of certain properties of electricity, which have of late years been
discovered to have an essential connection with the phenomena of
chemistry.
CAROLINE.
It is ELECTRICITY, if I recollect right, which comes next in our list of
simple substances?
MRS. B.
I have placed electricity in that list, rather from the necessity of
classing it somewhere, than from any conviction that it has a right to
that situation, for we are as yet so ignorant of its intimate nature,
that we are unable to determine, not only whether it is simple or
compound, but whether it is in fact a material agent; or, as Sir H. Davy
has hinted, whether it may not be merely a property inherent in matter.
As, however, it is necessary to adopt some hypothesis for the
explanation of the discoveries which this agent has enabled us to make,
I have chosen the opinion, at present most prevalent, which supposes the
existence of two kinds of electricity, distinguished by the names of
_positive_ and _negative_ electricity.
CAROLINE.
Well, I must confess, I do not feel nearly so interested in a science in
which so much uncertainty prevails, as in those which rest upon
established principles; I never was fond of electricity, because,
however beautiful and curious the phenomena it exhibits may be, the
theories, by which they were explained, appeared to me so various, so
obscure and inadequate, that I always remained dissatisfied. I was in
hopes that the new discoveries in electricity had thrown so great a
light on the subject, that every thing respecting it would now have been
clearly explained.
MRS. B.
That is a point which we are yet far from having attained. But, in spite
of the imperfection of our theories, you will be amply repaid by the
importance and novelty of the subject. The number of new facts which
have already been ascertained, and the immense prospect of discovery
which has lately been opened to us, will, I hope, ultimately lead to a
perfect elucidation of this branch of natural science; but at present
you must be contented with studying the effects, and in some degree
explaining the phenomena, without aspiring to a precise knowledge of the
remote cause of electricity.
You have already obtained some notions of electricity: in our present
conversation, therefore, I shall confine myself to that part of the
science which is of late discovery, and is more particularly connected
with chemistry.
It was a trifling and accidental circumstance which first gave rise to
this new branch of physical science. Galvani, a professor of natural
philosophy at Bologna, being engaged (about twenty years ago) in some
experiments on muscular irritability, observed, that when a piece of
metal was laid on the nerve of a frog, recently dead, whilst the limb
supplied by that nerve rested upon some other metal, the limb suddenly
moved, on a communication being made between the two pieces of metal.
EMILY.
How is this communication made?
MRS. B.
Either by bringing the two metals into contact, or by connecting them by
means of a metallic conductor. But without subjecting a frog to any
cruel experiments, I can easily make you sensible of this kind of
electric action. Here is a piece of zinc, (one of the metals I mentioned
in the list of elementary bodies)--put it _under_ your tongue, and this
piece of silver _upon_ your tongue, and let both the metals project a
little beyond the tip of the tongue--very well--now make the projecting
parts of the metals touch each other, and you will instantly perceive a
peculiar sensation.
EMILY.
Indeed I did, a singular taste, and I think a degree of heat: but I can
hardly describe it.
MRS. B.
The action of these two pieces of metal on the tongue is, I believe,
precisely similar to that made on the nerve of a frog. I shall not
detain you by a detailed account of the theory by which Galvani
attempted to account for this fact, as his explanation was soon
overturned by subsequent experiments, which proved that _Galvanism_ (the
name this new power had obtained) was nothing more than electricity.
Galvani supposed that the virtue of this new agent resided in the nerves
of the frog, but Volta, who prosecuted this subject with much greater
success, shewed that the phenomena did not depend on the organs of the
frog, but upon the electrical agency of the metals, which is excited by
the moisture of the animal, the organs of the frog being only a delicate
test of the presence of electric influence.
CAROLINE.
I suppose, then, the saliva of the mouth answers the same purpose as the
moisture of the frog, in exciting the electricity of the pieces of
silver and zinc with which Emily tried the experiment on her tongue.
MRS. B.
