689. _Protochloride of carbon_ does not conduct nor suffer decomposition.
In association with this substance, I submitted the _hydro-chloride of
carbon_ from olefiant gas and chlorine to the action of the electric
current; but it also refused to conduct or yield up its elements.
600. With regard to the exceptions (679.), upon closer examination some of
them disappear. Chloride of antimony (a compound of one proportional of
antimony and one and a half of chlorine) of recent preparation was put into
a tube (fig. 68.) (789.), and submitted when fused to the action of the
current, the positive electrode being of plumbago. No electricity passed,
and no appearance of decomposition was visible at first; but when the
positive and negative electrodes were brought very near each other in the
chloride, then a feeble action occurred and a feeble current passed. The
effect altogether was so small (although quite amenable to the law before
given (394.)), and so unlike the decomposition and conduction occurring in
all the other cases, that I attribute it to the presence of a minute
quantity of water, (for which this and many other chlorides have strong
attractions, producing hydrated chlorides,) or perhaps of a true
protochloride consisting of single proportionals (695, 796.).
691. _Periodide of mercury_ being examined in the same manner, was found
most distinctly to insulate whilst solid, but conduct when fluid, according
to the law of _liquido-conduction_ (402.); but there was no appearance of
decomposition. No iodine appeared at the _anode_, nor mercury or other
substance at the _cathode_. The case is, therefore, no exception to the
rule, that only compounds of single proportionals are decomposable; but it
is an exception, and I think the only one, to the statement, that all
bodies subject to the law of liquido-conduction are decomposable. I
incline, however, to believe, that a portion of protiodide of mercury is
retained dissolved in the periodide, and that to its slow decomposition the
feeble conducting power is due. Periodide would be formed, as a secondary
result, at the _anode_; and the mercury at the _cathode_ would also form,
as a secondary result, protiodide. Both these bodies would mingle with the
fluid mass, and thus no final separation appear, notwithstanding the
continued decomposition.
692. When _perchloride of mercury_ was subjected to the voltaic current, it
did not conduct in the solid state, but it did conduct when fluid. I think,
also, that in the latter case it was decomposed; but there are many
interfering circumstances which require examination before a positive
conclusion can be drawn[A].
[A] With regard to perchloride and periodide of mercury, see now 1340,
1341.--_Dec. 1838._
693. When the ordinary protoxide of antimony is subjected to the voltaic
current in a fused state, it also is decomposed, although the effect from
other causes soon ceases (402, 801.). This oxide consists of one
proportional of antimony and one and a half of oxygen, and is therefore an
exception to the general law assumed. But in working with this oxide and
the chloride, I observed facts which lead me to doubt whether the compounds
usually called the protoxide and the protochloride do not often contain
other compounds, consisting of single proportions, which are the true proto
compounds, and which, in the case of the oxide, might give rise to the
decomposition above described.
694. The ordinary sulphuret of antimony its considered as being the
compound with the smallest quantity of sulphur, and analogous in its
proportions to the ordinary protoxide. But I find that if it be fused with
metallic antimony, a new sulphuret is formed, containing much more of the
metal than the former, and separating distinctly, when fused, both from the
pure metal on the one hand, and the ordinary gray sulphuret on the other.
In some rough experiments, the metal thus taken up by the ordinary
sulphuret of antimony was equal to half the proportion of that previously
in the sulphuret, in which case the new sulphuret would consist of _single_
proportionals.
695. When this new sulphuret was dissolved in muriatic acid, although a
little antimony separated, yet it appeared to me that a true protochloride,
consisting of _single_ proportionals, was formed, and from that by
alkalies, &c., a true protoxide, consisting also of _single_ proportionals,
was obtainable. But I could not stop to ascertain this matter strictly by
analysis.
696. I believe, however, that there is such an oxide; that it is often
present in variable proportions in what is commonly called protoxide,
throwing uncertainty upon the results of its analysis, and causing the
electrolytic decomposition above described[A].
[A] In relation to this and the three preceding paragraphs, and also
801, see Berzelius's correction of the nature of the supposed now
sulphuret and oxide, Phil. Mag. 1836, vol. viii. 476: and for the
probable explanation of the effects obtained with the protoxide, refer
to 1340, 1341.--_Dec. 1838._
697. Upon the whole, it appears probable that all those binary compounds of
elementary bodies which are capable of being electrolyzed when fluid, but
not whilst solid, according to the law of liquido-conduction (394.),
consist of single proportionals of their elementary principles; and it may
be because of their departure from this simplicity of composition, that
boracic acid, ammonia, perchlorides, periodides, and many other direct
compounds of elements, are indecomposable.
698. With regard to salts and combinations of compound bodies, the same
simple relation does not appear to hold good. I could not decide this by
bisulphates of the alkalies, for as long as the second proportion of acid
remained, water was retained with it. The fused salts conducted, and were
decomposed; but hydrogen always appeared at the negative electrode.
699. A biphosphate of soda was prepared by heating, and ultimately fusing,
the ammonia-phosphate of soda. In this case the fused bisalt conducted, and
was decomposed; but a little gas appeared at the negative electrode; and
though I believe the salt itself was electrolyzed, I am not quite satisfied
that water was entirely absent.
