§ 9. _On a new Law of Electric Conduction._ § 10. _On Conducting Power
generally._
Received April 24,--Read May 23, 1833.
§ 9. _On a new Law of Electric Conduction._[A]
[A] In reference to this law see further considerations at 910. 1358.
1705.--_Dec. 1838._
380. It was during the progress of investigations relating to
electro-chemical decomposition, which I still have to submit to the Royal
Society, that I encountered effects due to a very _general law_ of electric
conduction not hitherto recognised; and though they prevented me from
obtaining the condition I sought for, they afforded abundant compensation
for the momentary disappointment, by the new and important interest which
they gave to an extensive part of electrical science.
381. I was working with ice, and the solids resulting from the freezing of
solutions, arranged either as barriers across a substance to be decomposed,
or as the actual poles of a voltaic battery, that I might trace and catch
certain elements in their transit, when I was suddenly stopped in my
progress by finding that ice was in such circumstances a non-conductor of
electricity; and that as soon as a thin film of it was interposed, in the
circuit of a very powerful voltaic battery, the transmission of electricity
was prevented, and all decomposition ceased.
382. At first the experiments were made with common ice, during the cold
freezing weather of the latter end of January 1833; but the results were
fallacious, from the imperfection of the arrangements, and the following
more unexceptionable form of experiment was adopted.
383. Tin vessels were formed, five inches deep, one inch and a quarter wide
in one direction, of different widths from three eighths to five eighths of
an inch in the other, and open at one extremity. Into these were fixed by
corks, plates of platina, so that the latter should not touch the tin
cases; and copper wires having previously been soldered to the plate, these
were easily connected, when required, with a voltaic pile. Then distilled
water, previously boiled for three hours, was poured into the vessels, and
frozen by a mixture of salt and snow, so that pure transparent solid ice
intervened between the platina and tin; and finally these metals were
connected with the opposite extremities of the voltaic apparatus, a
galvanometer being at the same time included in the circuit.
384. In the first experiment, the platina pole was three inches and a half
long, and seven eighths of an inch wide; it was wholly immersed in the
water or ice, and as the vessel was four eighths of an inch in width, the
average thickness of the intervening ice was only a quarter of an inch,
whilst the surface of contact with it at both poles was nearly fourteen
square inches. After the water was frozen, the vessel was still retained in
the frigorific mixture, whilst contact between the tin and platina
respectively was made with the extremities of a well-charged voltaic
battery, consisting of twenty pairs of four-inch plates, each with double
coppers. Not the slightest deflection of the galvanometer needle occurred.
385. On taking the frozen arrangement out of the cold mixture, and applying
warmth to the bottom of the tin case, so as to melt part of the ice, the
connexion with the battery being in the mean time retained, the needle did
not at first move; and it was only when the thawing process had extended so
far as to liquefy part of the ice touching the platina pole, that
conduction took place; but then it occurred effectually, and the
galvanometer needle was permanently deflected nearly 70°.
386. In another experiment, a platina spatula, five inches in length and
seven eighths of an inch in width, had four inches fixed in the ice, and
the latter was only three sixteenths of an inch thick between one metallic
surface and the other; yet this arrangement insulated as perfectly as the
former.
387. Upon pouring a little water in at the top of this vessel on the ice,
still the arrangement did not conduct; yet fluid water was evidently there.
This result was the consequence of the cold metals having frozen the water
where they touched it, and thus insulating the fluid part; and it well
illustrates the non-conducting power of ice, by showing how thin a film
could prevent the transmission of the battery current. Upon thawing parts
of this thin film, at _both_ metals, conduction occurred.
388. Upon warming the tin case and removing the piece of ice, it was found
that a cork having slipped, one of the edges of the platina had been all
but in contact with the inner surface of the tin vessel; yet,
notwithstanding the extreme thinness of the interfering ice in this place,
no sensible portion of electricity had passed.
389. These experiments were repeated many times with the same results. At
last a battery of fifteen troughs, or one hundred and fifty pairs of
four-inch plates, powerfully charged, was used; yet even here no sensible
quantity of electricity passed the thin barrier of ice.
