(1386.), the interval in air fails to be a measure, as yet, of the
insulating or resisting power of the gas in the vessel, yet we may for
present purposes take the mean interval as representing in some degree that
power. On examining these mean intervals as they are given in the third
column (1388.), it will be very evident, that gases, when employed as
dielectrics, have peculiar electrical relations to insulation, and
therefore to induction, very distinct from such as might be supposed to
depend upon their mere physical qualities of specific gravity or pressure.
1395. First, it is clear that at the _same pressure_ they are not alike,
the difference being as great as 37 and 110. When the small balls are
charged positively, and with the same surfaces and the same pressure,
muriatic acid gas has three times the insulating or restraining power
(1362.) of hydrogen gas, and nearly twice that of oxygen, nitrogen, or air.
1396. Yet it is evident that the difference is not due to specific gravity,
for though hydrogen is the lowest, and therefore lower than oxygen, oxygen
is much beneath nitrogen, or olefiant gas; and carbonic acid gas, though
considerably heavier than olefiant gas or muriatic acid gas, is lower than
either. Oxygen as a heavy, and olefiant as a light gas, are in strong
contrast with each other; and if we may reason of olefiant gas from
Harris's results with air (1365.), then it might be rarefied to two-thirds
its usual density, or to a specific gravity of 9.3 (hydrogen being 1), and
having neither the same density nor pressure as oxygen, would have equal
insulating powers with it, or equal tendency to resist discharge.
1397. Experiments have already been described (1291. 1292.) which show that
the gases are sensibly alike in their inductive capacity. This result is
not in contradiction with the existence of great differences in their
restraining power. The same point has been observed already in regard to
dense and rare air (1375.).
1398. Hence arises a new argument proving that it cannot be mere pressure
of the atmosphere which prevents or governs discharge (1377. 1378.), but a
specific electric quality or relation of the gaseous medium. Hence also
additional argument for the theory of molecular inductive action.
1399. Other specific differences amongst the gases may be drawn from the
preceding series of experiments, rough and hasty as they are. Thus the
positive and negative series of mean intervals do not give the same
differences. It has been already noticed that the negative numbers are
lower than the positive (1393.), but, besides that, the _order_ of the
positive and negative results is not the same. Thus, on comparing the mean
numbers (which represent for the present insulating tension,) it appears
that in air, hydrogen, carbonic acid, olefiant gas and muriatic acid, the
tension rose higher when the smaller ball was made positive than when
rendered negative, whilst in oxygen, nitrogen, and coal gas, the reverse
was the case. Now though the numbers cannot be trusted as exact, and though
air, oxygen, and nitrogen should probably be on the same side, yet some of
the results, as, for instance, those with muriatic acid, fully show a
peculiar relation and difference amongst gases in this respect. This was
further proved by making the interval in air 0.8 of an inch whilst muriatic
acid gas was in the vessel _a_; for on charging the small balls _s_ and S
positively, _all_ the discharge took place through the _air_; but on
charging them negatively, _all_ the discharge took place through the
_muriatic acid gas_.
1400. So also, when the conductor _n_ was connected _only_ with the
muriatic acid gas apparatus, it was found that the discharge was more
facile when the small ball _s_ was negative than when positive; for in the
latter case, much of the electricity passed off as brush discharge through
the air from the connecting wire _p_ but in the former case, it all seemed
to go through the muriatic acid.
1401. The consideration, however, of positive and negative discharge across
air and other gases will be resumed in the further part of this, or in the
next paper (1465. 1525.).
1402. Here for the present I must leave this part of the subject, which had
for its object only to observe how far gases agreed or differed as to their
power of retaining a charge on bodies acting by induction through them. All
the results conspire to show that Induction is an action of contiguous
molecules (1295. &c.); but besides confirming this, the first principle
placed for proof in the present inquiry, they greatly assist in developing
the specific properties of each gaseous dielectric, at the same time
showing that further and extensive experimental investigation is necessary,
and holding out the promise of new discovery as the reward of the labour
required.
* * * * *
1403. When we pass from the consideration of dielectrics like the gases to
that of bodies having the liquid and solid condition, then our reasonings
in the present state of the subject assume much more of the character of
mere supposition. Still I do not perceive anything adverse to the theory,
in the phenomena which such bodies present. If we take three insulating
dielectrics, as air, oil of turpentine, and shell-lac, and use the same
balls or conductors at the same intervals in these three substances,
increasing the intensity of the induction until discharge take place, we
shall find that it must be raised much higher in the fluid than for the
gas, and higher still in the solid than for the fluid. Nor is this
inconsistent with the theory; for with the liquid, though its molecules are
free to move almost as easily as those of the gas, there are many more
particles introduced into the given interval; and such is also the case
when the solid body is employed. Besides that with the solid, the cohesive
force of the body used will produce some effect; for though the production
of the polarized states in the particle of a solid may not be obstructed,
but, on the contrary, may in some cases be even favoured (1164. 1344.) by
its solidity or other circumstances, yet solidity may well exert an
influence on the point of final subversion, (just as it prevents discharge
in an electrolyte,) and so enable inductive intensity to rise to a much
higher degree.
1404. In the cases of solids and liquids too, bodies may, and most probably
do, possess specific differences as to their ability of assuming the
polarized state, and also as to the extent to which that polarity must rise
before discharge occurs. An analogous difference exists in the specific
inductive capacities already pointed out in a few substances (1278.) in the
last paper. Such a difference might even account for the various degrees of
insulating and conducting power possessed by different bodies, and, if it
should be found to exist, would add further strength to the argument in
favour of the molecular theory of inductive action.
* * * * *
1405. Having considered these various cases of sustained insulation in
non-conducting dielectrics up to the highest point which they can attain,
we find that they terminate at last in _disruptive discharge_; the peculiar
condition of the molecules of the dielectric which was necessary to the
continuous induction, being equally essential to the occurrence of that
effect which closes all the phenomena. This discharge is not only in its
appearance and condition different to the former modes by which the
lowering of the powers was effected (1320. 1343.), but, whilst really the
same in principle, varies much from itself in certain characters, and thus
presents us with the forms of _spark_, _brush_, and _glow_ (1359.). I will
first consider _the spark_, limiting it for the present to the case of
discharge between two oppositely electrified conducting surfaces.
_The electric spark or flash._
1406. The _spark_ is consequent upon a discharge or lowering of the
polarized inductive state of many dielectric particles, by a particular
action of a few of the particles occupying a very small and limited space;
all the previously polarized particles returning to their first or normal
condition in the inverse order in which they left it, and uniting their
powers meanwhile to produce, or rather to continue, (1417.--1436.) the
discharge effect in the place where the subversion of force first occurred.