700. Then a biborate of soda was prepared; and this, I think, is an
unobjectionable case. The salt, when fused, conducted, and was decomposed,
and gas appeared at both electrodes: even when the boracic acid was
increased to three proportionals, the same effect took place.
701. Hence this class of compound combinations does not seem to be subject
to the same simple law as the former class of binary combinations. Whether
we may find reason to consider them as mere solutions of the compound of
single proportionals in the excess of acid, is a matter which, with some
apparent exceptions occurring amongst the sulphurets, must be left for
decision by future examination.
702. In any investigation of these points, great care must be taken to
exclude water; for if present, secondary effects are so frequently produced
as often seemingly to indicate an electro-decomposition of substances, when
no true result of the kind has occurred (742, &c.).
703. It is evident that all the cases in which decomposition _does not
occur, may_ depend upon the want of conduction (677. 413.); but that does
not at all lessen the interest excited by seeing the great difference of
effect due to a change, not in the nature of the elements, but merely in
their proportions; especially in any attempt which may be made to elucidate
and expound the beautiful theory put forth by Sir Humphry Davy[A], and
illustrated by Berzelius and other eminent philosophers, that ordinary
chemical affinity is a mere result of the electrical attractions of the
particles of matter.
[A] Philosophical Transactions, 1807, pp. 32, 39; also 1826, pp. 387,
389.
¶ v. _On a new measure of Volta-electricity._
704. I have already said, when engaged in reducing common and voltaic
electricity to one standard of measurement (377.), and again when
introducing my theory of electro-chemical decomposition (504. 505. 510.),
that the chemical decomposing action of a current _is constant for a
constant quantity of electricity_, notwithstanding the greatest variations
in its sources, in its intensity, in the size of the _electrodes_ used, in
the nature of the conductors (or non-conductors (307.)) through which it is
passed, or in other circumstances. The conclusive proofs of the truth of
these statements shall be given almost immediately (783, &c.).
705. I endeavoured upon this law to construct an instrument which should
measure out the electricity passing through it, and which, being interposed
in the course of the current used in any particular experiment, should
serve at pleasure, either as a _comparative standard_ of effect, or as a
_positive measurer_ of this subtile agent.
706. There is no substance better fitted, under ordinary circumstances, to
be the indicating body in such an instrument than water; for it is
decomposed with facility when rendered a better conductor by the addition
of acids or salts; its elements may in numerous cases be obtained and
collected without any embarrassment from secondary action, and, being
gaseous, they are in the best physical condition for separation and
measurement. Water, therefore, acidulated by sulphuric acid, is the
substance I shall generally refer to, although it may become expedient in
peculiar cases or forms of experiment to use other bodies (843.).
707. The first precaution needful in the construction of the instrument was
to avoid the recombination of the evolved gases, an effect which the
positive electrode has been found so capable of producing (571.). For this
purpose various forms of decomposing apparatus were used. The first
consisted of straight tubes, each containing a plate and wire of platina
soldered together by gold, and fixed hermetically in the glass at the
closed extremity of the tube (Plate V. fig. 60.). The tubes were about
eight inches long, 0.7 of an inch in diameter, and graduated. The platina
plates were about an inch long, as wide as the tubes would permit, and
adjusted as near to the mouths of the tubes as was consistent with the safe
collection of the gases evolved. In certain cases, where it was required to
evolve the elements upon as small a surface as possible, the metallic
extremity, instead of being a plate, consisted of the wire bent into the
form of a ring (fig. 61.). When these tubes were used as measurers, they
were filled with the dilute sulphuric acid, inverted in a basin of the same
liquid (fig. 62.), and placed in an inclined position, with their mouths
near to each other, that as little decomposing matter should intervene as
possible; and also, in such a direction that the platina plates should be
in vertical planes (720).
708. Another form of apparatus is that delineated (fig. 63.). The tube is
bent in the middle; one end is closed; in that end is fixed a wire and
plate, _a_, proceeding so far downwards, that, when in the position
figured, it shall be as near to the angle as possible, consistently with
the collection, at the closed extremity of the tube, of all the gas evolved
against it. The plane of this plate is also perpendicular (720.). The other
metallic termination, _b_, is introduced at the time decomposition is to be
effected, being brought as near the angle as possible, without causing any
gas to pass from it towards the closed end of the instrument. The gas
evolved against it is allowed to escape.
709. The third form of apparatus contains both electrodes in the same tube;
the transmission, therefore, of the electricity, and the consequent
decomposition, is far more rapid than in the separate tubes. The resulting
gas is the sum of the portions evolved at the two electrodes, and the
instrument is better adapted than either of the former as a measurer of the
quantity of voltaic electricity transmitted in ordinary cases. It consists
of a straight tube (fig. 64.) closed at the upper extremity, and graduated,
through the sides of which pass platina wires (being fused into the glass),
which are connected with two plates within. The tube is fitted by grinding
into one mouth of a double-necked bottle. If the latter be one-half or
two-thirds full of the dilute sulphuric acid (706.), it will, upon
inclination of the whole, flow into the tube and fill it. When an electric
current is passed through the instrument, the gases evolved against the
plates collect in the upper portion of the tube, and are not subject to the
recombining power of the platina.