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Page 27
There are some metals, such as lead, for instance, which oxydise
readily, but this process stops short at the surface in contact with the
air, and so forms a coating which prevents the metal from further
oxydation; so that here, as in so many things else, strength is
connected with weakness.
_Electricity_.--This, in the most elementary view of it, is a more or
less attractive or repellant force latent in bodies, and which is
capable of being roused into action by the application of friction. It
is excited in a rod of glass by rubbing it with silk, and in a piece of
sealing-wax by rubbing it with flannel, though the effect is different
when we apply first the one and then the other to the same body. Thus,
_e.g._, if we apply the excited sealing-wax to a paper ring, or a
pith-ball, hung by a silk thread from a horizontal glass rod, it will,
after contact, repel it; and if, thereafter, we apply to it the excited
glass rod, it will attract it; or if we first apply the excited glass
rod to the paper ring, or pith-ball, it will, after contact, repel it;
and if thereafter we apply to it the excited sealing-wax, it will
attract it. The reason is, that when we once charge a body by contact
with either kind, it repels that kind, and attracts the opposite; if we
charge it from the glass, _i.e._, with vitreous electricity, it refuses
to have more, and is attracted to the sealing-wax; and if we charge it
from the sealing-wax, _i.e._, with resinous electricity, it refuses to
have more, and is attracted to the glass-rod; only it is to be observed
that, till the body is charged by either, it has an equal attraction for
both. From all which it appears that kindred electricities repel, and
opposite attract, each other.
Two pieces of gold leaf suspended from a metal rod, inserted at the top
of a glass shade full of perfectly pure, dry air, will separate if we
rub our foot on the carpet, and touch the top of the rod with one of our
fingers; for the motion of the body, as in walking, always excites
electricity, and it is this which, as it passes through the finger,
causes the phenomenon; though the least sensation of damp in the glass
would, by instantly draining off the electricity, defeat the experiment.
What happens in this case is, that one kind of electricity passes from
the finger to the leaves, while another kind, to make room for it,
passes from the leaf to the finger; and the leaves separate because they
are both more or less charged with the same kind of electricity, and
kindred electricities repel each other. Ribbons, particularly of white
silk, when well washed, are similarly susceptible of electrical
excitation; and they behave very much as the gold leaf does when they
are rubbed sharply through a piece of flannel. Gutta-percha is another
substance which, when similarly treated, is similarly affected.
This power is a very mysterious one, and of a nature to perplex even the
philosophic observer. Certain bodies, such as the metals, convey it, and
are called conductors; certain others, such as glass and porcelain,
arrest it, and are called insulators. It is for this reason that the
wires of the telegraph are supported by a non-conductor, for if not, the
electric current would pass into the earth by the first post and never
reach its final destination. Glass being an insulator, it was found
that, if a glass bottle was filled with water, and then corked up with a
cork, through which a nail was passed so that the top of it touched the
water, it would receive and retain a charge as long as it was held in
the hand; and this observation led to an invention of some account in
the subsequent applications of electricity, known, from the place of its
conception, as the Leyden jar. This is a glass jar, the inside of which
is coated with tinfoil, and the outside as far as the neck, and into
which, so as to touch the inside coating, a brass rod with a knob at
the top is inserted through a cork, which closes its mouth. By means of
this, in consequence of the isolation of the coatings by the glass,
electricity can, in a dry atmosphere, be condensed, and stored up and
husbanded till wanted.
A series of eggs, arranged in contact and in line, give occasion to a
pretty experiment. In consequence of the shells being non-conductors,
and the inside conducting, it happens that a current of electricity,
applied to the first of the series, will pass from one to another in a
succession of crackling sparks, in this way forcing itself through the
obstructing walls. This effect of electricity in making its way through
non-conducting obstructions accounts for the explosion which ensues when
a current of it comes in contact with a quantity of gunpowder; as it
also does for the fatal consequences which result when, on its way from
the atmosphere to the earth, it rushes athwart any resisting organic or
inorganic body.
_Magnetism_.--Unlike electricity, which acts with a shock and then
expires, magnetism is a constant quantity, and constant in its action;
and it has this singular property, that it can impart itself as a
permanent force to bodies previously without it. Thus, there being
natural magnets and artificial, we can, by passing a piece of steel over
a magnet, turn it into a strong magnet itself; although we can also,
when it is in the form of a horse-shoe, by a half turn round and then
rubbing it on the magnet, take away what it has acquired, and bring it
back to its original state. The magnetic property is very readily
imparted (by induction, as it is called) to soft iron, but when the iron
is removed from the magnetising body, it parts with the virtue as fast
as it acquired it. To obtain a substance that will retain the power
induced, we must make some other election; and hard steel is most
serviceable for conversion into a permanent magnet.
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