My impression is, that the few particles situated where discharge occurs
are not merely pushed apart, but assume a peculiar state, a highly exulted
condition for the time, i.e. have thrown upon them all the surrounding
forces in succession, and rising up to a proportionate intensity of
condition, perhaps equal to that of chemically combining atoms, discharge
the powers, possibly in the same manner as they do theirs, by some
operation at present unknown to us; and so the end of the whole. The
ultimate effect is exactly as if a metallic wire had been put into the
place of the discharging particles; and it does not seem impossible that
the principles of action in both cases, may, hereafter, prove to be the
same.
1407. The _path of the spark_, or of the discharge, depends on the degree
of tension acquired by the particles in the line of discharge,
circumstances, which in every common case are very evident and by the
theory easy to understand, rendering it higher in them than in their
neighbours, and, by exalting them first to the requisite condition, causing
them to determine the course of the discharge. Hence the selection of the
path, and the solution of the wonder which Harris has so well described[A]
as existing under the old theory. All is prepared amongst the molecules
beforehand, by the prior induction, for the path either of the electric
spark or of lightning itself.
[A] Nautical Magazine, 1834, p 229.
1408. The same difficulty is expressed as a principle by Nobili for voltaic
electricity, almost in Mr. Harris's words, namely[A], "electricity directs
itself towards the point where it can most easily discharge itself," and
the results of this as a principle he has well wrought out for the case of
voltaic currents. But the _solution_ of the difficulty, or the proximate
cause of the effects, is the same; induction brings the particles up to or
towards a certain degree of tension (1370.); and by those which first
attain it, is the discharge first and most efficiently performed.
[A] Bibliothèque Universelle, 1835, lix. 275.
1409. The _moment_ of discharge is probably determined by that molecule of
the dielectric which, from the circumstances, has its tension most quickly
raised up to the maximum intensity. In all cases where the discharge passes
from conductor to conductor this molecule must be on the surface of one of
them; but when it passes between a conductor and a nonconductor, it is,
perhaps, not always so (1453.). When this particle has acquired its maximum
tension, then the whole barrier of resistance is broken down in the line or
lines of inductive action originating at it, and disruptive discharge
occurs (1370.): and such an inference, drawn as it is from the theory,
seems to me in accordance with Mr. Harris's facts and conclusions
respecting the resistance of the atmosphere, namely, that it is not really
greater at any one discharging distance than another[A].
[A] Philosophical Transactions, 1834, pp. 227, 229.
1410. It seems probable, that the tension of a particle of the same
dielectric, as air, which is requisite to produce discharge, is a _constant
quantity_, whatever the shape of the part of the conductor with which it is
in contact, whether ball or point; whatever the thickness or depth of
dielectric throughout which induction is exerted; perhaps, even, whatever
the state, as to rarefaction or condensation of the dielectric; and
whatever the nature of the conductor, good or bad, with which the particle
is for the moment associated. In saying so much, I do not mean to exclude
small differences which may be caused by the reaction of neighbouring
particles on the deciding particle, and indeed, it is evident that the
intensity required in a particle must be related to the condition of those
which are contiguous. But if the expectation should be found to approximate
to truth, what a generality of character it presents! and, in the
definiteness of the power possessed by a particular molecule, may we not
hope to find an immediate relation to the force which, being electrical, is
equally definite and constitutes chemical affinity?
1411. Theoretically it would seem that, at the moment of discharge by the
spark in one line of inductive force, not merely would all the other lines
throw their forces into this one (1406.), but the lateral effect,
equivalent to a repulsion of these lines (1224. 1297.), would be relieved
and, perhaps, followed by a contrary action, amounting to a collapse or
attraction of these parts. Having long sought for some transverse force in
statical electricity, which should be the equivalent to magnetism or the
transverse force of current electricity, and conceiving that it might be
connected with the transverse action of the lines of inductive force,
already described (1297.), I was desirous, by various experiments, of
bringing out the effect of such a force, and making it tell upon the
phenomena of electro-magnetism and magneto-electricity[A].
[A] See further investigations of this subject, 1658-1666.
1709-1735.--_Dec. 1838._
1412. Amongst other results, I expected and sought for the mutual
affection, or even the lateral coalition of two similar sparks, if they
could be obtained simultaneously side by side, and sufficiently near to
each other. For this purpose, two similar Leyden jars were supplied with
rods of copper projecting from their balls in a horizontal direction, the
rods being about 0.2 of an inch thick, and rounded at the ends. The jars
were placed upon a sheet of tinfoil, and so adjusted that their rods, _a_
and _b_, were near together, in the position represented in plan at fig.
116: _c_ and _d_ were two brass balls connected by a brass rod and
insulated: _e_ was also a brass ball connected, by a wire, with the ground
and with the tinfoil upon which the Leyden jars were placed. By laying an
insulated metal rod across from _a_ to _b_, charging the jars, and removing
the rod, both the jars could be brought up to the same intensity of charge
(1370.). Then, making the ball _e_ approach the ball _d_, at the moment the
spark passed there, two sparks passed between the rods _n_, _o_, and the
ball _c_; and as far as the eye could judge, or the conditions determine,
they were simultaneous.
1413. Under these circumstances two modes of discharge took place; either
each end had its own particular spark to the ball, or else one end only was
associated by a spark with the ball, but was at the same time related to
the other end by a spark between the two.
1414. When the ball _c_ was about an inch in diameter, the ends _n_ and
_o_, about half an inch from it, and about 0.4 of an inch from each other,
the two sparks to the ball could be obtained. When for the purpose of
bringing the sparks nearer together, the ends, _n_ and _o_, were brought
closer to each other, then, unless very carefully adjusted, only one end
had a spark with the ball, the other having a spark to it; and the least
variation of position would cause either _n_ or _o_ to be the end which,
giving the direct spark to the ball, was also the one through, or by means
of which, the other discharged its electricity.
1415. On making the ball _c_ smaller, I found that then it was needful to
make the interval between the ends _n_ and _o_ larger in proportion to the
distance between them and the ball _c_. On making _c_ larger, I found I
could diminish the interval, and so bring the two simultaneous separate
sparks closer together, until, at last, the distance between them was not
more at the widest part than 0.6 of their whole length.
