_The same Subject continued._
No electric can be excited without producing electric appearances in the
body with which it is excited, provided that body be insulated; for this
insulated rubber will attract light bodies, give sparks, and make a
snapping noise, upon the approach of a conductor, as well as the excited
electric.
If an insulated conductor be pointed, or if a pointed conductor,
communicating with the earth, be held pretty near it, little or no
electric appearance will be exhibited, only a light will appear at each
of the points during the act of excitation, and a current of air will be
sensible from off them both.
The effect of pointed bodies is best explained on the supposition of the
electric matter in one body repelling that in another; and consequently
the electricity belonging to a body with a large surface making a
greater resistance to the entrance of foreign electricity than that
belonging to a smaller.
These two electricities, viz. that of the excited electric, and that of
the rubber, though similar to, are the reverse of, one another. A body
attracted by the one will be repelled by the other, and they will
attract, and in all respects act upon, one another more sensibly than
upon other bodies; so that two pieces of glass or silk possessed of
contrary electricities will cohere firmly together, and require a
considerable force to separate them.
These two electricities having been first discovered by producing one of
them from glass, and the other from amber, sealing-wax, sulphur, rosin,
&c. first obtained the names of _vitreous_ and _resinous_ electricity;
and it being afterwards imagined that one of them was a redundancy, and
the other a deficiency, of a supposed electric fluid, the former has
obtained the name of _positive_, and the latter that of _negative_,
electricity; and these terms are now principally in use.
Positive and negative electricity may be distinguished from each other
by the manner in which they appear at the points of bodies. From a
pointed body electrified positively, there issues a stream of light,
divided into denser streams, at the extremities; whereas, when the point
is electrified negatively, the light is more minutely divided, and
diffused equally. The former of these is called a _brush_, and the
latter a _star_.
If a conductor not insulated be brought within the atmosphere (that is
the sphere of action) of any electrified body, it acquires the
electricity opposite to that of the electrified body, and the nearer it
is brought, the stronger opposite electricity does it acquire, till the
one receive a spark from the other, and then the electricity of both
will be discharged.
The electric substance which separates the two conductors possessing
these two opposite kinds of electricity, is said to be _charged_.
Plates of glass are the most convenient for this purpose, and the
thinner the plate the greater is the charge it is capable of holding.
The conductors contiguous to each side of the glass are called their
_coating_.
Agreeably to the above-mentioned general principle, it is necessary that
one side of the charged glass have a communication with the rubber,
while the other receives the electricity from the conductor, or with the
conductor, while the other receives from the rubber.
It follows also, that the two sides of the plate thus charged are always
possessed of the two opposite electricities; that side which
communicates with the excited electric having the electricity of the
electric, and that which communicates with the rubber, that of the
rubber.
There is, consequently, a very eager attraction between these two
electricities with which the different sides of the plate are charged,
and when a proper communication is made by means of conductors, a flash
of electric light, attended with a report (which is greater or less in
proportion to the quantity of electricity communicated to them, and the
goodness of the conductors) is perceived between them, and the
electricity of both sides is thereby discharged.
The substance of the glass itself in, or upon, which these electricities
exist, is impervious to electricity, and does not permit them to unite;
but if they be very strong, and the plate of glass very thin, they will
force a passage through the glass. This, however, always breaks the
glass, and renders it incapable of another charge.
The flash of light, together with the explosion between the two opposite
sides of a charged electric, is generally called the _electric shock_,
on account of the disagreeable sensation it gives any animal whose body
is made use of to form the communication been them.
The electric shock is always found to perform the circuit from one side
of the charged glass to the other by the shortest passage through the
best conductors. Common communicated electricity also observes the same
rule in its transmission from one body to another.
It has not been found, that the electric shock takes up any sensible
space of time in being transmitted to the greatest distances.
The electric shock, as also the common electric spark, displaces the air
through which it passes; and if its passage from conductor to conductor
be interrupted by non-conductors of a moderate thickness, it will rend
and tear them in its passage, in such a manner as to exhibit the
appearance of a sudden expansion of the air about the center of the
shock.
If the electric circuit be interrupted, the electric matter, during the
discharge, will pass to any other body that lies near its path, and
instantly return. This may be called the _lateral explosion_. The effect
of this lateral explosion through a brass chain, when the quantity of
electricity is very great, will be the discolouring and partial burning
of the paper on which it lies.
If a great quantity of electricity be accumulated, as in a _battery_,
the explosion will pass over the surfaces of imperfect conductors
without entering them, and the effect will be a strong _concussion_ of
the substance. Also the electric matter thus accumulated and condensed
will, by its repulsion, form _concentric circles_, which will appear by
melting the surface of a flat piece of metal on which the explosion is
received.
If an electric shock, or strong spark, be made to pass through, or over,
the belly of a muscle, it forces it to contract, as in a convulsion.
If a strong shock be sent through a small animal body, it will often
deprive it instantly of life.
When the electric shock is very strong, it will give polarity to
magnetic needles, and sometimes it reverses their poles.
Great shocks, by which animals are killed, are said to hasten
putrefaction.
Electricity and lightning are in all respects the same thing; since
every effect of lightning may be imitated by electricity, and every
experiment in electricity may be made with lightning, brought down from
the clouds by means of insulated pointed rods of metal.