390. It seemed at first as if occasional departures from these effects
occurred; but they could always be traced to some interfering
circumstances. The water should in every instance be well-frozen; for
though it is not necessary that the ice should reach from pole to pole,
since a barrier of it about one pole would be quite sufficient to prevent
conduction, yet, if part remain fluid, the mere necessary exposure of the
apparatus to the air or the approximation of the hands, is sufficient to
produce, at the _upper surface_ of the water and ice, a film of fluid,
extending from the platina to the tin; and then conduction occurs. Again,
if the corks used to block the platina in its place are damp or wet within,
it is necessary that the cold be sufficiently well applied to freeze the
water in them, or else when the surfaces of their contact with the tin
become slightly warm by handling, that part will conduct, and the interior
being ready to conduct also, the current will pass. The water should be
pure, not only that unembarrassed results may be obtained, but also that,
as the freezing proceeds, a minute portion of concentrated saline solution
may not be formed, which remaining fluid, and being interposed in the ice,
or passing into cracks resulting from contraction, may exhibit conducting
powers independent of the ice itself.
391. On one occasion I was surprised to find that after thawing much of the
ice the conducting power had not been restored; but I found that a cork
which held the wire just where it joined the platina, dipped so far into
the ice, that with the ice itself it protected the platina from contact
with the melted part long after that contact was expected.
392. This insulating power of ice is not effective with electricity of
exalted intensity. On touching a diverged gold-leaf electrometer with a
wire connected with the platina, whilst the tin case was touched by the
hand or another wire, the electrometer was instantly discharged (419.).
393. But though electricity of an intensity so low that it cannot diverge
the electrometer, can still pass (though in very limited quantities
(419.),) through ice; the comparative relation of water and ice to the
electricity of the voltaic apparatus is not less extraordinary on that
account, Or less important in its consequences.
394. As it did not seem likely that this _law of the assumption of
conducting power during liquefaction, and loss of it during congelation_,
would be peculiar to water, I immediately proceeded to ascertain its
influence in other cases, and found it to be very general. For this purpose
bodies were chosen which were solid at common temperatures, but readily
fusible; and of such composition as, for other reasons connected with
electrochemical action, led to the conclusion that they would be able when
fused to replace water as conductors. A voltaic battery of two troughs, or
twenty pairs of four-inch plates (384.), was used as the source of
electricity, and a galvanometer introduced into the circuit to indicate the
presence or absence of a current.
395. On fusing a little chloride of lead by a spirit lamp on a fragment of
a Florence flask, and introducing two platina wires connected with the
poles of the battery, there was instantly powerful action, the galvanometer
was most violently affected, and the chloride rapidly decomposed. On
removing the lamp, the instant the chloride solidified all current and
consequent effects ceased, though the platina wires remained inclosed in
the chloride not more than the one-sixteenth of an inch from each other. On
renewing the heat, as soon as the fusion had proceeded far enough to allow
liquid matter to connect the poles, the electrical current instantly
passed.
396. On fusing the chloride, with one wire introduced, and then touching
the liquid with the other, the latter being cold, caused a little knob to
concrete on its extremity, and no current passed; it was only when the wire
became so hot as to be able to admit or allow of contact with the liquid
matter, that conduction took place, and then it was very powerful.
397. When chloride of silver and chlorate of potassa were experimented
with, in a similar manner, exactly the same results occurred.
398. Whenever the current passed in these cases, there was decomposition of
the substances; but the electro-chemical part of this subject I purpose
connecting with more general views in a future paper[A].
[A] In 1801, Sir H. Davy knew that "dry nitre, caustic potash, and
soda are conductors of galvanism when rendered fluid by a high degree
of heat," (Journals of the Royal Institution, 1802, p. 53,) but was
not aware of the general law which I have been engaged in developing.
It is remarkable, that eleven years after that, he should say, "There
are no fluids known except such as contain water, which are capable of
being made the medium of connexion between the metal or metals of the
voltaic apparatus."--Elements of Chemical Philosophy, p. 169.
399. Other substances, which could not be melted on glass, were fused by
the lamp and blowpipe on platina connected with one pole of the battery,
and then a wire, connected with the other, dipped into them. In this way
chloride of sodium, sulphate of soda, protoxide of lead, mixed carbonates
of potash and soda, &c. &c., exhibited exactly the same phenomena as those
already described: whilst liquid, they conducted and were decomposed;
whilst solid, though very hot, they insulated the battery current even when
four troughs were used.
