ether, which constitutes a part of it, and which more forcibly attracts
vitreous electric ether, from surrounding bodies which stand on it,
mixed with a less proportion of resinous ether, like an atmosphere, but
cannot unite with the resinous ether, which is combined with the glass.
And resin, on the contrary, holds within it, in combination, much
vitreous electric ether, which constitutes a part of it, and which more
forcibly attracts resinous electric ether from surrounding bodies, which
stand on it, mixed with a less proportion of vitreous ether, like an
atmosphere, but cannot unite with the vitreous ether which is combined
with the resin.
(5.) Hence the non-conductors of electricity are of two kinds, and
opposite to each other; the one class of the vitreous, and the other of
the resinous. But the most perfect conductor, such as metal, water and
charcoal, having neither kind of electric ether, _combined_ with them,
though _surrounded_ with both, suffer both kinds to pass through them
easily.
(6.) Great accumulation or condensation of the separate electric ethers,
attract each other so strongly that they will break a passage through
non-conducting bodies. Hence trees and stone walls are rent by
lightning.
(7.) When artificial or natural accumulations of these separate ethers
are in a very small quantity or intensity, they pass slowly and with
difficulty from one body to another, and require the best conductors for
this purpose. Whence many of the phenomena of the Torpedo, the Gymnotus,
and of Galvanism.
(8.) The electric ethers may be separately accumulated, by the contact
of conductors with non-conductors—by vicinity of the two ethers—by
heat—and by decomposition.
(9.) When these two ethers unite suddenly and with explosion, a
liberation of light and heat takes place, as in all chemical explosions.
Accordingly it is said that a _smell_ is perceptible from electric
sparks, and even a _taste_, which must be supposed to arise from new
combinations or decompositions.”
The theory founded on the principles above stated is supposed, by those
who adopt it, to solve many difficulties which can scarcely be accounted
for on the theory of Franklin.
Dr. Gibbes also adopts a chemical theory of electricity. He supposes
that oxygen gas is produced by the union of _positive electricity_ with
water; and hydrogen gas by the union of _negative electricity_ with
water: and that water, uniting in different proportions with the two
electricities, is the ponderable part of all the elastic fluids. He
asserts that by the _positive electricity_ metals are oxydated, and blue
vegetable colours reddened; and also that the acidifying effect of
electric commotions in the atmosphere, on weak fermented liquors, is
unquestionable.—On the other hand, according to this writer, by
_negative electricity_ the vegetable blue is restored, and the oxydated
metal revived.
These circumstances, among others, led Dr. Gibbes to conclude that when
hydrogen gas is produced by the affusion of water on red-hot metal, and
the metal is at the same time oxydated, a decomposition of _fire_ rather
than of _water_ has taken place; that the hot metal has parted with
negative electricity, which, uniting with a small proportion of the
water, has formed hydrogen gas; that a greater proportion of the water
has united with the positive electricity, and entered, as oxygen gas,
into combination with the metal. When the two gases are inflamed
together, the spark attracts to itself, in due proportions, the two
electricities contained in the two gases, which unite with explosion,
and produce fire. The water with which they were before combined is of
course deposited.
The reason why inflammable substances burn in oxygen gas, and not in
hydrogen, Dr. Gibbes supposes to be, that negative electricity greatly
prevails in all inflammable substances. Neither of the gases can be
inflamed separately, because fire depends on the union of the two
electricities; and such union cannot be effected unless both are present
in due proportion.
Dr. Gibbes supposes that the further illustration of the effects of the
two electricities, as chemical agents, will set aside some of the
leading doctrines of the Lavoisierian theory, and afford an easy
solution of certain phenomena which that theory cannot explain.