1416. Numerous sparks were then passed and carefully observed. They were
very rarely straight, but either curved or bent irregularly. In the average
of cases they were, I think, decidedly convex towards each other; perhaps
two-thirds presented more or less of this effect, the rest bulging more or
less outwards. I was never able, however, to obtain sparks which,
separately leaving the ends of the wires _n_ and _o_, conjoined into one
spark before they reached or communicated with the ball _c_. At present,
therefore, though I think I saw a tendency in the sparks to unite, I cannot
assert it as a fact.
1417. But there is one very interesting effect here, analogous to, and it
may be in part the same with, that I was searching for: I mean the
increased facility of discharge where the spark passes. For instance, in
the cases where one end, as _n_, discharged the electricity of both ends to
the ball _c_, fig. 116, the electricity of the other end _o_, had to pass
through an interval of air 1.5 times as great as that which it might have
taken, by its direct passage between the end and the ball itself. In such
cases, the eye could not distinguish, even by the use of Wheatstone's
means[A], that the spark from the end _n_, which contained both portions of
electricity, was a double spark. It could not have consisted of two sparks
taking separate courses, for such an effect would have been visible to the
eye; but it is just possible, that the spark of the first end _n_ and its
jar, passing at the smallest interval of time before that of the other _o_
had heated and expanded the air in its course, and made it so much more
favourable to discharge, that the electricity of the end _o_ preferred
leaping across to it and taking a very circuitous route, rather than the
more direct one to the ball. It must, however, be remarked, in answer to
this supposition, that the one spark between _d_ and _e_ would, by its
influence, tend to produce simultaneous discharges at _n_ and _o_, and
certainly did so, when no preponderance was given to one wire over the
other, as to the previous inductive effect (1414.).
[A] Philosophical Transactions, 1834, pp. 584, 585.
1418. The fact, however, is, that disruptive discharge is favourable to
itself. It is at the outset a case of tottering equilibrium: and if _time_
be an element in discharge, in however minute a proportion (1436.), then
the commencement of the act at any point favours its continuance and
increase there, and portions of power will be discharged by a course which
they would not otherwise have taken.
1419. The mere heating and expansion of the air itself by the first portion
of electricity which passes, must have a great influence in producing this
result.
1420. As to the result itself, we see its effect in every electric spark;
for it is not the whole quantity which passes that determines the
discharge, but merely that small portion of force which brings the deciding
molecule (1370.) up to its maximum tension; then, when its forces are
subverted and discharge begins, all the rest passes by the same course,
from the influence of the favouring circumstances just referred to; and
whether it be the electricity on a square inch, or a thousand square inches
of charged glass, the discharge is complete. Hereafter we shall find the
influence of this effect in the formation of brushes (1435.); and it is not
impossible that we may trace it producing the jagged spark and the forked
lightning.
* * * * *
1421. The characters of the electric spark in _different gases_ vary, and
the variation _may_ be due simply to the effect of the heat evolved at the
moment. But it may also be due to that specific relation of the particles
and the electric forces which I have assumed as the basis of a theory of
induction; the facts do not oppose such a view; and in that view the
variation strengthens the argument for molecular action, as it would seem
to show the influence of the latter in every part of the electrical effect
(1423. 1454.).
1422. The appearances of the sparks in different gases have often been
observed and recorded[A], but I think it not out of place to notice briefly
the following results; they were obtained with balls of brass, (platina
surfaces would have been better,) and at common pressures. In _air_, the
sparks have that intense light and bluish colour which are so well known,
and often have faint or dark parts in their course, when the quantity of
electricity passing is not great. In _nitrogen_, they are very beautiful,
having the same general appearance as in air, but have decidedly more
colour of a bluish or purple character, and I thought were remarkably
sonorous. In _oxygen_, the sparks were whiter than in air or nitrogen, and
I think not so brilliant. In _hydrogen_, they had a very fine crimson
colour, not due to its rarity, for the character passed away as the
atmosphere was rarefied (1459.)[B]. Very little sound was produced in this
gas; but that is a consequence of its physical condition[C]. In _carbonic
acid gas_, the colour was similar to that of the spark in air, but with a
little green in it: the sparks were remarkably irregular in form, more so
than in common air: they could also, under similar circumstances as to size
of ball, &c., be obtained much longer than in air, the gas showing a
singular readiness to cause the discharge in the form of spark. In
_muriatic acid gas_, the spark was nearly white: it was always bright
throughout, never presenting those dark parts which happen in air,
nitrogen, and some other gases. The gas was dry, and during the whole
experiment the surface of the glass globe within remained quite dry and
bright. In _coal gas_, the spark was sometimes green, sometimes red, and
occasionally one part was green and another red: black parts also occur
very suddenly in the line of the spark, i.e. they are not connected by any
dull part with bright portions, but the two seem to join directly one with
the other.
[A] See Van Marum's description of the Teylerian machine, vol. i. p.
112, and vol. ii. p. 196; also Ency. Britan., vol. vi., Article
Electricity, pp. 505, 507.
[B] Van Marum says they are about four times as large in hydrogen as
in air. vol. i. p. 122.
[C] Leslie. Cambridge Phil. Transactions, 267.
1423. These varieties of character impress my mind with a feeling, that
they are due to a direct relation of the electric powers to the particles
of the dielectric through which the discharge occurs, and are not the mere
results of a casual ignition or a secondary kind of action of the
electricity, upon the particles which it finds in its course and thrusts
aside in its passage (1454.).
1424. The spark may be obtained in media which are far denser than air, as
in oil of turpentine, olive oil, resin, glass, &c.: it may also be obtained
in bodies which being denser likewise approximate to the condition of
conductors, as spermaceti, water, &c. But in these cases, nothing occurs
which, as far as I can perceive, is at all hostile to the general views I
have endeavoured to advocate.
_The electrical brush._
1425. The _brush_ is the next form of disruptive discharge which I shall
consider. There are many ways of obtaining it, or rather of exalting its
characters; and all these ways illustrate the principles upon which it is
produced. If an insulated conductor, connected with the positive conductor
of an electrical machine, have a metal rod 0.3 of an inch in diameter
projecting from it outwards from the machine, and terminating by a rounded
end or a small ball, it will generally give good brushes; or, if the
machine be not in good action, then many ways of assisting the formation of
the brush can be resorted to; thus, the hand or any _large_ conducting
surface may be approached towards the termination to increase inductive
force (1374.): or the termination may be smaller and of badly conducting
matter, as wood: or sparks may be taken between the prime conductor of the
machine and the secondary conductor to which the termination giving brushes
belongs: or, which gives to the brushes exceedingly fine characters and
great magnitude, the air around the termination may be rarefied more or
less, either by heat or the air-pump; the former favourable circumstances
being also continued.
