[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. [B] Ibid. pp. 144, 145. 473. This opinion has, I think, been shown by other philosophers not to be accurate, though I do not know where to refer for a contradiction of it. Sir Humphry Davy himself said in 1801[A], that dry nitre, caustic potash and soda are conductors of galvanism when rendered fluid by a high degree of heat, but he must have considered them, or the nitre at least, as not suffering decomposition, for the statements above were made by him eleven years subsequently. In 1826 he also pointed out, that bodies not containing water, as _fused litharge_ and _chlorate of potassa_, were sufficient to form, with platina and zinc, powerful electromotive circles[B]; but he is here speaking of the _production_ of electricity in the pile, and not of its effects when evolved; nor do his words at all imply that any correction of his former distinct statements relative to _decomposition_ was required. [A] Journal of the Royal Institution, 1802, p. 53. [B] Philosophical Transactions, 1826, p. 406.