved by electric
agency into their constituent parts, and proved to be metallic oxides,
by that illustrious philosopher. All chemical changes produced by
electricity are accomplished on the same principle; and it appears that,
in general, combustible substances, metals, and alkalies go to the
negative wire, while acids and oxygen are evolved at the positive. The
transfer of these substances to the poles is not the least wonderful
effect of the Voltaic battery. Though the poles be at a considerable
distance from one another, nay, even in separate vessels, if a
communication be only established by a quantity of wet thread, as the
decomposition proceeds the component parts pass through the thread in an
invisible state, and arrange themselves at their respective poles.
According to Dr. Faraday, electro-chemical decomposition is simply a
case of the preponderance of one set of chemical affinities more
powerful in their nature over another set which are less powerful. And
in electro-chemical action of any kind produced by a continuous current,
the amount of action in a given time is nearly, if not rigorously,
proportional to the strength of the current. The great efficacy of
Voltaic electricity in chemical decomposition arises not from its
tension, but from the quantity set in motion and the continuance of its
action. Its agency appears to be most exerted on fluids and substances
which by conveying the electricity partially and imperfectly impede its
progress. But it is now proved to be as efficacious in the composition
as in the decomposition or analysis of bodies.
It had been observed that, when metallic solutions are subjected to
galvanic action, a deposition of metal, sometimes in the form of minute
crystals, takes place on the negative wire. By extending this principle,
and employing a very feeble Voltaic action, M. Becquerel has succeeded
in forming crystals of a great proportion of the mineral substances,
precisely similar to those produced by nature. The electric state of
metallic veins makes it possible that many natural crystals may have
taken their form from the action of electricity bringing their ultimate
particles, when in solution, within the narrow sphere of molecular
attraction. Both light and motion favour crystallization. Crystals which
form in different liquids are generally more abundant on the side of the
jar exposed to the light; and it is well known that still water, cooled
below 32°, starts into crystals of ice the instant it is agitated. A
feeble action is alone necessary, provided it be continued for a
sufficient time. Crystals formed rapidly are generally imperfect and
soft, and M. Becquerel found that even years of constant Voltaic action
were necessary for the crystallization of some of the hard substances.
If this law be general, how many ages may be required for the formation
of a diamond!
The deposition of metal from a metallic solution by galvanic electricity
has been most successfully applied to the arts of plating and gilding,
as well as to the more delicate process of copying medals and copper
plates. Indeed, not medals only, but any object of art or nature, may be
coated with precipitated metal, provided it be first covered with the
thinnest film of plumbago, which renders a non-conductor sufficiently
conducting to receive the metal. Photo-galvanic engraving depends upon
this. Gelatine mixed with bichromate of potash, nitrate of silver, and
iodide of potassium, is spread over a plate of glass, and when dry a
positive print is laid upon it with its face downwards, which, when
exposed to the sun, leaves its impression. When soaked in water the
gelatine swells around all those parts where the light had fallen, thus
forming an intaglio, a cast of which is taken in gutta-percha, which is
then coated with copper by the electro process, whence a copper plate in
relief is obtained.
Static electricity, on account of its high tension, passes through water
and other liquids as soon as it is formed, whatever the length of its
course may be. Voltaic electricity, on the contrary, is weakened by the
distance it has to traverse. Pure water is a very bad conductor; but ice
absolutely stops a current of Voltaic electricity altogether, whatever
be the power of the battery, although static or common electricity has
sufficient power to overcome its resistance. Dr. Faraday has discovered
that this property is not peculiar to ice; that, with a few exceptions,
bodies which do not conduct electricity when solid acquire that
property, and are immediately decomposed, when they become fluid, and,
in general, that decomposition takes place as soon as the solution
acquires the capacity of conduction, which has led him to suspect that
the power of conduction may be only a consequence of decomposition.
Heat increases the conducting power of some substances for Voltaic
electricity, and of the gases for both kinds. Dr. Faraday has given a
new proof of the connexion between heat and electricity, by showing
that, in general, when a solid, which is not a metal, becomes fluid, it
almost entirely loses its power of conducting heat, while it acquires a
capacity for conducting electricity in a high degree. M. Becquerel
regards the production of heat and that of electricity to be
concomitant; their dependence being such, that when one is increased the
other diminishes, and _vice versâ_, so that one may altogether disappear
with the increase of the other. For instance, when electricity
circulates in a metallic wire, the greater the heat produced, the less
the quantity of electricity which passes, and the contrary, so that the
affair proceeds as if electricity were converted into heat, and heat
into electricity. Again, in a closed galvanic circuit the sum of the
heat produced in the chemical action of the acidulated water upon the
zinc and in the conducting wire is constant, so that the quantity of
heat disengaged in the reaction is greater in proportion as less
electricity passes through the wire. These, and other circumstances,
prove such an intimate connexion between the production of heat and
electricity, that in the change of condition of substances the
electrical effects might disappear or be annulled by the calorific
effects.
The galvanic current affects all the senses: nothing can be more
disagreeable than the shock, which may even be fatal if the battery be
very powerful. A bright flash of light is perceived with the eyes shut,
when one of the wires touches the face, and the other the hand. By
touching the ear with one wire, and holding the other, strange noises
are heard; and an acid taste is perceived when the positive wire is
applied to the tip of the tongue, and the negative wire touches some
other part of it. By reversing the poles the taste becomes alkaline. It
renders the pale light of the glow-worm more intense. Dead animals are
roused by it, as if they started again into life, and it may ultimately
prove to be the cause of muscular action in the living.
Several fish possess the faculty of producing electrical effects. The
most remarkable are the gymnotus electricus, found in South America; and
the torpedo, a genus of ray, frequent in the Mediterranean. The
electrical action of the torpedo depends upon an apparatus apparently
analogous to the Voltaic pile, which the animal has the power of
charging at will, consisting of membranous columns filled throughout
with laminæ, separated from one another by a fluid. The absolute
quantity of electricity brought into circulation by the torpedo is so
great, that it effects the decomposition of water, has power sufficient
to make magnets, gives very severe shocks and the electric spark. It is
identical in kind with that of the galvanic battery, the electricity of
the under surface of the fish being the same with the negative pole, and
that in the upper surface the same with the positive pole. Its manner of
action is, however, somewhat different; for, although the evolution of
the electricity is continued for a sensible time, it is interrupted,
being communicated by a succession of discharges.