1426. The brush when obtained by a powerful machine on a ball about 0.7 of
an inch in diameter, at the end of a long brass rod attached to the
positive prime conductor, had the general appearance as to form represented
in fig. 117: a short conical bright part or root appeared at the middle
part of the ball projecting directly from it, which, at a little distance
from the ball, broke out suddenly into a wide brush of pale ramifications
having a quivering motion, and being accompanied at the same time with a
low dull chattering sound.
1427. At first the brush seems continuous, but Professor Wheatstone has
shown that the whole phenomenon consists of successive intermitting
discharges[A]. If the eye be passed rapidly, not by a motion of the head,
but of the eyeball itself, across the direction of the brush, by first
looking steadfastly about 10° or 15° above, and then instantly as much
below it, the general brush will be resolved into a number of individual
brushes, standing in a row upon the line which the eye passed over; each
elementary brush being the result of a single discharge, and the space
between them representing both the time during which the eye was passing
over that space, and that which elapsed between one discharge and another.
[A] Philosophical Transactions, 1834, p. 586.
1428. The single brushes could easily be separated to eight or ten times
their own width, but were not at the same time extended, i.e. they did not
become more indefinite in shape, but, on the contrary, less so, each being
more distinct in form, ramification, and character, because of its
separation from the others, in its effects upon the eye. Each, therefore,
was instantaneous in its existence (1436.). Each had the conical root
complete (1426.).
1429. On using a smaller ball, the general brush was smaller, and the
sound, though weaker, more continuous. On resolving the brush into its
elementary parts, as before, these were found to occur at much shorter
intervals of time than in the former case, but still the discharge was
intermitting.
1430. Employing a wire with a round end, the brush was still smaller, but,
as before, separable into successive discharges. The sound, though feebler,
was higher in pitch, being a distinct musical note.
1431. The sound is, in fact, due to the recurrence of the noise of each
separate discharge, and these, happening at intervals nearly equal under
ordinary circumstances, cause a definite note to be heard, which, rising in
pitch with the increased rapidity and regularity of the intermitting
discharges, gives a ready and accurate measure of the intervals, and so may
be used in any case when the discharge is heard, even though the
appearances may not be seen, to determine the element of _time_. So when,
by bringing the hand towards a projecting rod or ball, the pitch of the
tone produced by a brushy discharge increases, the effect informs us that
we have increased the induction (1374.), and by that means increased the
rapidity of the alternations of charge and discharge.
1432. By using wires with finer terminations, smaller brushes were
obtained, until they could hardly be distinguished as brushes; but as long
as _sound_ was heard, the discharge could be ascertained by the eye to be
intermitting; and when the sound ceased, the light became _continuous_ as a
glow (1359. 1405. 1526-1543.).
1433. To those not accustomed to use the eye in the manner I have
described, or, in cases where the recurrence is too quick for any
unassisted eye, the beautiful revolving mirror of Professor Wheatstone[A]
will be useful for such developments of condition as those mentioned above.
Another excellent process is to produce the brush or other luminous
phenomenon on the end of a rod held in the hand opposite to a charged
positive or negative conductor, and then move the rod rapidly from side to
side whilst the eye remains still. The successive discharges occur of
course in different places, and the state of things before, at, and after a
single coruscation or brush can be exceedingly well separated.
[A] Philosophical Transactions, 1834, pp. 581, 585.
1434. The _brush_ is in reality a discharge between a bad or a
non-conductor and either a conductor or another non-conductor. Under common
circumstances, the brush is a discharge between a conductor and air, and I
conceive it to take place in something like the following manner. When the
end of an electrified rod projects into the middle of a room, induction
takes place between it and the walls of the room, across the dielectric,
air; and the lines of inductive force accumulate upon the end in greater
quantity than elsewhere, or the particles of air at the end of the rod are
more highly polarized than those at any other part of the rod, for the
reasons already given (1374.). The particles of air situated in sections
across these lines of force are least polarized in the sections towards the
walls and most polarized in those nearer to the end of the wires (1369.):
thus, it may well happen, that a particle at the end of the wire is at a
tension that will immediately terminate in discharge, whilst in those even
only a few inches off, the tension is still beneath that point. But suppose
the rod to be charged positively, a particle of air A, fig. 118, next it,
being polarized, and having of course its negative force directed towards
the rod and its positive force outwards; the instant that discharge takes
place between the positive force of the particle of the rod opposite the
air and the negative force of the particle of air towards the rod, the
whole particle of air becomes positively electrified; and when, the next
instant, the discharged part of the rod resumes its positive state by
conduction from the surface of metal behind, it not only acts on the
particles beyond A, by throwing A into a polarized state again, but A
itself, because of its charged state, exerts a distinct inductive act
towards these further particles, and the tension is consequently so much
exalted between A and B, that discharge takes place there also, as well as
again between the metal and A.
1435. In addition to this effect, it has been shown, that, the act of
discharge having once commenced, the whole operation, like a case of
unstable equilibrium, is hastened to a conclusion (1370. 1418.), the rest
of the act being facilitated in its occurrence, and other electricity than
that which caused the first necessary tension hurrying to the spot. When,
therefore, disruptive discharge has once commenced at the root of a brush,
the electric force which has been accumulating in the conductor attached to
the rod, finds a more ready discharge there than elsewhere, and will at
once follow the course marked out as it were for it, thus leaving the
conductor in a partially discharged state, and the air about the end of the
wire in a charged condition; and the time necessary for restoring the full
charge of the conductor, and the dispersion of the charged air in a greater
or smaller degree, by the joint forces of repulsion from the conductor and
attraction towards the walls of the room, to which its inductive action is
directed, is just that time which forms the interval between brush and
brush (1420. 1427. 1431. 1447.).
1436. The words of this description are long, but there is nothing in the
act or the forces on which it depends to prevent the discharge being
_instantaneous_, as far as we can estimate and measure it. The
consideration of _time_ is, however, important in several points of view
(1418.), and in reference to disruptive discharge, it seemed from theory
far more probable that it might be detected in a brush than in a spark; for
in a brush, the particles in the line through which the discharge passes
are in very different states as to intensity, and the discharge is already
complete in its act at the root of the brush, before the particles at the
extremity of the ramifications have yet attained their maximum intensity.