400. Occasionally the substances were contained in small bent tubes of
green glass, and when fused, the platina poles introduced, one on each
side. In such cases the same general results as those already described
were procured; but a further advantage was obtained, namely, that whilst
the substance was conducting and suffering decomposition, the final
arrangement of the elements could be observed. Thus, iodides of potassium
and lead gave iodine at the positive pole, and potassium or lead at the
negative pole. Chlorides of lead and silver gave chlorine at the positive,
and metals at the negative pole. Nitre and chlorate; of potassa gave
oxygen, &c., at the positive, and alkali, or even potassium, at the
negative pole.
[Illustration]
401. A fourth arrangement was used for substances requiring very high
temperatures for their fusion. A platina wire was connected with one pole
of the battery; its extremity bent into a small ring, in the manner
described by Berzelius, for blowpipe experiments; a little of the salt,
glass, or other substance, was melted on this ring by the ordinary
blowpipe, or even in some cases by the oxy-hydrogen blowpipe, and when the
drop, retained in its place by the ring, was thoroughly hot and fluid, a
platina wire from the opposite pole of the battery was made to touch it,
and the effects observed.
402. The following are various substances, taken from very different
classes chemically considered, which are subject to this law. The list
might, no doubt, be enormously extended; but I have not had time to do more
than confirm the law by a sufficient number of instances.
First, _water_.
Amongst _oxides_;--potassa, protoxide of lead, glass of antimony, protoxide
of antimony, oxide of bismuth.
_Chlorides_ of potassium, sodium, barium, strontium, calcium, magnesium,
manganese, zinc, copper (proto-), lead, tin (proto-), antimony, silver.
_Iodides_ of potassium, zinc and lead, protiodide of tin, periodide of
mercury; _fluoride_ of potassium; _cyanide_ of potassium; _sulpho-cyanide_
of potassium.
_Salts._ Chlorate of potassa; nitrates of potassa, soda, baryta, strontia,
lead, copper, and silver; sulphates of soda and lead, proto-sulphate of
mercury; phosphates of potassa, soda, lead, copper, phosphoric glass or
acid phosphate of lime; carbonates of potassa and soda, mingled and
separate; borax, borate of lead, per-borate of tin; chromate of potassa,
bi-chromate of potassa, chromate of lead; acetate of potassa.
_Sulphurets._ Sulphuret of antimony, sulphuret of potassium made by
reducing sulphate of potassa by hydrogen; ordinary sulphuret of potassa.
Silicated potassa; chameleon mineral.
403. It is highly interesting in the instances of those substances which
soften before they liquefy, to observe at what period the conducting power
is acquired, and to what degree it is exalted by perfect fluidity. Thus,
with the borate of lead, when heated by the lamp upon glass, it becomes as
soft as treacle, but it did not conduct, and it was only when urged by the
blowpipe and brought to a fair red heat, that it conducted. When rendered
quite liquid, it conducted with extreme facility.
404. I do not mean to deny that part of the increased conducting power in
these cases of softening was probably due to the elevation of temperature
(432. 445.); but I have no doubt that by far the greater part was due to
the influence of the general law already demonstrated, and which in these
instances came gradually, instead of suddenly, into operation.
405. The following are bodies which acquired no conducting power upon
assuming the liquid state:--
Sulphur, phosphorus; iodide of sulphur, per-iodide of tin; orpiment,
realgar; glacial acetic acid, mixed margaric and oleic acids, artificial
camphor; caffeine, sugar, adipocire, stearine of cocoa-nut oil, spermaceti,
camphor, naphthaline, resin, gum sandarach, shell lac.
406. Perchloride of tin, chloride of arsenic, and the hydrated chloride of
arsenic, being liquids, had no sensible conducting power indicated by the
galvanometer, nor were they decomposed.
407. Some of the above substances are sufficiently remarkable as exceptions
to the general law governing the former cases. These are orpiment, realgar,
acetic acid, artificial camphor, per-iodide of tin, and the chlorides of
tin and arsenic. I shall have occasion to refer to these cases in the paper
on Electro-chemical Decomposition.
408. Boracic acid was raised to the highest possible temperature by an
oxy-hydrogen flame (401.), yet it gained no conducting powers sufficient to
affect the galvanometer, and underwent no apparent voltaic decomposition.
It seemed to be quite as bad a conductor as air. Green bottle-glass, heated
in the same manner, did not gain conducting power sensible to the
galvanometer. Flint glass, when highly heated, did conduct a little and
decompose; and as the proportion of potash or oxide of lead was increased
in the glass, the effects were more powerful. Those glasses, consisting of
boracic acid on the one hand, and oxide of lead or potassa on the other,
show the assumption of conducting power upon fusion and the accompanying
decomposition very well.