_Æpinus’ Theory of Electricity._
Mr. Æpinus, of the imperial academy of Petersburgh, has attempted to
class the phenomena of electricity and magnetism in a mathematical
method. In the course of his works he gives some views of the subject
which are new and highly ingenious, and as some good judges suppose,
calculated to surmount many difficulties, and to answer many questions,
which occur in considering the Franklinian theory. The leading
principles of his plan are comprehended in the following propositions.
1. Its particles repel each other, with a force decreasing as the
squares of the distances increase.
2. Its particles attract the particles of some ingredients in all other
bodies, with a force decreasing according to the same law, with an
increase of distance; and that this attraction is mutual.
3. The electric fluid is dispersed in the pores of other bodies, and
moves with various degrees of facility through the pores of different
kinds of matter. In those bodies which we call non-electrics, such as
water or metals, it moves without any perceivable obstruction; but in
glass, resin, and all bodies called electrics, it moves with very great
difficulty, or is altogether immoveable.
4. The phenomena of electricity are of two kinds: 1. Such as arise from
the actual motion of the fluid, from a body containing more, to one
containing less of it. 2. Such as do not immediately arise from this
transference, but are instances of its attraction and repulsion.—
These principles are applied at great length, and with a pleasing degree
of precision, by the ingenious theorist, to the Leyden phial, and to the
various phenomena of electric attraction and repulsion. It will be
readily seen that Æpinus adopts, in substance, the theory of Franklin,
of which, in some particulars, he presents new and more satisfactory
views than the American philosopher. In the sixty first volume of the
Philosophical Transactions, there is a dissertation by the Hon. Mr.
Cavendish on this subject, which he considers as an extension and more
accurate application of Æpinus’s theory.
CHAP. III.
_The Franklinian Theory of Electricity._
We are now to give Dr. Franklin’s theory of plus and minus, or positive
and negative electricity, and adduce facts, to shew how far this theory
will go to explain the different phenomena.
The Doctor supposed that all the operations in electricity, depended
upon one fluid, _sui generis_, extremely subtile and elastic.—That there
subsists a very strong repulsion between the particles of this fluid, in
regard to one another, and as strong an attraction, with regard to other
matter.—Thus one quantity of electric matter will repel another quantity
of the same, but will attract, and be attracted by, any terrestrial
matter that happens to be near it. The pores of all bodies are supposed
to be full of this subtile fluid; and when its equilibrium is not
disturbed, that is, when there is neither more nor less of it in a body
than its natural share, or than it is capable of retaining by its own
attraction, the fluid does not manifest itself to our senses. The action
of the rubber upon an electric disturbs this equilibrium, occasioning a
redundancy of the fluid in one place, and a deficiency of it in another.
This equilibrium being forcibly disturbed, the mutual repulsion of the
particles of the fluid is necessarily exerted to restore it. If two
bodies be both of them over-charged, the electric atmospheres repel one
another, and both the bodies recede from each other, to places where the
fluid is less dense.—For as there is supposed to be a mutual attraction
between all bodies and the electric matter, such bodies as are
electrified must go along with their atmospheres. If both bodies are
exhausted of their natural share of this fluid, they are both attracted
by the denser fluid, existing either in the atmosphere contiguous to
them, or in other neighbouring bodies; which occasions them still to
recede from one another, as if they were over-charged.
Dr. Franklin’s theory has gained the greatest reputation, from the easy
solution it affords of all the phenomena of the Leyden phial. The fluid
is supposed to move with the greatest ease in bodies which are
conductors; but with extreme difficulty in _electrics per se_; in so
much that glass is absolutely impermeable to it. It is also supposed
that all electrics, and particularly glass, on account of the smallness
of their pores, do at all times contain an exceedingly great, and always
an equal quantity of this fluid; so that no more can be thrown into any
one part of any electric substance, except the same quantity go out at
another, and the gain be exactly equal to the loss. These things being
premised, the phenomena of charging and discharging a plate of glass, or
a Leyden phial, may be easily solved. In the usual manner of
electrifying by a smooth glass globe or cylinder, all the electric
matter is supplied by the rubber, from all the bodies which communicate
with it. If it be made to communicate with nothing but one of the
coatings of a glass plate, while the prime-conductor is connected with
the other, that side of the glass which communicates with the rubber,
must necessarily be exhausted in order to supply the conductor, which
must convey the whole of it to the coating with which it is connected.
