_Of Electricity._
Electricity is a property belonging to, or capable of being communicated
to, all substances whatever; and whereas by some of them it is
transmitted with great ease, and by others with much difficulty, they
have been divided into two classes, and denominated _conductors_ or
_non-conductors_ of electricity. Also the latter receiving this power by
friction, and other means, are termed _electrics_, and the former
_non-electrics_.
Metals of all kinds, and water, are conductors, though in very different
degrees; so also is charcoal. All other substances, and also a perfect
vacuum, are non-conductors of electricity. But many of these substances,
when they are made very hot, as glass, resin, baked wood, and perhaps
all the rest on which the experiment can be made in this state, are
conductors.
It is the property of all kinds of electrics, when they are rubbed by
bodies different from themselves, to attract light substances of all
kinds, to exhibit an appearance of _light_, attended with a particular
_sound_, on the approach of any conductor; and if the nostrils are
presented, they are affected with a _smell_ like that of phosphorus.
This attraction is most easily explained by supposing that electricity
is produced by a fluid exceedingly elastic, or repulsive of itself, and
attracted by all other substances.
An electric exhibiting the appearances above mentioned, is said to be
_excited_, and some of them, particularly the _tourmaline_, are excited
by heating and cooling, as well as by friction. It appears, however,
that excitation consists in the mere transferring of electricity from
one substance to another, and that the great source of electricity is in
the earth. On this account it is necessary to the considerable
excitation of any electric, that the substance against which it is
rubbed (hence termed _the rubber_) have a communication with the earth,
by means of conductors; for if the rubber be _insulated_, that is cut
off from all communication with the earth by means of electrics, the
friction has but little effect.
When insulated bodies have been attracted by, and brought into contact
with, an excited electric, they begin to be repelled by it, and also to
repel one another; nor will they be attracted again till they have been
brought into contact with some conductor communicating with the earth;
but after this they will be attracted as at first.
If conductors be _insulated_, electric powers may be communicated to
them by the approach of excited electrics, or the contact of other
electrified bodies. They will then attract light bodies, and give
sparks, &c. like the excited electrics themselves.
When electricity is strongly communicated to insulated animal bodies,
the pulse is quickened, and perspiration increased; and if they receive,
or part with, their electricity on a sudden, a painful sensation is felt
at the place of communication. But what is more extraordinary, is, that
the influence of the brain and nerves upon the muscles seems to be of an
electric nature.
This is one of the last and most important of all philosophical
discoveries. I shall, therefore, give the result of all the observations
that have hitherto been made on the subject, in a _series of
propositions_, drawn up by an intelligent friend, who has given much
more attention to it than I have done.
1. The nerve of the limb of an animal being laid bare, and surrounded
with a piece of sheet lead, or of tinfoil, if a communication be formed
between the nerve thus armed and any of the neighbouring muscles, by
means of a piece of zinc, strong contractions will be produced in the
limb.
2. If a portion of the nerve which has been laid bare be armed as above,
contractions will be produced as powerfully, by forming the
communication between the armed and bare part of the nerve, as between
the armed part and muscle.
3. A similar effect is produced by arming a nerve and simply touching
the armed part of the nerve with the metallic conductor.
4. Contractions will take place if a muscle be armed, and a
communication be formed by means of the conductor between it and a
neighbouring nerve. The same effect will be produced if the
communication be formed between the armed muscle and another muscle,
which is contiguous to it.
5. Contractions may be produced in the limb of an animal by bringing the
pieces of metal into contact with each other at some distance from the
limb, provided the latter make part of a line of communication between
the two metallic conductors.
The experiment which proves this is made in the following manner. The
amputated limb of an animal being placed upon a table, let the operator
hold with one hand the principal nerve, previously laid bare, and in the
other let him hold a piece of zinc; let a small plate of lead or silver
be then laid upon the table, at some distance from the limb, and a
communication be formed, by means of water, between the limb and the
part of the table where the metal is lying. If the operator touch the
piece of silver with the zinc, contractions will be produced in the limb
the moment that the metals come into contact with each other. The same
effect will be produced if the two pieces of metal be previously placed
in contact, and the operator touch one of them with his finger. This
fact was discovered by Mr. William Cruikshank.
6. Contractions can be produced in the amputated leg of a frog, by
putting it into water, and bringing the two metals into contact with
each other at a small distance from the limb.
7. The influence which has passed through, and excited contractions in,
one limb, may be made to pass through, and excite contractions in,
another limb. In performing this experiment it is necessary to attend to
the following circumstances: let two amputated limbs of a frog be taken;
let one of them be laid upon a table, and its foot be folded in a piece
of silver; let a person lift up the nerve of this limb with a silver
probe, and another person hold in his hand a piece of zinc, with which
he is to touch the silver including the foot; let the person holding the
zinc in one hand catch with the other the nerve of the second limb, and
he who touches the nerve of the first limb is to hold in his other hand
the foot of the second; let the zinc now be applied to the silver
including the foot of the first limb, and contractions will immediately
be excited in both limbs.
8. The heart is the only involuntary muscle in which contractions can be
excited by these experiments.
9. Contractions are produced more strongly, the farther the coating is
placed from the origin of the nerve.
10. Animals which were almost dead have been found to be considerably
revived by exciting this influence.
11. When these experiments are repeated upon an animal that has been
killed by opium, or by the electric shock, very slight contractions are
produced; and no contractions whatever will take place in an animal that
has been killed by corrosive sublimate, or that has been starved to
death.
12. Zinc appears to be the best exciter when applied to gold, silver,
molybdena, steel, or copper. The latter metals, however, excite but
feeble contractions when applied to each other. Next to zinc, in contact
with these metals, tin and lead, and silver and lead, appear to be the
most powerful exciters.
At least two kinds of fishes, the _torpedo_ and the _electrical eel_,
have a voluntary power of giving so strong a shock to the water in which
they swim, as to affect fishes and other animals which come near them;
and by a conducing communication between different parts of these
fishes, an electric shock may be given exactly like that of the Leyden
phial, which will be described hereafter; and if the communication be
interrupted, a flash of electric light will be perceived.
The growth of vegetables is also quickened by electricity.