1437. I consider _brush_ discharge as probably a successive effect in this
way. Discharge begins at the root (1426. 1553.), and, extending itself in
succession to all parts of the single brush, continues to go on at the root
and the previously formed parts until the whole brush is complete; then, by
the fall in intensity and power at the conductor, it ceases at once in all
parts, to be renewed, when that power has risen again to a sufficient
degree. But in a _spark_, the particles in the line of discharge being,
from the circumstances, nearly alike in their intensity of polarization,
suffer discharge so nearly at the same moment as to make the time quite
insensible to us.
1438. Mr. Wheatstone has already made experiments which fully illustrate
this point. He found that the brush generally had a sensible duration, but
that with his highest capabilities he could not detect any such effect in
the spark[A]. I repeated his experiment on the brush, though with more
imperfect means, to ascertain whether I could distinguish a longer duration
in the stem or root of the brush than in the extremities, and the
appearances were such as to make me think an effect of this kind was
produced.
[A] Philosophical Transactions, 1836, pp. 586, 590.
1439. That the discharge breaks into several ramifications, and by them
passes through portions of air alike, or nearly alike, as to polarization
and the degree of tension the particles there have acquired, is a very
natural result of the previous state of things, and rather to be expected
than that the discharge should continue to go straight out into space in a
single line amongst those particles which, being at a distance from the end
of the rod, are in a lower state of tension than those which are near: and
whilst we cannot but conclude, that those parts where the branches of a
single brush appear, are more favourably circumstanced for discharge than
the darker parts between the ramifications, we may also conclude, that in
those parts where the light of concomitant discharge is equal, there the
circumstances are nearly equal also. The single successive brushes are by
no means of the same particular shape even when they are observed without
displacement of the rod or surrounding objects (1427. 1433.), and the
successive discharges may be considered as taking place into the mass of
air around, through different roads at each brush, according as minute
circumstances, such as dust, &c. (1391. 1392.), may have favoured the
course by one set of particles rather than another.
1440. Brush discharge does not essentially require any current of the
medium in which the brush appears: the current almost always occurs, but is
a consequence of the brush, and will be considered hereafter (1562-1610.).
On holding a blunt point positively charged towards uninsulated water, a
star or glow appeared on the point, a current of air passed from it, and
the surface of the water was depressed; but on bringing the point so near
that sonorous brushes passed, then the current of air instantly ceased, and
the surface of the water became level.
1441. The discharge by a brush is not to all the particles of air that are
near the electrified conductor from which the brush issues; only those
parts where the ramifications pass are electrified: the air in the central
dark parts between them receives no charge, and, in fact, at the time of
discharge, has its electric and inductive tension considerably lowered. For
consider fig. 128 to represent a single positive brush;--the induction
before the discharge is from the end of the rod outwards, in diverging
lines towards the distant conductors, as the walls of the room, &c., and a
particle at _a_ has polarity of a certain degree of tension, and tends with
a certain force to become charged; but at the moment of discharge, the air
in the ramifications _b_ and _d_, acquiring also a positive state, opposes
its influence to that of the positive conductor on _a_, and the tension of
the particle at _a_ is therefore diminished rather than increased. The
charged particles at _b_ and _d_ are now inductive bodies, but their lines
of inductive action are still outwards towards the walls of the room; the
direction of the polarity and the tendency of other particles to charge
from these, being governed by, or in conformity with, these lines of force.
1442. The particles that are charged are probably very highly charged, but,
the medium being a non-conductor, they cannot communicate that state to
their neighbours. They travel, therefore, under the influence of the
repulsive and attractive forces, from the charged conductor towards the
nearest uninsulated conductor, or the nearest body in a different state to
themselves, just as charged particles of dust would travel, and are then
discharged; each particle acting, in its course, as a centre of inductive
force upon any bodies near which it may come. The travelling of these
charged particles when they are numerous, causes wind and currents, but
these will come into consideration under _carrying discharge_ (1319. 1562.
&c.).
1443. When air is said to be electrified, and it frequently assumes this
state near electrical machines, it consists, according to my view, of a
mixture of electrified and unelectrified particles, the latter being in
very large proportion to the former. When we gather electricity from air,
by a flame or by wires, it is either by the actual discharge of these
particles, or by effects dependent on their inductive action, a case of
either kind being produceable at pleasure. That the law of equality between
the two forces or forms of force in inductive action is as strictly
preserved in these as in other cases, is fully shown by the fact, formerly
stated (1173. 1174.), that, however strongly air in a vessel might be
charged positively, there was an exactly equal amount of negative force on
the inner surface of the vessel itself, for no residual portion of either
the one or the other electricity could be obtained.
1444. I have nowhere said, nor does it follow, that the air is charged only
where the luminous brush appears. The charging may extend beyond those
parts which are visible, i.e. particles to the right or left of the lines
of light may receive electricity, the parts which are luminous being so
only because much electricity is passing by them to other parts (1437.);
just as in a spark discharge the light is greater as more electricity
passes, though it has no necessary relation to the quantity required to
commence discharge (1370. 1420.). Hence the form we see in a brush may by
no means represent the whole quantity of air electrified; for an invisible
portion, clothing the visible form to a certain depth, may, at the same
time, receive its charge (1552.).
1445. Several effects which I have met with in muriatic acid gas tend to
make me believe, that that gaseous body allows of a dark discharge. At the
same time, it is quite clear from theory, that in some gases, the reverse
of this may occur, i.e. that the charging of the air may not extend even so
far as the light. We do not know as yet enough of the electric light to be
able to state on what it depends, and it is very possible that, when
electricity bursts forth into air, all the particles of which are in a
state of tension, light may be evolved by such as, being very near to, are
not of, those which actually receive a charge at the time.
1446. The further a brush extends in a gas, the further no doubt is the
charge or discharge carried forward; but this may vary between different
gases, and yet the intensity required for the first moment of discharge not
vary in the same, but in some other proportion. Thus with respect to
nitrogen and muriatic acid gases, the former, as far as my experiments have
proceeded, produces far finer and larger brushes than the latter (1458.
1462.), but the intensity required to commence discharge is much higher for
the muriatic acid than the nitrogen (1395.). Here again, therefore, as in
many other qualities, specific differences are presented by different
gaseous dielectrics, and so prove the special relation of the latter to the
act and the phenomena of induction.
