d it appeared to me that, when
fused, such acid was not a conductor, nor decomposed; but I had not enough
of the dry acid in my possession to allow me to decide the point
satisfactorily. My belief is, that when sulphur appears during the action
of the pile on sulphuric acid, it is the result of a secondary action, and
that the acid itself is not electrolyzable (757.).

[A] De la Rive.

682. Phosphoric acid is, I believe, also in the same condition; but I have
found it impossible to decide the point, because of the difficulty of
operating on fused anhydrous phosphoric acid. Phosphoric acid which has
once obtained water cannot be deprived of it by heat alone. When heated,
the hydrated acid volatilizes. Upon subjecting phosphoric acid, fused upon
the ring end of a wire (401.), to the action of the voltaic apparatus, it
conducted, and was decomposed; but gas, which I believe to be hydrogen, was
always evolved at the negative electrode, and the wire was not affected as
would have happened had phosphorus been separated. Gas was also evolved at
the positive electrode. From all the facts, I conclude it was the water and
not the acid which was decomposed.

683. _Arsenic acid_. This substance conducted, and was decomposed; but it
contained water, and I was unable at the time to press the investigation so
as to ascertain whether a fusible anhydrous arsenic acid could be obtained.
It forms, therefore, at present no exception to the general result.

684. Nitrous acid, obtained by distilling nitrate of lead, and keeping it
in contact with strong sulphuric acid, was found to conduct and decompose
slowly. But on examination there were strong reasons for believing that
water was present, and that the decomposition and conduction depended upon
it. I endeavoured to prepare a perfectly anhydrous portion, but could not
spare the time required to procure an unexceptionable result.

685. Nitric acid is a substance which I believe is not decomposed directly
by the electric current. As I want the facts in illustration of the
distinction existing between primary and secondary decomposition, I will
merely refer to them in this place (752.).

686. That these mineral acids should confer facility of conduction and
decomposition on water, is no proof that they are competent to favour and
suffer these actions in themselves. Boracic acid does the same thing,
though not decomposable. M. de la Rive has pointed out that chlorine has
this power also