409. I was very anxious to try the general experiment with sulphuric acid,
of about specific gravity 1.783, containing that proportion of water which
gives it the power of crystallizing at 40° Fahr.; but I found it impossible
to obtain it so that I could be sure the whole would congeal even at 0°
Fahr. A ten-thousandth part of water, more or less than necessary, would,
upon cooling the whole, cause a portion of uncongealable liquid to
separate, and that remaining in the interstices of the solid mass, and
moistening the planes of division, would prevent the correct observation of
the phenomena due to entire solidification and subsequent liquefaction.
410. With regard to the substances on which conducting power is thus
conferred by liquidity, the degree of power so given is generally very
great. Water is that body in which this acquired power is feeblest. In the
various oxides, chlorides, salts, &c. &c., it is given in a much higher
degree. I have not had time to measure the conducting power in these cases,
but it is apparently some hundred times that of pure water. The increased
conducting power known to be given to water by the addition of salts, would
seem to be in a great degree dependent upon the high conducting power of
these bodies when in the liquid state, that state being given them for the
time, not by heat but solution in the water[A].
[A] See a doubt on this point at 1356.--_Dec. 1838._
411. Whether the conducting power of these liquefied bodies is a
consequence of their decomposition or not (413.), or whether the two
actions of conduction and decomposition are essentially connected or not,
would introduce no difference affecting the probable accuracy of the
preceding statement.
412. This _general assumption of conducting power_ by bodies as soon as
they pass from the solid to the liquid state, offers a new and
extraordinary character, the existence of which, as far as I know, has not
before been suspected; and it seems importantly connected with some
properties and relations of the particles of matter which I may now briefly
point out.
413. In almost all the instances, as yet observed, which are governed by
this law, the substances experimented with have been those which were not
only compound bodies, but such as contain elements known to arrange
themselves at the opposite poles; and were also such as could be
_decomposed_ by the electrical current. When conduction took place,
decomposition occurred; when decomposition ceased, conduction ceased also;
and it becomes a fair and an important question, Whether the conduction
itself may not, wherever the law holds good, be a consequence not merely of
the capability, but of the act of decomposition? And that question may be
accompanied by another, namely, Whether solidification does not prevent
conduction, merely by chaining the particles to their places, under the
influence of aggregation, and preventing their final separation in the
manner necessary for decomposition?
414. But, on the other hand, there is one substance (and others may occur),
the _per-iodide of mercury_, which, being experimented with like the others
(400.), was found to insulate when solid, and to acquire conducting power
when fluid; yet it did not seem to undergo decomposition in the latter
case.
415. Again, there are many substances which contain elements such as would
be expected to arrange themselves at the opposite poles of the pile, and
therefore in that respect fitted for decomposition, which yet do not
conduct. Amongst these are the iodide of sulphur, per-iodide of zinc,
per-chloride of tin, chloride of arsenic, hydrated chloride of arsenic,
acetic acid, orpiment, realgar, artificial camphor, &c.; and from these it
might perhaps be assumed that decomposition is dependent upon conducting
power, and not the latter upon the former. The true relation, however, of
conduction and decomposition in those bodies governed by the general law
which it is the object of this paper to establish, can only be
satisfactorily made out from a far more extensive series of observations
than those I have yet been able to supply[A].
[A] See 673, &c. &c.--_Dec. 1838._
416. The relation, under this law, of the conducting power for electricity
to that for heat, is very remarkable, and seems to imply a natural
dependence of the two. As the solid becomes a fluid, it loses almost
entirely the power of conduction for heat, but gains in a high degree that
for electricity; but as it reverts hack to the solid state, it gains the
power of conducting heat, and loses that of conducting electricity. If,
therefore, the properties are not incompatible, still they are most
strongly contrasted, one being lost as the other is gained. We may hope,
perhaps, hereafter to understand the physical reason of this very
extraordinary relation of the two conducting powers, both of which appear
to be directly connected with the corpuscular condition of the substances
concerned.
417. The assumption of conducting power and a decomposable condition by
liquefaction, promises new opportunities of, and great facilities in,
voltaic decomposition. Thus, such bodies as the oxides, chlorides,
cyanides, sulpho-cyanides, fluorides, certain vitreous mixtures, &c. &c.,
may be submitted to the action of the voltaic battery under new
circumstances; and indeed I have already been able, with ten pairs of
plates, to decompose common salt, chloride of magnesium, borax, &c. &c.,
and to obtain sodium, magnesium, boron, &c., in their separate states.
