Encyclopaedia Britannica > Volume 8, DIA-England

S72
ELECTRICITY.
Experi¬
ments of
jEpinus.
History, in their secondary crystals which occurs in tourmaline.
Mr Brard discovered that pyro-electricity was a property
of the axinite ; and more recently Sir David Brewster has
detected it, as we shall afterwards see, in a variety of other
minerals.
In repeating and extending the experiments of Haiiy,
Sir David Brewster discovered that various artificial salts
were pyro-electrical; and he mentions the tartrate of pot¬
ash and soda, and the tartaric acid, as exhibiting this pro¬
perty in a very strong degree. He likewise made many
experiments with the tourmaline when cut into thin slices,
and reduced to the finest powder, in which state each
atom preserved its pyro-electricity; and he has shown
that scolezite and mesolite, even when deprived of their
water of crystallization, and reduced to powder, preserve
their property of becoming electrical by heat. When
this white powder is heated and stirred about by any
substance whatever, it collects in masses like new fal¬
len snow, and adheres to the body with which it is stir¬
red.
In addition to his experiments on the tourmaline, iEpi-
nus made several on the electricity of melted sulphur;
and, in conjunction with Wilcke, he investigated the sub¬
ject of electric atmospheres, and discovered a beautiful
method of charging a plate of air by suspending large
wooden boards coated with tin, and having their surfaces
near each other, and parallel. iEpinus, however, has been
principally distinguished by his ingenious theory of elec¬
tricity, which he has explained and illustrated in a separate
work which appeared at St Petersburg in 1759. This
theory is founded on the following principles. 1. The
particles of the electric fluid repel each other with a force
decreasing as the distance increases. 2. The particles of
the electric fluid attract the particles of all bodies, and
are attracted by them, with a force obeying the same law.
3. The electric fluid exists in the pores of bodies; and
while it moves without any obstruction in non-electrics,
such as metals, water, &c., it moves with extreme diffi¬
culty in electrics, such as glass, rosin, &c. 4. Electrical
phenomena are produced, either by the transference of the
fluid from a body containing more to another containing
less of it, or from its attraction and repulsion when no
transference takes place.
Electricity The electricity of fishes, like that of minerals, now be¬
ef fishes. gan to excite very general attention. The ancients, as
we have seen, were acquainted with the benumbing power
of the torpedo, but it was not till 1676 that modern natu¬
ralists attended to this remarkable property. The Ara¬
bians had long before given this fish the name of raad or
lightning; but Redi was the first who communicated the
fact that the shock was conveyed to the fisherman by
means of the line and rod which connected him with the
fish. Lorenzini published engravings of its electrical or¬
gans ; Reaumur described the electrical properties of the
fish; Kasmpfer compared the effects which it produced
to lightning; but Bancroft was the first person who dis¬
tinctly suspected that the effects of the torpedo were elec¬
trical. In 1773 Mr Walsh and Dr Ingenhouz proved,
by many curious experiments, that the shock of the tor¬
pedo was an electrical one; and Dr Blunter examined
and described the anatomical structure of its electrical
organs. Humboldt, Gay Lussac, and M. Geoffroy, pur¬
sued the subject with success; and Mr Cavendish con¬
structed an artificial torpedo, by which he was able to pro¬
duce artificially the actions of the living animal. The sub¬
ject has been more recently investigated by Dr Todd, Sir
Humphry Davy, and Dr John Davy.
The power of giving electric shocks has been discover¬
ed also in the Gymnotus Electricus, the Silurus Electri-
cus, the Trichiurus Indicus, and the Tetraodon Electricus.
The most interesting and the best known of these singu- pi t
lar fishes is the Gymnotus or Surinam eel. Its electrical v_»"v
organs have been minutely described by Hunter and
Geoffroy; Dr Williamson, Dr Gordon, and Mr Walsh
have published interesting details of its electrical powers;
and Humboldt has more recently given the most roman¬
tic account of the combats which are carried on in South
America between the gymnoti and the wild horses in the
vicinity of Calabozo.
Among the modern cultivators of electricity, our coun-MrCa
tryman, the late Mr Cavendish, is entitled to a distin-dbh, b
guished place. Before he had any knowledge of the1^31^
theory of iEpinus, he had composed and communicated to1810,
the Royal Society a theory of electrical phenomena near¬
ly the same as that of the German philosopher. As Mr
Cavendish, however, had carried the theory much farther,
and considered it under a more accurate point of view,
he did not hesitate to give his paper to the world.
Mr Cavendish made some accurate experiments on the
relative conducting power of different substances. He
found that the electric fluid experiences as much resist¬
ance in passing through a column of water one inch
long, as it does in passing through an iron wire of the
same diameter 400,000,000 inches long, and hence he
concludes that rain or distilled water conducts 400,000,000
times more than iron wire. He found that the water, or
a solution of one part of salt in one of water, conducts a
hundred times better than fresh water ; and that a satu¬
rated solution of sea-salt conducts seven hundred and
twenty times better than fresh water. Mr Cavendish
likewise determined by nice experiments that the quan¬
tity of electricity in coated glass of a certain area in¬
creased with the thinness of the glass; and that in dif¬
ferent coated plates the quantity was as the area of the
coated surface directly, and as the thickness of the glass
inversely.
Although electricity had been employed as a chemical Chemii
agent in the oxidation and fusion of metals, yet it is to electric
Mr Cavendish that we owe the first of those brilliant en¬
quiries which have done so much for the advancement of
modern chemistry. By means of the electric spark he
succeeded in decomposing atmospheric air. By using dif¬
ferent proportions of oxygen and hydrogen, and examin¬
ing the product which they formed after explosion with
the electric spark, he obtained a proportion when the pro¬
duct was pure water. He was equally successful in the
more difficult experiment of exploding oxygen and nitro¬
gen ; but when he combined seven measures of oxygen
with three measures of nitrogen, he obtained from their
explosion nitric acid. As several foreigners had failed
in repeating this interesting experiment, Mr Cavendish,
aided by Mr Gilpin, exhibited it publicly before the lead¬
ing members of the Royal Society on the 6th of Decem¬
ber 1787.
The decomposition of water by the electric spark was
first effected by MM. Paets, Troostwyk, and Deiman;
and improved methods of doing it were discovered and
used by Dr Pearson, Mr Cuthbertson, and Dr Wol¬
laston.
As a chemical agent, however, electricity was now des-Ca w
lined to transfer its supremacy to another science. ^^etaiceie
great discovery made by Galvani in 1790, that the con-tricitVt
tact of metals produced muscular contraction in frogs, and
the invention of the Voltaic pile, in 1800, by M. Volta
of Como, have led to the establishment of a new science,
called Galvanism or Voltaic Electricity, which, though now
proved to be identical with common electricity, requires to
be treated in a separate article. The chemical effects ot
the Voltaic pile far transcended those of ordinary electrici¬
ty, and enabled Sir Humphry Davy to decompose the earths