nt.

678. When a compound contains such elements as are known to pass towards
the opposite extremities of the voltaic pile, still the proportions in
which they are present appear to be intimately connected with capability in
the compound of suffering or resisting decomposition. Thus, the
protochloride of tin readily conducts, and is decomposed (402.), but the
perchloride neither conducts nor is decomposed (406.). The protiodide of
tin is decomposed when fluid (402.); the periodide is not (405.). The
periodide of mercury when fused is not decomposed (691.), even though it
does conduct. I was unable to contrast it with the protiodide, the latter
being converted into mercury and periodide by heat.

679. These important differences induced me to look more closely to certain
binary compounds, with a view of ascertaining whether a _law_ regulating
the _decomposability_ according to some _relation of the proportionals or
equivalents_ of the elements, could be discovered. The proto compounds
only, amongst those just referred to, were decomposable; and on referring
to the substances quoted to illustrate the force and generality of the law
of conduction and decomposition which I discovered (402.), it will be found
that all the oxides, chlorides, and iodides subject to it, except the
chloride of antimony and the periodide of mercury, (to which may now
perhaps be added corrosive sublimate,) are also decomposable, whilst many
per compounds of the same elements, not subject to the law, were not so
(405. 406.).

680. The substances which appeared to form the strongest exceptions to this
general result were such bodies as the sulphuric, phosphoric, nitric,
arsenic, and other acids.

681. On experimenting with sulphuric acid, I found no reason to believe
that it was by itself a conductor of, or decomposable by, electricity,
although I had previously been of that opinion (552.). When very strong it
is a much worse conductor than if diluted[A]. If then subjected to the
action of a powerful battery, oxygen appears at the _anode_, or positive
electrode, although much is absorbed (728.), and hydrogen and sulphur
appear at the _cathode_, or negative electrode. Now the hydrogen has with
me always been pure, not sulphuretted, and has been deficient in proportion
to the sulphur present, so that it is evident that when decomposition
occurred water must have been decomposed. I endeavoured to make the
experiment with anhydrous sulphuric acid; an