By this operation, therefore, the electric fluid becomes almost entirely
exhausted from one side of the plate, while it is as much accumulated on
the other; and the discharge is made by the electric fluid rushing, as
soon as an opportunity is given it by means of proper conductors, from
the side which was overloaded to that which was exhausted.
It is not however necessary to this theory, that the same individual
particles of electric matter which were thrown upon one side of the
plate, should make the whole circuit of the intervening conductors,
especially in very great distances, so as actually to arrive at the
exhausted side. It may be sufficient to suppose, that the additional
quantity of fluid displaces and occupies the place of an equal portion
of the natural quantity of fluid, belonging to those conductors in the
circuit which lay contiguous to the charged side of the glass. This
displaced fluid may drive forwards an equal quantity of the same matter
in the next conductor; and thus the progress may continue, till the
exhausted side of the glass is supplied by the fluid naturally existing
in the conductors contiguous to it.
To account for the velocity with which electricity passes through good
conductors, Dr. Franklin compares the electricity in the conductors, to
a wire in the bore of a tube, which it exactly fills.—If one end of this
wire be moved forward, every other part of it will move in the same
direction, and at the same instant.
Dr. Priestley says, it may be thought a difficulty upon this hypothesis,
that one of the sides of a glass plate cannot be exhausted, without the
other receiving more than its natural share; particularly as the
particles of this fluid are supposed to be repulsive of one another. But
it must be considered, that the attraction of the glass is sufficient to
retain even the large quantity of electric fluid which is natural to it,
against all attempts to withdraw it, unless that eager attraction can be
satisfied by the admission of an equal quantity from some other quarter.
When this opportunity of a supply is given by connecting one of the
coatings with the rubber and the other with the conductor, the two
attempts, to introduce more of the fluid into one of the sides, and to
subtract some from the other, are made, in a manner, at the same
instant. The action of the rubber tends to disturb the equilibrium of
the fluid in the glass; and no sooner has a spark quitted one of the
sides to go to the rubber, than it is supplied by the conductor on the
other; and the difficulty with which these additional particles move in
the substance of the glass, effectually prevents its reaching the
opposite exhausted side. It is not said, however, but that either side
of the glass may give or receive a small quantity of the electric fluid,
without altering the quantity on the opposite side. It is only a very
considerable part of the charge that is meant, when one side is said to
be filled while the other is exhausted.
The above is the substance of the theory most generally received. It
depends upon the following principles.
1. All terrestrial substances, as well as the atmosphere which surrounds
the earth, are full of electric matter.
2. Glass, and other electric substances, though they contain a great
deal of electric matter, are nevertheless impermeable by it.
3. This electric matter violently repels itself, and attracts all other
matter.
4. By the excitation of an electric, the equilibrium of the fluid
contained in it is disturbed, and one part of it is overloaded with
electricity, while the other contains too little.
5. Conducting substances are permeable to the electric matter through
their whole substance, and do not conduct it merely over their surface.
6. Positive electricity is when a body has too much of the electric
fluid, and negative electricity, when it has too little.
Of these positions we shall now adduce those proofs, drawn from
different facts, which seem in the strongest manner to confirm them.
I. “All terrestrial substances, as well as the atmosphere which
surrounds the earth, are full of electric matter.” The proofs of this
are very easy. There is no place of the earth or sea where the electric
fire may not be collected, by making a communication between it and the
rubber of an electric machine. Therefore, considering that the whole
earth is moist, and that moisture is a conductor of electricity, and
that every part of the earth must thus communicate with another, it is
certain that the electric matter must diffuse itself as far as the
moisture of the earth reaches; and this may reasonably be supposed to be
to the very centre.
