440 
ELECTRICITY. 
Silver and zinr, or gold and zinc, form the mod per¬ 
fect galvanic combination. All metalic bodies are ex¬ 
cellent condudtors of eledtricity ; but, when oxydated, 
they become non-condudtors. In this change ot capacity 
electricity is given out. A piece of zinc, placed in a 
weak folution of fulphuric acid, is fubjedted to but a 
trifling alteration, compared to the change which enfues 
when it is brought in contadt with a piece of filver. 
This curious phenomenon was firft noticed by Dr. Allie; 
and was by him advanced as an argument in favour of 
galvanifm being the refult of a chemical operation. 
A plate of filver, and a plate of zinc, with a piece of 
cloth, or of any bibulous fubftance, moiftened in water, 
placed between them, conftitute a fimple galvanic com¬ 
bination. In this cafe the zinc becomes oxydated on the 
portion of its furface which is in contadt with the wet 
medium. If a prepared frog be fubftituted for the 
moiftened cloth, the humidity on its furface will produce 
fimilar eftedts on the zinc ; and when the circuit is com¬ 
pleted between the filverand zinc, by any fubftance which 
is a condtrdtor of eledtricity, the mr.fcles of the frog will 
be thrown into adtion. Thefe contractions will be renew¬ 
ed every time the circuit is completed. By a prepared frog^ 
is undet'ftood, a frog divided into two portions by a pair ot 
feiftars, through the middle.of its body and Ipine, the 
vifeera being removed, and the lower part ikinned. The 
fciatic nerves rife very high up in the fpine in this animal, 
and are very difcernible when it is thus arranged. When 
it is intended to arm the nerves, after the introdudtion of 
a pair of pointed feiftars beneath them, the fpine is cut 
through without dividing them. A portion of the infe¬ 
rior part of the fpine ftiould be afterwards feparated; fo 
as to leave room for the nerves to be covered by a piece 
of tin-foil. This is what is denominated armings or coat, 
ing, the nerves. In fome experiments, it is more conve¬ 
nient to feparate the lower extremities, and employ the 
crural nerve. 
If a ftngle metal be employed, it rnuft be of fuch a de- 
feription as not to be eaftly added upon by water. Thus, 
,jf a filver Qup, nearly filled with a weak folution of nitric 
acid, be placed in a glafs velfel containing common water, 
a fimple galvanic combination will be formed ; feeing 
that the iniide of the filver veflel will become oxydated 
by the acid, while its external furface will not undergo 
any change. If a prepared frog be made to form the cir¬ 
cuit between the two fluids, immediately on its comple¬ 
tion the convulfions will be produced. The more adiive 
the fluid which is employed, the more powerful will be 
the eftedts. Thus, fait water is better than common wa¬ 
ter; and dcid and water than fait and water. Volta 
aferibes this difference to tlie condudting powers of the 
fluids, and not to any changes induced in the metallic 
bodies employed. 
After a certain period, the metal the moft liable to oxy- 
dation has its furface oxydated ; and till this is removed 
the galvanic operation ceafes. Whenever a metal be¬ 
comes oxydated, its capacity foreledtricity is diminiftied: 
oxygen cannot enter into union with a metallic lubftance, 
without the eledtricity which conftituted a portion.of the 
metal being given out. A metal which is in a ftate of 
giving out electricity, will evince politive figns of the 
eledtric fluid, and occafion a fenfible eledtrofeope to di¬ 
verge with the fame ftate of eledtricity as is produced by 
excited glafs. 
In the feries of metals, of gold, filver, copper, iron, 
tin, lead, zinc, each will become pofttive by its combi¬ 
nation with that which precedes it, and negative with 
that by which it is followed. Thus, gold and filver form 
a fimple galvanic combination. If a fluid be employed 
which will adt on the filver, and not on the gold, as hap¬ 
pens with the nitric acid ; in this cafe the filver will be 
the oxydable metal, and will therefore give out a portion 
of its combined eledtricity. This equally applies to cop¬ 
per and iron, to iron and tin, &c. Silver, when com¬ 
bined with gold, exhibits pofttive figns; but when it is 
arranged with copper, it evinces negative figns. 