1447. To sum up these considerations respecting the character and condition
of the brush, I may state that it is a spark to air; a diffusion of
electric force to matter, not by conduction, but disruptive discharge, a
dilute spark which, passing to very badly conducting matter, frequently
discharges but a small portion of the power stored up in the conductor; for
as the air charged reacts on the conductor, whilst the conductor, by loss
of electricity, sinks in its force (1435.), the discharge quickly ceases,
until by the dispersion of the charged air and the renewal of the excited
conditions of the conductor, circumstances have risen up to their first
effective condition, again to cause discharge, and again to fall and rise,
1448. The brush and spark gradually pass into one another, Making a small
ball positive by a good electrical machine with a large prime conductor,
and approaching a large uninsulated discharging ball towards it, very
beautiful variations from the spark to the brush may be obtained. The
drawings of long and powerful sparks, given by Van Marum[A], Harris[B], and
others, also indicate the same phenomena. As far as I have observed,
whenever the spark has been brushy in air of common pressures, the whole of
the electricity has not been discharged, but only portions of it, more or
less according to circumstances; whereas, whenever the effect has been a
distinct spark throughout the whole of its course, the discharge has been
perfect, provided no interruption had been made to it elsewhere, in the
discharging circuit, than where the spark occurred.
[A] Description of the Teylerian machine, vol. i. pp. 28. 32.; vol.
ii. p. 226, &c.
[B] Philosophical Transactions, 1834, p. 213.
1449. When an electrical brush from an inch to six inches in length or more
is issuing into free air, it has the form given, fig. 117. But if the hand,
a ball, of any knobbed conductor be brought near, the extremities of the
coruscations turn towards it and each other, and the whole assumes various
forms according to circumstances, as in figs. 119, 120, and 121. The
influence of the circumstances in each case is easily traced, and I might
describe it here, but that I should be ashamed to occupy the time of the
Society in things so evident. But how beautifully does the curvature of the
ramifications illustrate the curved form of the lines of inductive force
existing previous to the discharge! for the former are consequences of the
latter, and take their course, in each discharge, where the previous
inductive tension had been raised to the proper degree. They represent
these curves just as well as iron filings represent magnetic curves, the
visible effects in both cases being the consequences of the action of the
forces in _the places where_ the effects appear. The phenomena, therefore,
constitute additional and powerful testimony (1216. 1230.) to that already
given in favour both of induction through dielectrics in curved lines
(1231.), and of the lateral relation of these lines, by an effect
equivalent to a repulsion producing divergence, or, as in the cases
figured, the bulging form.
1450. In reference to the theory of molecular inductive action, I may also
add, the proof deducible from the long brushy ramifying spark which, may be
obtained between a small ball on the positive conductor of an electrical
machine, and a larger one at a distance (1448. 1504.). What a fine
illustration that spark affords of the previous condition of _all_ the
particles of the dielectric between the surfaces of discharge, and how
unlike the appearances are to any which would be deduced from the theory
which assumes inductive action to be action at a distance, in straight
lines only; and charge, as being electricity retained upon the surface of
conductors by the mere pressure of the atmosphere!
* * * * *
1451. When the brush is obtained in rarefied air, the appearances vary
greatly, according to circumstances, and are exceedingly beautiful.
Sometimes a brush may be formed of only six or seven branches, these being
broad and highly luminous, of a purple colour, and in some parts an inch or
more apart: by a spark discharge at the prime conductor (1455.) single
brushes may be obtained at pleasure. Discharge in the form of a brush is
favoured by rarefaction of the air, in the same manner and for the same
reason as discharge in the form of a spark (1375.); but in every case there
is previous induction and charge through the dielectric, and polarity of
its particles (1437.), the induction being, as in any other instance,
alternately raised by the machine and lowered by the discharge. In certain
experiments the rarefaction was increased to the utmost degree, and the
opposed conducting surfaces brought as near together as possible without
producing glow (1529.): the brushes then contracted in their lateral
dimensions, and recurred so rapidly as to form an apparently continuous arc
of light from metal to metal. Still the discharge could be observed to
intermit (1427.), so that even under these high conditions, induction
preceded each single brush, and the tense polarized condition of the
contiguous particles was a necessary preparation for the discharge itself.
1452. The brush form of disruptive discharge may be obtained not only in
air and gases, but also in much denser media. I procured it in _oil of
turpentine_ from the end of a wire going through a glass tube into the
fluid contained in a metal vessel. The brush was small and very difficult
to obtain; the ramifications were simple, and stretched out from each
other, diverging very much. The light was exceedingly feeble, a perfectly
dark room being required for its observation. When a few solid particles,
as of dust or silk, were in the liquid, the brush was produced with much
greater facility.
1453. The running together or coalescence of different lines of discharge
(1412.) is very beautifully shown in the brush in air. This point may
present a little difficulty to those who are not accustomed to see in every
discharge an equal exertion of power in opposite directions, a positive
brush being considered by such (perhaps in consequence of the common phrase
_direction of a current_) as indicating a breaking forth in different
directions of the original force, rather than a tendency to convergence and
union in one line of passage. But the ordinary case of the brush may be
compared, for its illustration, with that in which, by holding the knuckle
opposite to highly excited glass, a discharge occurs, the ramifications of
a brush then leading from the glass and converging into a spark on the
knuckle. Though a difficult experiment to make, it is possible to obtain
discharge between highly excited shell-lac and the excited glass of a
machine: when the discharge passes, it is, from the nature of the charged
bodies, brush at each end and spark in the middle, beautifully illustrating
that tendency of discharge to facilitate like action, which I have
described in a former page (1418.).
1454. The brush has _specific characters_ in different gases, indicating a
relation to the particles of these bodies even in a stronger degree than
the spark (1422. 1423.). This effect is in strong contrast with the
non-variation caused by the use of different substances as _conductors_
from which the brushes are to originate. Thus, using such bodies as wood,
card, charcoal, nitre, citric acid, oxalic acid, oxide of lead, chloride of
lead, carbonate of potassa, potassa fusa, strong solution of potash, oil of
vitriol, sulphur, sulphuret of antimony, and hæmatite, no variation in the
character of the brushes was obtained, except that (dependent upon their
effect as better or worse conductors) of causing discharge with more or
less readiness and quickness from the machine[A].
[A] Exception must, of course, be made of those cases where the root
of the brush, becoming a spark, causes a little diffusion or even
decomposition of the matter there, and so gains more or less of a
particular colour at that part.