The case is equally clear with regard to the atmosphere. The extract
from Mr. Cavallo’s journal, given in the chapter upon atmospheric
electricity, is a sufficient proof that the atmosphere is full of
electric matter.
II. “Glass, and other electric substances, though they contain a great
deal of electric matter, are nevertheless impermeable by it.” The
principal arguments for the impermeability of glass by the electric
fluid are drawn from the phenomena of the Leyden phial. It is very plain
that there is, in charging this phial, an expulsion of fire from the
outside, at the same time that it is thrown upon the inside. This
appears from numberless experiments, but is most readily observable in
the following. Let a coated phial be set upon an insulating stand, and
the knob of another phial brought near its coating. As soon as sparks
are discharged from the prime-conductor to the knob of the first, an
equal number will be observed to proceed from its coating to the knob of
the second. This is very remarkable, and an unphilosophical observer
will scarce ever fail to conclude, that the fire runs directly through
the substance of the glass. Dr. Franklin however concludes that it does
not, because there is a very great accumulation of electricity on the
inside of the glass, which discovers itself by a violent flash and
explosion, when a communication is made between the outside and inside
coatings. But it must be confessed, there is here no other reason for
concluding the glass to be impermeable, than the _probability_ that the
electric matter is accumulated on one side of the glass and deficient on
the other.
Another argument against the permeability of glass and other electrics
is, that coated phials can receive but a very slight charge when their
outside coating is insulated, and this can be effected only with a very
powerful machine.
III. “The electric fluid violently repels itself, and attracts all other
matter.” The proofs of this position have been so abundantly given in
the course of this work, particularly in the chapter on electric
attraction and repulsion, that we think it entirely superfluous to
repeat them here.
IV. “By the excitation of an electric, the equilibrium of the fluid is
disturbed, and one part of it is overloaded with electricity, while the
other contains too little.” This position must be considered as entirely
hypothetical, as the manner in which the electric fluid is collected by
the excitation of glass, or any other electric substance, has not yet
been satisfactorily explained.
V. “Conducting bodies are permeable by the electric fluid, through the
whole of their substance, and do not conduct it merely over their
surface.” Take a wire of any kind of metal, and cover part of it with
some electric substance, as rosin, sealing-wax, &c. then discharge a jar
through it, and it will be found that it conducts as well as without the
electric coating. This, says Mr. Cavallo, proves that the electric
matter passes through the substance of the metal, and not over the
surface. A wire, adds he, continued through a vacuum is also a
convincing proof of this assertion.
VI. “Positive electricity is an accumulation, or too great a quantity of
the electric matter contained in a body; and negative electricity is
when there is too little.” This position, like the fourth, must be
considered as hypothetical—the peculiar nature of the electric fluid not
admitting of experiments to prove, or to disprove it.
APPENDIX.
NUMBER I.
_A description of the Cement used for electrical purposes._
The best cement for electrical purposes is made by melting two parts of
rosin, two of bee’s-wax, and one of brick-dust, or red ochre, together.
This method of making cement is much preferable to that of rosin alone,
as it is not so brittle, and at the same time it insulates equally well.
NUMBER II.
_A Composition for Coating Cylinders or Globes._
The most approved composition for lining glass cylinders or globes, is
made with four parts of Venice turpentine, one part of rosin, and one of
bee’s-wax.—They must be boiled together for about two hours over a slow
fire, and stirred very frequently; afterwards the composition is left to
cool, when it is fit for use.
If a cylinder or globe is to be lined with this mixture, a sufficient
quantity must be pulverised, and introduced into the glass; then by
holding the glass near the fire, the composition is melted, and by a
little skill may be spread over all its internal surface, to about the
thickness of a wafer.—The glass, however, must be heated very gradually,
otherwise, there is danger of its breaking in the operation.
