Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY. 625
me. iias suspended to it a weight so as to stretch it consider- The reader who has perused with attention our chapter Phenome-
ami ably, the wire will be increased in length, in place of being on Electric Light, will recognise in these experiments the
aws diminished. This effect is by no means inconsistent with origin of those beautiful results which have been obtained
^ose already described. The heat, which, as we shall after- by Fusinieri, by passing the electric shock from a metallic Diffu^ion
£ansion.war(js see, jg aiwayS eVolved during the transmission of an ball to a polished metallic surface; and the diffusion ofofmetals
electric charge, produces a softness short of fluidity, which solid bodies into metallic vapour, as it maybe called, is into va-
allows the extending force to overcome the absolute tena- finely illustrated in the following experiments. Take three pour,
city of the wire. strips of window glass, each about three inches long and
It is very obvious that the contraction of the wire in one one wide, and having placed two narrow strips of gold leaf
dimension in these experiments was owing to its expansion or leaf brass between them, so that the ends of the gold
in a direction at right angles to the length, in the same leaf project a little beyond the glass, transmit the charge
manner as a piece of caoutchouc extended in one direction of a large Leyden jar through the gold leaf. I he gold leaf
is shortened in the other. Mr Nairne indeed observed that will be found to be melted by the shock, and driven into the
the wire had increased in thickness ; but though he used a surface of the glass. The outer plates of glass are gene-
pair of scales which turned with one eighth of a grain, he rally broken in this experiment, and the middle one, which
could not observe any change in the weight of the wire. frequently remains entire, has an indelible metallic stain
The same phenomenon takes place in fluid metals. If, upon each of its surfaces. This stain is obviously the me-
for example, we fill a capillary or thermometer tube with tallic vapour of the gold driven into the pores of the glass,
mercury, and transmit through the mercurial column an The dispersion of gold or silver into a metallic vapoui
electrical charge, the metal will suffer such a degree of ex- may be exhibited in another manner. Let a strip of gold
pansion as to burst the tube to pieces. or silver leaf, or Dutch metal, be fixed with gum to toe
When the body which transmits the shock has a less surface of a piece of paper, and be placed in such a manner
conducting power than metals, the tendency to expand between the forceps of an universal discharger, that a
will of course be still greater. If a little water, for example, strong electrical charge may be passed through it. Ihe
is placed in a glass tube, and a shock passed through the metallic strip will entirely disappear, in consequence of
water, the tube will burst by the expansion of the fluid; having been dispersed into a vapour or powder, pait of
and the experiment will succeed even if a common drink- which remains in a state of oxidation on the paper, which,
ine glass is filled with water and substituted for the tube, from this cause, receives a greenish-brown tinge.
Beccaria placed a drop of water in the centre of a solid Ihe metallic colours thus obtained have been employe
glass-ball, and burst the ball by transmitting a shock for impressing ornamental figures upon paper or silk. In
through the fluid drop. This experiment was beautifully order to do this, trace the outline of the figures on thick
varied by the Italian philosopher, who constructed a small drawing paper, and having cut it out as in stincil plates,
mortar, and having put a ball into it, he placed behind the place it on the silk or paper intended to be ornamented,
ball a drop of water, so as to lie between the two wires When a gold leaf is laid upon it, and a. card above the
which passed through the side of the mortar. When an gold leaf, the whole is placed in a press or beneath a weight,
electric charge was sent through the two wires, the ex- and an electrical charge sent through it; the metallic stain
pansion sustained by the water drove out the ball with is limited to the portion of the drawing paper that is cut
great velocity. M. Lullin gave the ball a still greater im- away, and consequently any outline figure may be readily
pulsion by substituting a drop of oil for the drop of water, impressed upon the ground employed to receive it.
Even when the conductor is air, a violent expansive ef- Dr Franklin was the first person who impressed metallic
feet is created during the transmission of the electric shock, stains upon glass by electrical discharges. Tine gold com-
This effect is well shown by fitting a cork cap into an ivory municated a reddish stain and silver a greenish one, and
mortar having a cavity an inch deep and half an inch wide, the metallic vapour, when driven into the pores of glass,
When a shock is sent through the wire in this cavity, it is was able to resist the action of the strongest aqua regia.
expanded so suddenly as to drive out the cork with great
violence.
When the electric shock is made to pass through solid Sect. II.— On ihe Chemical Changes produced by Electric
bodies which are imperfect conductors, such as wood, stone, city on Inorganic Bodies.
sugar, and glass, they will be broken by the expansion which ... ^ • i
is produced. In the case of glass not very thick, it will be The effects of electricity as a chemical agent are strik-Chemica
broken into innumerable pieces. When the glass is so thick ingly displayed in its power of evolving heat, and conse‘Educed
as to resist the shock, it is marked with vivid prismatic co- quently of inflaming and fusing bodies, and lt.s.PowerLot ky eiectri.
lours, which Mr Henley supposes to be thin laminae of promoting chemical composition and decomposition. ^ecity.
the glass separated from one another by the shock. influence of electricity in producing combustion ma} e
The expansive effect created by the shock is finely exhi- shown by several beautiful experiments. # .
bited by dipping a clean brass chain in melted rosin, and Eocp. 1. do light a candle by electricity. Having wrap vo ution
laying it upon paper. If the charge of thirty-two square ped some loose cotton wool round the extremity of a longot ea .
feet of coated glass is sent through it, the resinous coating brass pin or wire, roll the cotton in the powder of white
will be driven off from every part of the chain, which will or yellow rosin. Bring the naked end of the wne into
be entirely cleared of it. contact with the external coating, while the cotton end is
When a clean uncoated brass chain is laid upon a plate applied to the brass knob of a charged jar, and the rosin
of glass, and a charge of thirty-two square feet passed and cotton will be instantly inflamed,
through it, the glass will be marked in every part of its sur- By dipping the cotton in oil of turpentine, and using a
face where it was touched by the chain, every marking large jar, the cotton may be inflamed m a similar manner ;
having the width and colour of the link. The metal could and its inflammation will be promoted by strewing upon it
be scraped off the glass at the outside of the mark, but some fine brass dust.
it was actually driven in other places into the pores of the Exp. 2. To light a candle in another way. rus a
glass. Dr Priestley, who made this interesting experiment, wire up through the middle of the candle to within a
produced a similar effect upon glass with a silver chain, short distance of the wick, and having connected the out-
and small pieces of other metals. side coating of a charged jar, by means of a chain, with the
VOL. VIII. K