470.  On using solutions of iodide of potassium, acetate of lead, &c., similar effects were obtained; but as they were all consistent with the results above described, I refrain from describing the appearances minutely.

471.  These cases of electro-chemical decomposition are in their nature exactly of the same kind as those affected under ordinary circumstances by the voltaic battery, notwithstanding the great differences as to the presence or absence, or at least as to the nature of the parts usually called poles; and also of the final situation of the elements eliminated at the electrified boundary surfaces (467.).  They indicate at once an internal action of the parts suffering decomposition, and appear to show that the power which is effectual in separating the elements is exerted there, and not at the poles.  But I shall defer the consideration of this point for a short time (493. 518.), that I may previously consider another supposed condition of electro-chemical decomposition[A].

[A] I find (since making and describing these results,) from a note to Sir Humphry Davy’s paper in the Philosophical Transactions, 1807, p. 31, that that philosopher, in repeating Wollaston’s experiment of the decomposition of water by common electricity (327. 330.) used an arrangement somewhat like some of those I have described.  He immersed a guarded platina point connected with the machine in distilled water, and dissipated the electricity from the water into the air by moistened filaments of cotton.  In this way he states that he obtained oxygen and hydrogen separately from each other.  This experiment, had I known of it, ought to have been quoted in an earlier series of these Researches (342.); but it does not remove any of the objections I have made to the use of Wollaston’s apparatus as a test of true chemical action (331.).

P ii. Influence of Water in Electro-chemical Decomposition.

472.  It is the opinion of several philosophers, that the presence of water is essential in electro-chemical decomposition, and also for the evolution of electricity in the voltaic battery itself.  As the decomposing cell is merely one of the cells of the battery, into which particular substances are introduced for the purpose of experiment, it is probable that what is an essential condition in the one case is more or less so in the other.  The opinion, therefore, that water is necessary to decomposition, may have been founded on the statement made by Sir Humphry Davy, that “there are no fluids known, except such as contain water, which are capable of being made the medium of connexion between the metals or metal of the voltaic apparatus[A]:”  and again, “when any substance rendered fluid by heat, consisting of water, oxygen, and inflammable or metallic matter, is exposed to those wires, similar phenomena (of decomposition) occur[B].”

  [A] Elements of Chemical Philosophy, p. 160, &c.