1455. The following are a few of the effects I observed in different gasses
at the positively charged surfaces, and with atmospheres varying in their
pressure. The general effect of rarefaction was the same for all the gases:
at first, sparks passed; these gradually were converted into brushes, which
became larger and more distinct in their ramifications, until, upon further
rarefaction, the latter began to collapse and draw in upon each other, till
they formed a stream across from conductor to conductor: then a few lateral
streams shot out towards the glass of the vessel from the conductors; these
became thick and soft in appearance, and were succeeded by the full
constant glow which covered the discharging wire. The phenomena varied with
the size of the vessel (1477.), the degree of rarefaction, and the
discharge of electricity from the machine. When the latter was in
successive sparks, they were most beautiful, the effect of a spark from a
small machine being equal to, and often surpassing, that produced by the
_constant_ discharge of a far more powerful one.
1456. _Air._--Fine positive brushes are easily obtained in air at common
pressures, and possess the well-known purplish light. When the air is
rarefied, the ramifications are very long, filling the globe (1477.); the
light is greatly increased, and is of a beautiful purple colour, with an
occasional rose tint in it.
1457. _Oxygen._--At common pressures, the brush is very close and
compressed, and of a dull whitish colour. In rarefied oxygen, the form and
appearance are better, the colour somewhat purplish, but all the characters
very poor compared to those in air.
1458. _Nitrogen_ gives brushes with great facility at the positive surface,
far beyond any other gas I have tried: they are almost always fine in form,
light, and colour, and in rarefied nitrogen, are magnificent. They surpass
the discharges in any other gas as to the quantity of light evolved.
1459. _Hydrogen_, at common pressures, gave a better brush than oxygen, but
did not equal nitrogen; the colour was greenish gray. In rarefied hydrogen,
the ramifications were very fine in form and distinctness, but pale in
colour, with a soft and velvety appearance, and not at all equal to those
in nitrogen. In the rarest state of the gas, the colour of the light was a
pale gray green.
1460. _Coal gas._--The brushes were rather difficult to produce, the
contrast with nitrogen being great in this respect. They were short and
strong, generally of a greenish colour, and possessing much of the spark
character: for, occurring on both the positive and negative terminations,
often when there was a dark interval of some length between the two
brushes, still the quick, sharp sound of the spark was produced, as if the
discharge had been sudden through this gas, and partaking, in that respect,
of the character of a spark. In rare coal gas, the brush forms were better,
but the light very poor and the colour gray.
1461. _Carbonic acid gas_ produces a very poor brush at common pressures,
as regards either size, light, or colour; and this is probably connected
with the tendency which this gas has to discharge the electricity as a
spark (1422.). In rarefied carbonic acid, the brush is better in form, but
weak as to light, being of a dull greenish or purplish line, varying with
the pressure and other circumstances.
1462. _Muriatic acid gas._--It is very difficult to obtain the brush in
this gas at common pressures. On gradually increasing the distance of the
rounded ends, the sparks suddenly ceased when the interval was about an
inch, and the discharge, which was still through the gas in the globe, was
silent and dark. Occasionally a very short brush could for a few moments be
obtained, but it quickly disappeared. Even when the intermitting spark
current (1455.) from the machine was used, still I could only with
difficulty obtain a brush, and that very short, though I used rods with
rounded terminations (about 0.25 of an inch in diameter) which had before
given them most freely in air and nitrogen. During the time of this
difficulty with the muriatic gas, magnificent brushes were passing off from
different parts of the machine into the surrounding air. On rarefying the
gas, the formation of the brush was facilitated, but it was generally of a
low squat form, very poor in light, and very similar on both the positive
and negative surfaces. On rarefying the gas still more, a few large
ramifications were obtained of a pale bluish colour, utterly unlike those
in nitrogen.
* * * * *
1463. In all the gases, the different forms of disruptive discharge may be
linked together and gradually traced from one extreme to the other, i.e.
from the spark to the glow (1405. 1526.), or, it may be, to a still further
condition to be called dark discharge (1544-1560.); but it is,
nevertheless, very surprising to see what a specific character each keeps
whilst under the predominance of the general law. Thus, in muriatic acid,
the brush is very difficult to obtain, and there comes in its place almost
a dark discharge, partaking of the readiness of the spark action. Moreover,
in muriatic acid, I have _never_ observed the spark with any dark interval
in it. In nitrogen, the spark readily changes its character into that of
brush. In carbonic acid gas, there seems to be a facility to occasion spark
discharge, whilst yet that gas is unlike nitrogen in the facility of the
latter to form brushes, and unlike muriatic acid in its own facility to
continue the spark. These differences add further force, first to the
observations already made respecting the spark in various gases (1422.
1423.), and then, to the proofs deducible from it, of the relation of the
electrical forces to the particles of matter.
1464. The peculiar characters of nitrogen in relation to the electric
discharge (1422. 1458.) must, evidently, have an important influence over
the form and even the occurrence of lightning. Being that gas which most
readily produces coruscations, and, by them, extends discharge to a greater
distance than any other gas tried, it is also that which constitutes
four-fifths of our atmosphere; and as, in atmospheric electrical phenomena,
one, and sometimes both the inductive forces are resident on the particles
of the air, which, though probably affected as to conducting power by the
aqueous particles in it, cannot be considered as a good conductor; so the
peculiar power possessed by nitrogen, to originate and effect discharge in
the form of a brush or of ramifications, has, probably, an important
relation to its electrical service in nature, as it most seriously affects
the character and condition of the discharge when made. The whole subject
of discharge from and through gases is of great interest, and, if only in
reference to atmospheric electricity, deserves extensive and close
experimental investigation.
_Difference of discharge at the positive and negative conducting surfaces._
1465. I have avoided speaking of this well-known phenomenon more than was
quite necessary, that I might bring together here what I have to say on the
subject. When the brush discharge is observed in air at the positive and
negative surfaces, there is a very remarkable difference, the true and full
comprehension of which would, no doubt, be of the utmost importance to the
physics of electricity; it would throw great light on our present subject,
i.e. the molecular action of dielectrics under induction, and its
consequences; and seems very open to, and accessible by, experimental
inquiry.
1466. The difference in question used to be expressed in former times by
saying, that a point charged positively gave brushes into the air, whilst
the same point charged negatively gave a star. This is true only of bad
conductors, or of metallic conductors charged intermittingly, or otherwise
controlled by collateral induction. If metallic points project _freely_
into the air, the positive and negative light upon them differ very little
in appearance, and the difference can be observed only upon close
examination.