§ 10. _On Conducting Power generally._[A]
[A] In reference to this § refer to 983 in series viii., and the
results connected with it.--_Dec. 1838._
418. It is not my intention here to enter into an examination of all the
circumstances connected with conducting power, but to record certain facts
and observations which have arisen during recent inquiries, as additions to
the general stock of knowledge relating to this point of electrical
science.
419. I was anxious, in the first place, to obtain some idea of the
conducting power of ice and solid salts for electricity of high tension
(392.), that a comparison might be made between it and the large accession
of the same power gained upon liquefaction. For this purpose the large
electrical machine (290.) was brought into excellent action, its conductor
connected with a delicate gold-leaf electrometer, and also with the platina
inclosed in the ice (383.), whilst the tin case was connected with the
discharging train (292.). On working the machine moderately, the gold
leaves barely separated; on working it rapidly, they could be opened nearly
two inches. In this instance the tin case was five-eighths of an inch in
width; and as, after the experiment, the platina plate was found very
nearly in the middle of the ice, the average thickness of the latter had
been five-sixteenths of an inch, and the extent of surface of contact with
tin and platina fourteen square inches (384.). Yet, under these
circumstances, it was but just able to conduct the small quantity of
electricity which this machine could evolve (371.), even when of a tension
competent to open the leaves two inches; no wonder, therefore, that it
could not conduct any sensible portion of the electricity of the troughs
(384.), which, though almost infinitely surpassing that of the machine in
quantity, had a tension so low as not to be sensible to an electrometer.
420. In another experiment, the tin case was only four-eighths of an inch
in width, and it was found afterwards that the platina had been not quite
one-eighth of an inch distant in the ice from one side of the tin vessel.
When this was introduced into the course of the electricity from the
machine (419.), the gold leaves could be opened, but not more than half an
inch; the thinness of the ice favouring the conduction of the electricity,
and permitting the same quantity to pass in the same time, though of a much
lower tension.
421. Iodide of potassium which had been fused and cooled was introduced
into the course of the electricity from the machine. There were two pieces,
each about a quarter of an inch in thickness, and exposing a surface on
each side equal to about half a square inch; these were placed upon platina
plates, one connected with the machine and electrometer (419.), and the
other with the discharging train, whilst a fine platina wire connected the
two pieces, resting upon them by its two points. On working the electrical
machine, it was possible to open the electrometer leaves about two-thirds
of an inch.
422. As the platina wire touched only by points, the facts show that this
salt is a far better conductor than ice; but as the leaves of the
electrometer opened, it is also evident with what difficulty conduction,
even of the small portion of electricity produced by the machine, is
effected by this body in the solid state, when compared to the facility
with which enormous quantities at very low tensions are transmitted by it
when in the fluid state.
423. In order to confirm these results by others, obtained from the voltaic
apparatus, a battery of one hundred and fifty plates, four inches square,
was well-charged: its action was good; the shock from it strong; the
discharge would _continue_ from copper to copper through four-tenths of an
inch of air, and the gold-leaf electrometer before used could be opened
nearly a quarter of an inch.
424. The ice vessel employed (420.) was half an inch in width; as the
extent of contact of the ice with the tin and platina was nearly fourteen
square inches, the whole was equivalent to a plate of ice having a surface
of seven square inches, of perfect contact at each side, and only one
fourth of an inch thick. It was retained in a freezing mixture during the
experiment.
425. The order of arrangement in the course of the electric current was as
follows. The positive pole of the battery was connected by a wire with the
platina plate in the ice; the plate was in contact with the ice, the ice
with the tin jacket, the jacket with a wire, which communicated with a
piece of tin foil, on which rested one end of a bent platina wire (312.),
the other or decomposing end being supported on paper moistened with
solution of iodide of potassium (316.): the paper was laid flat on a
platina spatula connected with the negative end of the battery. All that
part of the arrangement between the ice vessel and the decomposing wire
point, including both these, was insulated, so that no electricity might
pass through the latter which had not traversed the former also.