As all metals, with the exception of gold, undergo 
fome fmall changes on the application of an oxydating 
fluid, the moft perfedt galvanic combination is confti¬ 
tuted, when one of the metals undergoes the greateft 
change, and the other the leaft. When zinc and copper 
are employed with a folution of a muriat of ammonia, 
the muriatic acid adts upon the zinc, and a portion of am¬ 
monia attacks the copper. In this way the galvanic ef. 
fedt is not fo powerful as if gold had been employed. 
All fubftances which are condudtors of electricity are 
likevvife condudtors of galvanifm, and in precifely the 
fame order: the moft perfedt condudtors being metallic 
fubftances; next charcoal, mul'cular flefli, &c. nearly in 
the following order: 
Conductors. —All the metals.—Minerals containing 
metals not oxydated.—Vegetable coal.—Mineral coal.— 
Carbonated blend.—Aluminous fchift. — Inflammable 
fchift.—Siliceous fubftances containing carbon.—Grey 
manganefe, arifing from the carbon it contains.—Muf- 
cular fled).—Membranes, nerves, ligaments, and blood- 
veflels, whether moift or dry.—Morellsand mufhrooms.—i 
White of an egg.—Water, blood, juices of plants.™ 
Cellular tilTiie of vegetables.—Spirit of wine.—Wine, 
beer, acids, alkaline folutions, foft foap, and flame, 
(doubted by fome.) 
Non-conductors.— Oxyds of metals.—Oxyds of 
carbon. — Gates. — Periofteum. —Hair. — Epidermis and 
cuticle.—Fibres of wood.—Glafs.—Amber. —White of 
an egg hardened.—Oil, refins, and gums. 
Volta divides the condudtors of eledtricity into two 
clafles, dry, and moift. The dry condudtors he ranks in 
the firft clafs ; and the moift condudtors in the fecond. 
When a moift condudtor and a dry condudtor are brought 
into contadt, a difturbance of the eledtrical fluid takes 
place. The mode in which this difturbance is produced, 
he does not venture to conjedture ; and it is oftentimes 
fo low as fcarcely to be detedted by any means at prefent 
known. 
Mr. Pepys has contrived an ingenious apparatus to be 
adapted to Bennet’s eledtrolcope, fhewn at O, fig. 24, in 
the Electricity Plate III. for the purpofe of evincing all 
fuch fmall dates of eledtricity, developed in galvanic 
experiments. Witli this view, inftead of the tin-foil 
coatings within.fide the glafs, lie has fubftituted two 
plates, n,e, forming an acute angle, which, by a regulat¬ 
ing ferew, can be adjufted to any required diftance from 
the gold leaves. The angular part is fecured to the bot¬ 
tom ; the open part perpendicularly upwards ; and the 
gold leaves, reprefented oppofite c, hang between them. 
By this ingenious mode of approximating the coatings to 
the gold leaves, the refiftance being diminiftied, a weaker 
intenfity of eledtricity fuffices for their difturbance.—■ 
Owing to its fuperior fallibility, this inftrument is now 
generally denominated the Galvanometer. 
Mr. Read, well known by his valuable obfervations ©n 
atmofpherical eledtricity, is the firft who applied the 
galvanic condenfer to this eledtrofeope ; to the end that 
it fhould alfo evince fmaller intenfities of eledtricity. 
The very minute portion of the fluid given out by the 
ftngle contadt of two different metals, does not produce 
any difturbance of the gold leaves ; but when feveral mi¬ 
nute portions are accumulated, a feparation of the leaves 
will take place. The eledtrofeope, in its fimple ftate, 
will be as much charged the firft time as if the contadt 
had been made a tlioufand times, and cannot therefore 
acquire a greater quantity of the fluid than fuffices to 
place it in equilibria with the metallic plates. This por¬ 
tion being inadequate to the production of any divergency 
of the gold leaves, Mr. Read applied the principle of the 
eledtrical doubler to the above inftrument, by which 
means he was enabled to charge an intervening plate of 
air. By thus accumulating every minute portion of the 
eledtri* 