Precisely. It does not appear, however, necessary that the fluid used
for this purpose should be of an animal nature. Water, and acids very
much diluted by water, are found to be the most effectual in promoting
the developement of electricity in metals; and, accordingly, the
original apparatus which Volta first constructed for this purpose,
consisted of a pile or succession of plates of zinc and copper, each
pair of which was connected by pieces of cloth or paper impregnated with
water; and this instrument, from its original inconvenient structure and
limited strength, has gradually arrived at its present state of power
and improvement, such as is exhibited in the Voltaic battery. In this
apparatus, a specimen of which you see before you (PLATE VI. fig. 1.),
the plates of zinc and copper are soldered together in pairs, each pair
being placed at regular distances in wooden troughs and the interstices
being filled with fluid.
[Illustration: Plate VI. p. 151.
Fig. 1. Voltaic Battery.
Fig. 2.
Fig. 4.
Fig. 1. 2. & 4. Voltaic Batteries
Fig. 3. Electrical Machine.
A the Cylinder.
B the Conductor.
R the Rubber.
C the Chain.]
CAROLINE.
Though you will not allow us to enquire into the precise cause of
electricity, may we not ask in what manner the fluid acts on the metals
so as to produce it?
MRS. B.
The action of the fluid on the metals, whether water or acid be used, is
entirely of a chemical nature. But whether electricity is excited by
this chemical action, or whether it is produced by the contact of the
two metals, is a point upon which philosophers do not yet perfectly
agree.
EMILY.
But can the mere contact of two metals, without any intervening fluid,
produce electricity?
MRS. B.
Yes, if they are afterwards separated. It is an established fact, that
when two metals are put in contact, and afterwards separated, that which
has the strongest attraction for oxygen exhibits signs of positive, the
other of negative electricity.
CAROLINE.
It seems then but reasonable to infer that the power of the Voltaic
battery should arise from the contact of the plates of zinc and copper.
MRS. B.
It is upon this principle that Volta and Sir H. Davy explain the
phenomena of the pile; but notwithstanding these two great authorities,
many philosophers entertain doubts on the truth of this theory. The
principal difficulty which occurs in explaining the phenomena of the
Voltaic battery on this principle, is, that two such plates show no
signs of different states of electricity whilst in contact, but only on
being separated after contact. Now in the Voltaic battery, those plates
that are in contact always continue so, being soldered together: and
they cannot therefore receive a succession of charges. Besides, if we
consider the mere disturbance of the balance of electricity by the
contact of the plates, as the sole cause of the production of Voltaic
electricity, it remains to be explained how this disturbed balance
becomes an inexhaustible source of electrical energy, capable of pouring
forth a constant and copious supply of electrical fluid, though without
any means of replenishing itself from other sources. This subject, it
must be owned, is involved in too much obscurity to enable us to speak
very decidedly in favour of any theory. But, in order to avoid
perplexing you with different explanations, I shall confine myself to
one which appears to me to be least encumbered with difficulties, and
most likely to accord with truth.*
This theory supposes the electricity to be excited by the chemical
action of the acid on the zinc; but you are yet such novices in
chemistry, that I think it will be necessary to give you some previous
explanation of the nature of this action.
All metals have a strong attraction for oxygen, and this element is
found in great abundance both in water and in acids. The action of the
diluted acid on the zinc consists therefore in its oxygen combining with
it, and dissolving its surface.
[Footnote *: This mode of explaining the phenomena of the Voltaic
pile is called the _chemical theory_ of electricity, because it
ascribes the cause of these phenomena to certain chemical changes
which take place during their appearance. In the preceding edition
of this work, the same theory was presented in a more elaborate,
but less easy form than it is in this. The mode of viewing the
subject which is here sketched was long since suggested by Dr.
Bostock, of whose theory, however, this is by no means to be
considered as a complete statement.]
CAROLINE.
In the same manner I suppose as we saw an acid dissolve copper?
MRS. B.
Yes; but in the Voltaic battery the diluted acid is not strong enough to
produce so complete an effect; it acts only on the surface of the zinc,
to which it yields its oxygen, forming upon it a film or crust, which is
a compound of the oxygen and the metal.
EMILY.
