Encyclopaedia Britannica > Volume 8, DIA-England

600 ELECTRICIT Y.
Phenome- coal was held nearly horizontally, so that the carbonic
na and ac;c{ which was generated could rise only by ascending
, ^aws- along the base of the charcoal, which was now vertical, no
sensible effect icas obtained ; and when the lateral as well
as the upper surface of the charcoal, placed vertically,
was lighted, an uncertain result was obtained.
In order to determine the electricity of the charcoal
itself, M. Pouillet places the base of the cylinder upon
the disc of the condenser; and after lighting the upper
end of it, and keeping up the fire by a gentle blast of air,
the condenser was charged, and showed that the electri¬
city taken by the charcoal was negative. When the char¬
coal burnt on all its surface, or when it touched the con¬
denser only in a few points, no electrical effects were ob¬
served. In the last of these cases a small quantity of
electricity only can pass by a small number of points, and
in the first case the positive electricity of the ascending
carbonic acid was recombined with the negative electri¬
city. In order to produce intense and rapid electrical
effects, several cylinders of charcoal, of the same height,
should be placed on their ends, and near each other, upon
a sufficiently large plate of brass; and when all the cylin¬
ders are made to burn at their upper ends, and their
united columns of carbonic acid received by another brass
plate communicating with the condenser, and raised a few
inches, or even a foot, above it, a strong charge of positive
electricity will in a few seconds be communicated to the
brass plate. When the electricity of the charcoal is re¬
quired, we have only to unite the condenser to the brass
plate upon which the burning cylinders are placed, and
in a few seconds the condenser will be abundantly char¬
ged with negative electricity.
When the combustion is maintained by a current of
oxygen, the electricity is not only much more intense,
but is much more quickly developed ; and the gold leaves
of the condenser separate to their maximum divergency
in an instant. The first point, however, to be attended
to in every form of the experiment, is to burn only the
upper horizontal surface, so that the carbonic acid forms
and ascends in a moment, and without touching any other
body till it deposits its electricity on the brass plate. So
essential is this condition, that if we burn even a deep
cavity on the circumference of a vertical cylinder of char¬
coal, and do this even with a jet of oxygen, the electrical
indications are sometimes positive and sometimes negative,
just as the electricity of the gas or the charcoal predomi¬
nates.
Experi- M. Pouillet next entered upon the more arduous in-
mentsof vestigation of determining whether or not electricity is
Pouillet. produced by change of condition or chemical affinity.
Volta had supposed that carbon, in becoming gaseous, ab¬
sorbed the positive and left to the remaining solid parts the
negative electricity which we find in them. M. Pouillet,
on the contrary, supposed that if electricity is disengag¬
ed from two elements which combine, positive electricity
would be given out by the one and negative by the other;
and that when these elements separate, each of them re¬
quired to take up the fluid which they had lost.
Combus- By forming combinations unaccompanied by changes of
tion of condition, M. Pouillet resolved this question. He first
hydrogen. j.rje[} t]iat 0f oxygen and hydrogen. The flame of hydro¬
gen, like charcoal, gave electricity, sometimes strong and
sometimes feeble, sometimes posfft'w and sometimes nega¬
tive ; and it was some time before he discovered the cause
of these discrepancies. That the gases are not very good
conductors of electricity, he found by the following very
curious experiment. Having set a very small spirit-lamp
upon a common electroscope, and about five or six feet
above it a feebly charged body, such as a stick of electrified
rosin or a plate of glass, he observed that the gold leaves
diverged greatly, though the same charged body could pro- Phenon
duce no divergence if held even so near as an inch to the naam
electroscope without flame. This apparatus enabled our Laws,
author to discover the smallest trace of electricity. If'^Y>
we turn the plate of an electrifying machine, the air of
the room is electrified; and the flame which ascends in
that air is charged at the moment with electricity of the
same name. A pile in action electrifies the air in the
same manner, as the flame of the electroscope proves. A
charcoal fire, or even a lighted candle, developes carbonic
acid electrified positively, which is shown also by the elec¬
troscope. The atmospheric air, in short, is always electri¬
fied ; and if it enters a room by any opening, it will pre¬
serve itself in an electrified state so- long as to affect the
results of experiments on stnall quantities of electricity.
These causes of error being excluded, M. Pouillet re¬
peated his experiments on the combustion of hydrogen.
The gas was emitted from a glass tube, and the flame,
which was vertical, was about three inches long and four
or five lines broad.. The brass plate was now set aside,
and the electricity conducted to the condenser by a pla-
tina wire, whose end is coiled into a spiral. The spire
is vertical, and the circumvolutions are sometimes so
large as to surround the flame without touching it, and
sometimes so small as to be completely enveloped in the
interior of the flame. When we approach this flame from
the exterior outline of the spire, and keep it ten milli¬
meters distant, we obtain indications of positive electri¬
city. As the distance of the flame diminishes, the elec¬
tricity becomes more and more intense; but when the
flame touches the spire, the electricity becomes weak, and
its nature uncertain. The same thing is observed when
the flame passes to the interior of the spire, and in the
direction of its own axis. Hence there exists round the
apparent flame of the hydrogen a sort of atmosphere, more
than ten millimeters in thickness, charged with positive
electricity. Positive electricity being thus developed in
the combustion of hydrogen, Pouillet tried to discover
the negative electricity which must have been set free.
He placed a small spiral in the centre of the flame, and
when it was enveloped on all sides, negative electricity
was collected by the condenser. If we plunge the spire
half way into the bright part of the flame, no electricity is
manifested. Hence it follows that the inside and outside Electrn
of the flame are in opposite electrical states, the formerof^nu
being negative and the latter positive, and that there is
an intermediate layer of the flame where the electricity
disappears. On these facts M. Pouillet thus reasons. In
the thickness of the exterior atmosphere of the flame,
when the positive electricity appears, the combination of
oxygen and hydrogen is not effected, for the hydrogen
cannot arrive there. The electricity is therefore commu¬
nicated, and it must come from the oxygen which predo¬
minates on the outside, and which envelopes in some mea¬
sure all the jet of hydrogen. This combined oxygen
must therefore disengage positive electricity, which com¬
municates itself to the neighbouring strata of air sufficient¬
ly heated to conduct it. In like manner the hydrogen pre¬
dominates in the interior of the flame, and the negative
- electricity must be disengaged from the hydrogen which
burns, and which it communicates to the excess of un¬
combined hydrogen. If this view is correct, it is proba¬
ble that, at a certain distance above the flame, the two op¬
posite electricities ought no longer to appear, as they must
have combined; and this is proved to be the case by the
fruitless attempt to collect electricity at a distance suffi¬
ciently great above the vertical flame. At the distance,
however, of a few inches, other phenomena appear. The
two electrical fluids appear there in the same quantity,
but they are not recombined; for if we present a solder-