426. Under these circumstances, it was found that, a pale brown spot of
iodine was slowly formed under the decomposing platina point, thus
indicating that ice could conduct a little of the electricity evolved by a
voltaic battery charged up to the degree of intensity indicated by the
electrometer. But it is quite evident that notwithstanding the enormous
quantity of electricity which the battery could furnish, it was, under
present circumstances, a very inferior instrument to the ordinary machine;
for the latter could send as much through the ice as it could carry, being
of a far higher intensity, i.e. able to open the electrometer leaves half
an inch or more (419. 420.).
427. The decomposing wire and solution of iodide of potassium were then
removed, and replaced by a very delicate galvanometer (205.); it was so
nearly astatic, that it vibrated to and fro in about sixty-three beats of a
watch giving one hundred and fifty beats in a minute. The same feebleness
of current as before was still indicated; the galvanometer needle was
deflected, but it required to break and make contact three or four times
(297.), before the effect was decided.
428. The galvanometer being removed, two platina plates were connected with
the extremities of the wires, and the tongue placed between them, so that
the whole charge of the battery, so far as the ice would let it pass, was
free to go through the tongue. Whilst standing on the stone floor, there
was shock, &c., but when insulated, I could feel no sensation. I think a
frog would have been scarcely, if at all, affected.
429. The ice was now removed, and experiments made with other solid bodies,
for which purpose they were placed under the end of the decomposing wire
instead of the solution of iodide of potassium (125.). For instance, a
piece of dry iodide of potassium was placed on the spatula connected with
the negative pole of the battery, and the point of the decomposing wire
placed upon it, whilst the positive end of the battery communicated with
the latter. A brown spot of iodine very slowly appeared, indicating the
passage of a little electricity, and agreeing in that respect with the
results obtained by the use of the electrical machine (421.). When the
galvanometer was introduced into the circuit at the same time with the
iodide, it was with difficulty that the action of the current on it could
be rendered sensible.
430. A piece of common salt previously fused and solidified being
introduced into the circuit was sufficient almost entirely to destroy the
action on the galvanometer. Fused and cooled chloride of lead produced the
same effect. The conducting power of these bodies, _when fluid_, is very
great (395. 402.).
431. These effects, produced by using the common machine and the voltaic
battery, agree therefore with each other, and with the law laid down in
this paper (394.); and also with the opinion I have supported, in the Third
Series of these Researches, of the identity of electricity derived from
different sources (360.).
432. The effect of heat in increasing the conducting power of many
substances, especially for electricity of high tension, is well known. I
have lately met with an extraordinary case of this kind, for electricity of
low tension, or that of the voltaic pile, and which is in direct contrast
with the influence of heat upon metallic bodies, as observed and described
by Sir Humphry Davy[A].
[A] Philosophical Transactions, 1821, p. 131.
433. The substance presenting this effect is sulphuret of silver. It was
made by fusing a mixture of precipitated silver and sublimed sulphur,
removing the film of silver by a file from the exterior of the fused mass,
pulverizing the sulphuret, mingling it with more sulphur, and fusing it
again in a green glass tube, so that no air should obtain access during the
process. The surface of the sulphuret being again removed by a file or
knife, it was considered quite free from uncombined silver.
434. When a piece of this sulphuret, half an inch in thickness, was put
between surfaces of platina, terminating the poles of a voltaic battery of
twenty pairs of four-inch plates, a galvanometer being also included in the
circuit, the needle was slightly deflected, indicating a feeble conducting
power. On pressing the platina poles and sulphuret together with the
fingers, the conducting power increased as the whole became warm. On
applying a lamp under the sulphuret between the poles, the conducting power
rose rapidly with the heat, and at last-the galvanometer needle jumped into
a fixed position, and the sulphuret was found conducting in the manner of a
metal. On removing the lamp and allowing the heat to fall, the effects were
reversed, the needle at first began to vibrate a little, then gradually
left its transverse direction, and at last returned to a position very
nearly that which it would take when no current was passing through the
galvanometer.
435. Occasionally, when the contact of the sulphuret with the platina poles
was good, the battery freshly charged, and the commencing temperature not
too low, the mere current of electricity from the battery was sufficient to
raise the temperature of the sulphuret; and then, without any application
of extraneous heat, it went on increasing conjointly in temperature and
conducting power, until the cooling influence of the air limited the
effects. In such cases it was generally necessary to cool the whole
purposely, to show the returning series of phenomena.
