n against them.

675. Amongst powerful compounds which are not decomposed, boracic acids
stand prominent (408.). Then again, the iodide of sulphur, and the
chlorides of sulphur, phosphorus, and carbon, are not decomposable under
common circumstances, though their elements are of a nature which would
lead to a contrary expectation. Chloride of antimony (402. 690.), the
hydro-carbons, acetic acid, ammonia, and many other bodies undecomposable
by the voltaic pile, would seem to be formed by an affinity sufficiently
strong to indicate that the elements were so far contrasted in their nature
as to sanction the expectation that, the pile would separate them,
especially as in some cases of mere solution (530. 544.), where the
affinity must by comparison be very weak, separation takes place[A].

[A] With regard to solution, I have met with some reasons for
supposing that it will probably disappear as a cause of transference,
and intend resuming the consideration at a convenient opportunity.

676. It must not be forgotten, however, that much of this difficulty, and
perhaps the whole, may depend upon the absence of conducting power, which,
preventing the transmission of the current, prevents of course the effects
due to it. All known compounds being non-conductors when solid, but
conductors when liquid, are decomposed, with _perhaps_ the single exception
at present known of periodide of mercury (679. 691.)[A]; and even water
itself, which so easily yields up its elements when the current passes, if
rendered quite pure, scarcely suffers change, because it then becomes a
very bad conductor.

[A] See now, 1340, 1341.--_Dec. 1838._

677. If it should hereafter be proved that the want of decomposition in
those cases where, from chemical considerations, it might be so strongly
expected (669, 672. 674.), is due to the absence or deficiency of
conducting power, it would also at the same time be proved that
decomposition _depends_ upon conduction, and not the latter upon the former
(413.); and in water this seems to be very nearly decided. On the other
hand, the conclusion is almost irresistible, that in electrolytes the power
of transmitting the electricity across the substance is _dependent_ upon
their capability of suffering decomposition; taking place only whilst they
are decomposing, and being proportionate to the quantity of elements
separated (821.). I may not, however, stop to discuss this point
experimentally at prese