Since there is so strong a chemical attraction between oxygen and
metals, I suppose they are naturally in different states of electricity?
MRS. B.
Yes; it appears that all metals are united with the positive, and that
oxygen is the grand source of the negative electricity.
CAROLINE.
Does not then the acid act on the plates of copper, as well as on those
of zinc?
MRS. B.
No; for though copper has an affinity for oxygen, it is less strong than
that of zinc; and therefore the energy of the acid is only exerted upon
the zinc.
It will be best, I believe, in order to render the action of the Voltaic
battery more intelligible, to confine our attention at first to the
effect produced on two plates only. (PLATE VI. fig. 2.)
If a plate of zinc be placed opposite to one of copper, or any other
metal less attractive of oxygen, and the space between them (suppose of
half an inch in thickness), be filled with an acid or any fluid capable
of oxydating the zinc, the oxydated surface will have its capacity for
electricity diminished, so that a quantity of electricity will be
evolved from that surface. This electricity will be received by the
contiguous fluid, by which it will be transmitted to the opposite
metallic surface, the copper, which is not oxydated, and is therefore
disposed to receive it; so that the copper plate will thus become
positive, whilst the zinc plate will be in the negative state.
This evolution of electrical fluid however will be very limited; for as
these two plates admit of but very little accumulation of electricity,
and are supposed to have no communication with other bodies, the action
of the acid, and further developement of electricity, will be
immediately stopped.
EMILY.
This action, I suppose, can no more continue to go on, than that of a
common electrical machine, which is not allowed to communicate with
other bodies?
MRS. B.
Precisely; the common electrical machine, when excited by the friction
of the rubber, gives out both the positive and negative electricities.--
(PLATE VI. Fig. 3.) The positive, by the rotation of the glass cylinder,
is conveyed into the conductor, whilst the negative goes into the
rubber. But unless there is a communication made between the rubber and
the ground, but a very inconsiderable quantity of electricity can be
excited; for the rubber, like the plates of the battery, has too small a
capacity to admit of an accumulation of electricity. Unless therefore
the electricity can pass out of the rubber, it will not continue to go
into it, and consequently no additional accumulation will take place.
Now as one kind of electricity cannot be given out without the other,
the developement of the positive electricity is stopped as well as that
of the negative, and the conductor therefore cannot receive a succession
of charges.
CAROLINE.
But does not the conductor, as well as the rubber, require a
communication with the earth, in order to get rid of its electricity?
MRS. B.
No; for it is susceptible of receiving and containing a considerable
quantity of electricity, as it is much larger than the rubber, and
therefore has a greater capacity; and this continued accumulation of
electricity in the conductor is what is called a charge.
EMILY.
But when an electrical machine is furnished with two conductors to
receive the two electricities, I suppose no communication with the earth
is required?
MRS. B.
Certainly not, until the two are fully charged; for the two conductors
will receive equal quantities of electricity.
CAROLINE.
I thought the use of the chain had been to convey the electricity _from_
the ground into the machine?
MRS. B.
That was the idea of Dr. Franklin, who supposed that there was but one
kind of electricity, and who, by the terms positive and negative (which
he first introduced), meant only different quantities of the same kind
of electricity. The chain was in that case supposed to convey
electricity _from_ the ground through the rubber into the conductor. But
as we have adopted the hypothesis of two electricities, we must consider
the chain as a vehicle to conduct the negative electricity into the
earth.
EMILY.
And are both kinds of electricity produced whenever electricity is
excited?
MRS. B.
Yes, invariably. If you rub a tube of glass with a woollen cloth, the
glass becomes positive, and the cloth negative. If, on the contrary, you
excite a stick of sealing-wax by the same means, it is the rubber which
becomes positive, and the wax negative.
But with regard to the Voltaic battery, in order that the acid may act
freely on the zinc, and the two electricities be given out without
interruption, some method must be devised, by which the plates may part
with their electricities as fast as they receive them. --Can you think
of any means by which this might be effected?
EMILY.
Would not two chains or wires, suspended from either plate to the
ground, conduct the electricities into the earth, and thus answer the
purpose?
MRS. B.