436. Occasionally, also, the effects would sink of themselves, and could
not be renewed until a fresh surface of the sulphuret had been applied to
the positive pole. This was in consequence of peculiar results of
decomposition, to which I shall have occasion to revert in the section on
Electro-chemical Decomposition, and was conveniently avoided by inserting
the ends of two pieces of platina wire into the opposite extremities of a
portion of sulphuret fused in a glass tube, and placing this arrangement
between the poles of the battery.
437. The hot sulphuret of silver conducts sufficiently well to give a
bright spark with charcoal, &c. &c., in the manner of a metal.
438. The native grey sulphuret of silver, and the ruby silver ore, both
presented the same phenomena. The native malleable sulphuret of silver
presented precisely the same appearances as the artificial sulphuret.
439. There is no other body with which I am acquainted, that, like
sulphuret of silver, can compare with metals in conducting power for
electricity of low tension when hot, but which, unlike them, during
cooling, loses in power, whilst they, on the contrary, gain. Probably,
however, many others may, when sought for, be found[A].
[A] See now on this subject, 1340, 1341.--_Dec. 1838._
440. The proto-sulphuret of iron, the native per-sulphuret of iron,
arsenical sulphuret of iron, native yellow sulphuret of copper and iron,
grey artificial sulphuret of copper, artificial sulphuret of bismuth, and
artificial grey sulphuret of tin, all conduct the voltaic battery current
when cold, more or less, some giving sparks like the metals, others not
being sufficient for that high effect. They did not seem to conduct better
when heated, than before; but I had not time to enter accurately into the
investigation of this point. Almost all of them became much heated by the
transmission of the current, and present some very interesting phenomena in
that respect. The sulphuret of antimony does not conduct the same current
sensibly either hot or cold, but is amongst those bodies acquiring
conducting power when fused (402.). The sulphuret of silver and perhaps
some others decompose whilst in the solid state; but the phenomena of this
decomposition will be reserved for its proper place in the next series of
these Researches.
441. Notwithstanding the extreme dissimilarity between sulphuret of silver
and gases or vapours, I cannot help suspecting the action of heat upon them
to be the same, bringing them all into the same class as conductors of
electricity, although with those great differences in degree, which are
found to exist under common circumstances. When gases are heated, they
increase in conducting power, both for common and voltaic electricity
(271.); and it is probable that if we could compress and condense them at
the same time, we should still further increase their conducting power.
Cagniard de la Tour has shown that a substance, for instance water, may be
so expanded by heat whilst in the liquid state, or condensed whilst in the
vaporous state, that the two states shall coincide at one point, and the
transition from one to the other be so gradual that no line of demarcation
can be pointed out[A]; that, in fact, the two states shall become
one;--which one state presents us at different times with differences in
degree as to certain properties and relations; and which differences are,
under ordinary circumstances, so great as to be equivalent to two different
states.
[A] Annales de Chimie, xxi. pp. 127, 178.
442. I cannot but suppose at present that at that point where the liquid
and the gaseous state coincide, the conducting properties are the same for
both; but that they diminish as the expansion of the matter into a rarer
form takes place by the removal of the necessary pressure; still, however,
retaining, as might be expected, the capability of having what feeble
conducting power remains, increased by the action of heat.
443. I venture to give the following summary of the conditions of electric
conduction in bodies, not however without fearing that I may have omitted
some important points[A].
[A] See now in relation to this subject, 1320--1242.--_Dec. 1838._
444. All bodies conduct electricity in the same manner from metals to lac
and gases, but in very different degrees.
445. Conducting power is in some bodies powerfully increased by heat, and
in others diminished, yet without our perceiving any accompanying essential
electrical difference, either in the bodies or in the changes occasioned by
the electricity conducted.
446. A numerous class of bodies, insulating electricity of low intensity,
when solid, conduct it very freely when fluid, and are then decomposed by
it.
447. But there are many fluid bodies which do not sensibly conduct
electricity of this low intensity; there are some which conduct it and are
not decomposed; nor is fluidity essential to decomposition[A].
[A] See the next series of these Experimental Researches.
448. There is but one body yet discovered[A] which, insulating a voltaic
current when solid, and conducting it when fluid, is not decomposed in the
latter case (414.).
[A] It is just possible that this case may, by more delicate
experiment, hereafter disappear. (See now, 1340, 1341, in relation to
this note.--_Dec. 1838._)
449. There is no strict electrical distinction of conduction which can, as
yet, be drawn between bodies supposed to be elementary, and those known to
be compounds.
_Royal Institution,
April 15, 1833_.