NUMBER III.
_To make the best kind of Amalgam for exciting Electrics.—_
Any metal dissolved in mercury or quick-silver will answer the purpose
very well, thus two parts of quick-silver with one of tin-foil, or Aurum
Mosaicum, have been used to advantage. But the most powerful composition
for an amalgam, is zinc and mercury, in the proportion of one part of
the latter, with five of the former, to which may be added a little
bee’s-wax or tallow, the proper way of preparing this amalgam is the
following.—Let the quick-silver be heated, to about the degree of
boiling water, and let the zinc also be melted in an iron ladle. Pour
the heated quick-silver into a wooden box, and immediately afterwards
pour the melted zinc into it likewise. Then shut the box, and shake it
about for some time. You must now wait till the amalgam is cool, or
nearly so, and then mix a little bee’s-wax, or mutton-suet with it, by
trituration.
NUMBER IV.
_The preparation of electrical Paint._
The electrician will very frequently have occasion to make use of paint,
both for ornament, and convenience. We shall therefore describe a
pigment, which, while it looks very well, insulates the instrument, and
answers a variety of other purposes.—If a red colour is wished, let a
piece of red sealing-wax be dissolved in a sufficient quantity of highly
rectified spirits of wine, then let the substance which you intend to
colour be warmed, after which the paint may be laid on by means of a
hair pencil. Care should be taken to render the instrument clean and
dry, especially if it be a glass one.—Two or three coats of this paint,
will generally answer every purpose. If a black colour is preferred,
black sealing-wax may be used.
If the outside coating of a jar is desired to be coloured, common oil
paint will do much better than that above described, for here insulation
is not required; a covering of some paint or other is always necessary,
in order to prevent the amalgam, which is often scattered about the
table where the apparatus is placed, from corroding the tin-foil with
which the jar is covered.
NUMBER V.
_To make the Artificial Bolognian Stone._
“Calcine some common oyster-shells, by keeping them in a good coal fire,
for half an hour; let the purest part of the calx be pulverised and
sifted. Mix with three parts of this powder, one part of flowers of
sulphur. Let this mixture be rammed into a crucible of about an inch and
a half in depth, till it be almost full; and let it be placed in the
middle of the fire, where it must be kept red hot for an hour at least,
and then set it by to cool: when cold turn it out of the crucible; and
cutting or breaking it to pieces, scrape off, upon trial, the brightest
parts; which, if good phosphorus, will be a white powder.”
EPITOME
OF
GALVANISM.
CHAP. I.
_A Short account of the discovery of Galvanism._
This part of our subject has been called _animal electricity_, by the
greater part of those persons who have written upon it;—but this name
seems to be improper; for, as an author of reputation on the subject,
remarks, “it has by no means been proved that these phenomena depend
either upon electricity or animal life.” While this is the case, it is
certainly best to distinguish this science by the name of its inventor
Louis Galvani. He was an Italian, and professor of anatomy at Bologna,
when he made the discovery of Galvanism, which was entirely accidental,
as will appear in the following account.
Whilst Galvani was one day employed in dissecting a frog, in a room
where some of his friends were amusing themselves with electrical
experiments, one of them happened to draw a spark from the conductor, at
the same time that the professor touched one of the nerves of the
animal. The consequence was, that the animal’s whole body was instantly
shaken by a violent convulsion. Astonished at the phenomenon, and at
first imagining that it might be owing to his having wounded the nerve,
the professor pricked it with the point of his knife, to assure himself
whether or not this was the case; but no motion of the frog’s body was
produced. He now touched the nerve with the instrument as at first, and
directed a spark to be taken at the same time from the machine, on which
the contractions were renewed. Upon a third trial the animal remained
motionless; but observing that he held his knife by the handle, which
was made of ivory, he changed it for a metallic one, and immediately the
movements took place, which never was the case when he used an electric,
or non-conducting substance.