It would answer the purpose of carrying off the electricity, I admit;
but recollect, that though it is necessary to find a vent for the
electricity, yet we must not lose it, since it is the power which we are
endeavouring to obtain. Instead, therefore, of conducting it into the
ground, let us make the wires, from either plate, meet: the two
electricities will thus be brought together, and will combine and
neutralize each other; and as long as this communication continues, the
two plates having a vent for their respective electricities, the action
of the acid will go on freely and uninterruptedly.
EMILY.
That is very clear, so far as two plates only are concerned; but I
cannot say I understand how the energy of the succession of plates, or
rather pairs of plates, of which the Galvanic trough is composed, is
propagated and accumulated throughout a battery?
MRS. B.
In order to shew you how the intensity of the electricity is increased
by increasing the number of plates, we will examine the action of four
plates; if you understand these, you will readily comprehend that of any
number whatever. In this figure (PLATE VI. Fig. 4.), you will observe
that the two central plates are united; they are soldered together, (as
we observed in describing the Voltaic trough,) so as to form but one
plate which offers two different surfaces, the one of copper, the other
of zinc.
Now you recollect that, in explaining the action of two plates, we
supposed that a quantity of electricity was evolved from the surface of
the first zinc plate, in consequence of the action of the acid, and was
conveyed by the interposed fluid to the copper plate, No. 2, which thus
became positive. This copper plate communicates its electricity to the
contiguous zinc plate, No. 3, in which, consequently, some accumulation
of electricity takes place. When, therefore, the fluid in the next cell
acts upon the zinc plate, electricity is extricated from it in larger
quantity, and in a more concentrated form, than before. This
concentrated electricity is again conveyed by the fluid to the next pair
of plates, No. 4 and 5, when it is farther increased by the action of
the fluid in the third cell, and so on, to any number of plates of which
the battery may consist; so that the electrical energy will continue to
accumulate in proportion to the number of double plates, the first zinc
plate of the series being the most negative, and the last copper plate
the most positive.
CAROLINE.
But does the battery become more and more strongly charged, merely by
being allowed to stand undisturbed?
MRS. B.
No, for the action will soon stop, as was explained before, unless a
vent be given to the accumulated electricities. This is easily done,
however, by establishing a communication by means of the wires
(Fig. 1.), between the two ends of the battery: these being brought into
contact, the two electricities meet and neutralize each other, producing
the shock and other effects of electricity; and the action goes on with
renewed energy, being no longer obstructed by the accumulation of the
two electricities which impeded its progress.
EMILY.
Is it the union of the two electricities which produces the electric
spark?
MRS. B.
Yes; and it is, I believe, this circumstance which gave rise to Sir H.
Davy’s opinion that caloric may be a compound of the two electricities.
CAROLINE.
Yet surely caloric is very different from the electrical spark?
MRS. B.
The difference may consist probably only in intensity: for the heat of
the electric spark is considerably more intense, though confined to a
very minute spot, than any heat we can produce by other means.
EMILY.
Is it quite certain that the electricity of the Voltaic battery is
precisely of the same nature as that of the common electrical machine?
MRS. B.
Undoubtedly; the shock given to the human body, the spark, the
circumstance of the same substances which are conductors of the one
being also conductors of the other, and of those bodies, such as glass
and sealing-wax, which are non-conductors of the one, being also
non-conductors of the other, are striking proofs of it. Besides, Sir H.
Davy has shewn in his Lectures, that a Leyden jar, and a common electric
battery, can be charged with electricity obtained from a Voltaic
battery, the effect produced being perfectly similar to that obtained by
a common machine.
Dr. Wollaston has likewise proved that similar chemical decompositions
are effected by the electric machine and by the Voltaic battery; and has
made other experiments which render it highly probable, that the origin
of both electricities is essentially the same, as they show that the
rubber of the common electrical machine, like the zinc in the Voltaic
battery, produces the two electricities by combining with oxygen.
CAROLINE.
But I do not see whence the rubber obtains oxygen, for there is neither
acid nor water used in the common machine, and I always understood that
the electricity was excited by the friction.