1467. The effect varies exceedingly under different circumstances, but, as
we must set out from some position, may perhaps be stated thus: if a
metallic wire with a rounded termination in free air be used to produce the
brushy discharge, then the brushes obtained when the wire is charged
negatively are very poor and small, by comparison with those produced when
the charge is positive. Or if a large metal ball connected with the
electrical machine be charged _positively_, and a fine uninsulated point be
gradually brought towards it, a star appears on the point when at a
considerable distance, which, though it becomes brighter, does not change
its form of a star until it is close up to the ball: whereas, if the ball
be charged negatively, the point at a considerable distance has a star on
it as before; but when brought nearer, (in my case to the distance of 1-1/2
inch,) a brush formed on it, extending to the negative ball; and when still
nearer, (at 1/8 of an inch distance,) the brush ceased, and bright sparks
passed. These variations, I believe, include the whole series of
differences, and they seem to show at once, that the negative surface tends
to retain its discharging character unchanged, whilst the positive surface,
under similar circumstances, permits of great variation.
1468. There are several points in the character of the negative discharge
to air which it is important to observe. A metal rod, 0.3 of an inch in
diameter, with a rounded end projecting into the air, was charged
negatively, and gave a short noisy brush (fig. 122.). It was ascertained
both by sight (1427. 1433.) and sound (1431.), that the successive
discharges were very rapid in their recurrence, being seven or eight times
more numerous in the same period, than those produced when the rod was
charged positively to an equal degree. When the rod was positive, it was
easy, by working the machine a little quicker, to replace the brush by a
glow (1405. 1463.), but when it was negative no efforts could produce this
change. Even by bringing the hand opposite the wire, the only effect was to
increase the number of brush discharges in a given period, raising at the
same time the sound to a higher pitch.
1469. A point opposite the negative brush exhibited a star, and as it was
approximated caused the size and sound of the negative brush to diminish,
and, at last, to cease, leaving the negative end silent and dark, yet
effective as to discharge.
1470. When the round end of a smaller wire (fig. 123.) was advanced towards
the negative brush, it (becoming positive by induction) exhibited the quiet
glow at 8 inches distance, the negative brush continuing. When nearer, the
pitch of the sound of the negative brush rose, indicating quicker
intermittences (1431.); still nearer, the positive end threw off
ramifications and distinct brushes; at the same time, the negative brush
contracted in its lateral directions and collected together, giving a
peculiar narrow longish brush, in shape like a hair pencil, the two brushes
existing at once, but very different in their form and appearance, and
especially in the more rapid recurrence of the negative discharges than of
the positive. On using a smaller positive wire for the same experiment, the
glow first appeared on it, and then the brush, the negative brush being
affected at the same time; and the two at one distance became exceedingly
alike in appearance, and the sounds, I thought, were in unison; at all
events they were in harmony, so that the intermissions of discharge were
either isochronous, or a simple ratio existed between the intervals. With a
higher action of the machine, the wires being retained unaltered, the
negative surface became dark and silent, and a glow appeared on the
positive one. A still higher action changed the latter into a spark. Finer
positive wires gave other variations of these effects, the description of
which I must not allow myself to go into here.
1471. A thinner rod was now connected with the negative conductor in place
of the larger one (1468.), its termination being gradually diminished to a
blunt point, as in fig. 124; and it was beautiful to observe that,
notwithstanding the variation of the brush, the same general order of
effects was produced. The end gave a small sonorous negative brush, which
the approach of the hand or a large conducting surface did not alter, until
it was so near as to produce a spark. A fine point opposite to it was
luminous at a distance; being nearer it did not destroy the light and sound
of the negative brush, but only tended to have a brush produced on itself,
which, at a still less distance, passed into a spark joining the two
surfaces.
1472. When the distinct negative and positive brushes are produced
simultaneously in relation to each other in air, the former almost always
has a contracted form, as in fig. 125, very much indeed resembling the
figure which the positive brush itself has when influenced by the lateral
vicinity of positive parts acting by induction. Thus a brush issuing from a
point in the re-entering angle of a positive conductor has the same
compressed form (fig. 126.).
1473. The character of the negative brush is not affected by the chemical
nature of the substances of the conductors (1454.), but only by their
possession of the conducting power in a greater or smaller degree.
1474. Rarefaction of common air about a negative ball or blunt point
facilitated the development of the negative brush, the effect being, I
think, greater than on a positive brush, though great on both. Extensive
ramifications could be obtained from a ball or end electrified negatively
to the plate of the air-pump on which the jar containing it stood.
1475. A very important variation of the relative forms and conditions of
the positive and negative brush takes place on varying the dielectric in
which they are produced. The difference is so very great that it points to
a specific relation of this form of discharge to the particular gas in
which it takes place, and opposes the idea that gases are but obstructions
to the discharge, acting one like another and merely in proportion to their
pressure (1377.).
1476. In _air_, the superiority of the positive brush is well known (1467.
1472.). In _nitrogen_, it is as great or even greater than in air (1458.).
In _hydrogen_, the positive brush loses a part of its superiority, not
being so good as in nitrogen or air; whilst the negative brush does not
seem injured (1459.). In _oxygen_, the positive brush is compressed and
poor (1457); whilst the negative did not become less: the two were so alike
that the eye frequently could not tell one from the other, and this
similarity continued when the oxygen was gradually rarefied. In _coal gas_,
the brushes are difficult of production as compared to nitrogen (1460.),
and the positive not much superior to the negative in its character, either
at common or low pressures. In _carbonic acid gas_, this approximation of
character also occurred. In _muriatic acid gas_, the positive brush was
very little better than the negative, and both difficult to produce (1462.)
as compared with the facility in nitrogen or air.
1477. These experiments were made with rods of brass about a quarter of an
inch thick having rounded ends, these being opposed in a glass globe 7
inches in diameter, containing the gas to be experimented with. The
electric machine was used to communicate directly, sometimes the positive,
and sometimes the negative state, to the rod in connection with it.
1478. Thus we see that, notwithstanding there is a general difference in
favour of the superiority of the positive brush over the negative, that
difference is at its maximum in nitrogen and air; whilst in carbonic acid,
muriatic acid, coal gas, and oxygen, it diminishes, and at last almost
disappears. So that in this particular effect, as in all others yet
examined, the evidence is in favour of that view which refers the results
to a direct relation of the electric forces with the molecules of the
matter concerned in the action (1421. 1423. 1463.). Even when special
phenomena arise under the operation of the general law, the theory adopted
seems fully competent to meet the case.
1479. Before I proceed further in tracing the probable cause of the
difference between the positive and negative brush discharge, I wish to
know the results of a few experiments which are in course of preparation:
and thinking this Series of Researches long enough, I shall here close it
with the expectation of being able in a few weeks to renew the inquiry, and
entirely redeem my pledge (1306.).
_Royal Institution,
Dec. 23rd, 1837._