Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY.
585
PI, jme* It has been the general opinion of philosophers that the
n> n<l electric spark was the electric fluid itself, or, as Biot ex-
lv9, presses it, “ a modification of electricity itself, which had
the faculty of becoming light at a certain degree of ac-
“rf'0? cumulation." ’ * ' *1 ’ " ’
This eminent writer, however, considered
ithis opinion as erroneous, and has devoted a whole chap¬
ter to prove that electric light has the same origin as the
light disengaged from air by mechanical pressure, “ and
that it is purely the effect of the compression produced on
the air by the explosion of electricity” In order to estab-
Thyof lish this theory, M. Biot has stated, on the authority of
H.iot. several experiments, “ that the intensity of electric light
depends always on the ratio which exists between the
quantity of electricity transmitted and the resistance of
the medium; and he has shown, by an experiment with
Kinnersley’s air thermometer, “ that at each spark the
air of the cylinder, driven by the repulsive force, presses
on the surface of mercury, which rises suddenly in the
small tube, and falls back again immediately after the ex¬
plosion.” “ This indication,” he adds, “ proves the sepa¬
ration produced between the particles of the mass of air
where the electricity passes; and from what we know of
its extreme velocity, it is certain that the particles ex¬
posed immediately to its shock ought in the first moment
to sustain individually all the effect of the compression.
They ought, then, from this cause alone to disengage light
as when they are subjected to any other mechanical pres¬
sure. Thus one part at least of the electric light is neces¬
sarily due to this cause; and this being the case, there
is no experiment which can lead us to conjecture that it
is not all due to this cause.”1
These arguments, whatever may be their weight, carry
no conviction to our mind. When we possess two series
of accurate experiments, by which it is proved that light
produced by mechanical pressure in air and gases, of
different bulks, and of different degrees of temperature,
rarefaction, and condensation, has the same colour, the
same composition, and the same general character, as the
light produced by electricity in passing through air and
gases under the very same circumstances, we shall regard
this theory as deserving of consideration.
M. Biot, anticipating the objection that electrical light
is produced in the best vacuum, replies, that a vacuum
such as we can produce is filled with vapours and gas,
and that the barometric one is filled with mercurial va¬
pour. Still, however, the light produced is not produced
by air, and it should be shown that mechanical pressure
is capable of eliciting light in such a vacuum; at all
events, the light ought to bear some proportion to the
degree of rarefaction, whereas Sir H. Davy obtained a
bright light in the best vacuum with mercury, and the
same light in the best vacuum with melted tin.
Anticipating another objection from the fact that the
electric spark, when intense, passes through water, M. Biot
gives a double reply, Is#, that the water itself is probably
compressible, and therefore, we presume, gives out light
during its compression ; and, 2c%, that water always con¬
tains in combination a certain quantity of air, which may
also contribute to the result. It would be desirable, there¬
fore, to ascertain if water, and wrater with much air, give
out light by mechanical compression.
In explaining the fact that electrical light is violet when
the electricity is feeble, and of a brilliant whiteness when
Jt is produced by a violent discharge, M. Biot remarks
I that “ this variation of tints discloses still further the ori¬
gin of the light; for we observe it in substances which
burn according as the combustion is slow or rapid ; that is
to say, according as the oxygen of the air which this Phenome-
combustion absorbs is more or less rapidly condensed. na and
The light which sulphur disengages when it begins to in- t Laws,
flame is as violet as that of feeble electricity, but that of
sulphur in vigorous combustion is white.” The views upon
which this argument is founded are themselves hypothe¬
tical. It is by no means made out that the colour passes
from violet to white as the intensity of combustion in¬
creases ; and, in the very case of sulphur referred to by
M. Biot, Sir John Herschel has proved, that when it is
inflamed in a white hot crucible, it gives out neither blue,
green, nor red rays, but solely homogeneous yellow light, of
a very definite refrangibility, and which contains almost
none of the elements of white light.
A more philosophical view of the probable origin of Sir H.
electrical light has been hinted at by Sir Humphry Davy, Davy,s
in his paper of 1822, already quoted. “ The circumstance,” theory,
says he, “ that the intensity of the electrical light in the
mercurial vacuum diminishes as it is cooled to a certain
point, when the vapour must be of infinitely small density,
and is then stationary, seems strongly opposed to the idea
that it (electrical light) is owing to any permanent vapour
emitted constantly by the mercury. The results with tin
must be regarded as more equivocal; because, as this sub¬
stance cannot be boiled in vacuo, it may be always sus¬
pected to have emitted a small quantity of the rare air
or gas to which it has been exposed ; yet, supposing this
circumstance, such gas must be at least as highly expand¬
ed as the vapour from cooled mercury, and can hardly be
supposed capable of affording the dense light which the
passage of the charged Leyden phial through the vacuum
produces.
“ When the intense heat produced by electricity is con¬
sidered, and the strong attractive powers of differently
electrified surfaces, and the rapidity of the changes of
state, it does not seem at all improbable that the super¬
ficial particles of bodies, which, when detached by the
repulsive power of heat, form vapour, may be likewise de¬
tached by electrical powers, and that they may produce
luminous appearances in a vacuum, free from all other
matter, by the annihilation of their opposite electrical state.
“ In common cases of electrical action, the quantity of
the heat generated by the annihilation of the different
states depends upon the nature of the matter on which it
acts ; and in cases where electrical sparksare taken in fluids,
vapour or gas is always generated; and in elastic fluids,
the intensity of the light is always greater the denser the
medium.”2
About the same time that Sir H. Davy was occupied Discoveries
with these researches, Dr Fusinieri was engaged in those°.f Pr.*'u'
beautiful experiments on electric light which have addedsinieri*
so greatly to our knowledge of its nature and origin. The
results of these experiments, which seem to have been
commenced in 1821, were published in successive years.
In 1825 there appeared, in the Journal of Pavia, a most
interesting communication, of which the following is a
brief abstract, relative to the transport of ponderable mat¬
ter in the electrical discharges of ordinary machines ; and,
in 1831, another of equal importance on the transport of
ponderable substances by lightning.
Dr Fusinieri has proved that the electric spark which
issues from a brass conductor, and traverses air, contains
brass in the state of fusion, and incandescent molecules
of zinc.
When the spark issues from a globe of silver, it contains
in its passage through air silver in fusion, and incandes¬
cent molecules of the same metal.
VOL. VIII.
1 Traite de Physique, tom. ii. p. 463.
* Phil. Trans. 1822, p. 72-73-
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