MRS. B.
It appears that by friction the rubber obtains oxygen from the
atmosphere, which is partly composed of that element. The oxygen
combines with the amalgam of the rubber, which is of a metallic nature,
much in the same way as the oxygen of the acid combines with the zinc in
the Voltaic battery, and it is thus that the two electricities are
disengaged.
CAROLINE.
But, if the electricities of both machines are similar, why not use the
common machine for chemical decompositions?
MRS. B.
Though its effects are similar to those of the Voltaic battery, they are
incomparably weaker. Indeed Dr. Wollaston, in using it for chemical
decompositions, was obliged to act upon the most minute quantities of
matter, and though the result was satisfactory in proving the similarity
of its effects to those of the Voltaic battery, these effects were too
small in extent to be in any considerable degree applicable to chemical
decomposition.
CAROLINE.
How terrible, then, the shock must be from a Voltaic battery, since it
is so much more powerful than an electrical machine!
MRS. B.
It is not nearly so formidable as you think; at least it is by no means
proportional to the chemical effect. The great superiority of the
Voltaic battery consists in the large _quantity_ of electricity that
passes; but in regard to the _rapidity_ or _intensity_ of the charge, it
is greatly surpassed by the common electrical machine. It would seem
that the shock or sensation depends chiefly upon the intensity; whilst,
on the contrary, for chemical purposes, it is quantity which is
required. In the Voltaic battery, the electricity, though copious, is so
weak as not to be able to force its way through the fluid which
separates the plates, whilst that of a common machine will pass through
any space of water.
CAROLINE.
Would not it be possible to increase the intensity of the Voltaic
battery till it should equal that of the common machine?
MRS. B.
It can actually be increased till it imitates a weak electrical machine,
so as to produce a visible spark when accumulated in a Leyden jar. But
it can never be raised sufficiently to pass through any considerable
extent of air, because of the ready communication through the fluids
employed.
By increasing the number of plates of a battery, you increase its
_intensity_, whilst, by enlarging the dimensions of the plates, you
augment its _quantity_; and, as the superiority of the battery over the
common machine consists entirely in the quantity of electricity
produced, it was at first supposed that it was the size, rather than the
number of plates that was essential to the augmentation of power. It
was, however, found upon trial, that the quantity of electricity
produced by the Voltaic battery, even when of a very moderate size, was
sufficiently copious, and that the chief advantage in this apparatus was
obtained by increasing the intensity, which, however, still falls very
short of that of the common machine.
I should not omit to mention, that a very splendid, and, at the same
time, most powerful battery, was, a few years ago, constructed under the
direction of Sir H. Davy, which he repeatedly exhibited in his course of
electro-chemical lectures. It consists of two thousand double plates of
zinc and copper, of six square inches in dimensions, arranged in troughs
of Wedgwood-ware, each of which contains twenty of these plates. The
troughs are furnished with a contrivance for lifting the plates out of
them in a very convenient and expeditious manner.*
[Footnote *: A model of this mode of construction is exhibited in
PLATE XIII. Fig. 1.]
CAROLINE.
Well, now that we understand the nature of the action of the Voltaic
battery, I long to hear an account of the discoveries to which it has
given rise.
MRS. B.
You must restrain your impatience, my dear, for I cannot with any
propriety introduce the subject of these discoveries till we come to
them in the regular course of our studies. But, as almost every
substance in nature has already been exposed to the influence of the
Voltaic battery, we shall very soon have occasion to notice its effects.
CONVERSATION VI.
ON OXYGEN AND NITROGEN.
MRS. B.
To-day we shall examine the chemical properties of the ATMOSPHERE.
CAROLINE.
I thought that we were first to learn the nature of OXYGEN, which come
next in our table of simple bodies?
MRS. B.
And so you shall; the atmosphere being composed of two principles,
OXYGEN and NITROGEN, we shall proceed to analyse it, and consider its
component parts separately.
EMILY.
I always thought that the atmosphere had been a very complicated fluid,
composed of all the variety of exhalations from the earth.
MRS. B.