549.  It may be expressed as a general consequence, that the more directly bodies are opposed to each other in chemical affinity, the more ready is their separation from each other in cases of electro-chemical decomposition, i.e. provided other circumstances, as insolubility, deficient conducting power, proportions, &c., do not interfere.  This is well known to be the case with water and saline solutions; and I have found it to be equally true with dry chlorides, iodides, salts, &c., rendered subject to electro-chemical decomposition by fusion (402.).  So that in applying the voltaic battery for the purpose of decomposing bodies not yet resolved into forms of matter simpler than their own, it must be remembered, that success may depend not upon the weakness, or failure upon the strength, of the affinity by which the elements sought for are held together, but contrariwise; and then modes of application may be devised, by which, in association with ordinary chemical powers, and the assistance of fusion (394. 417.), we may be able to penetrate much further than at present into the constitution of our chemical elements.

550.  Some of the most beautiful and surprising cases of electro-chemical decomposition and transfer which Sir Humphry Davy described in his celebrated paper[A], were those in which acids were passed through alkalies, and alkalies or earths through acids[B]; and the way in which substances having the most powerful attractions for each other were thus prevented from combining, or, as it is said, had their natural affinity destroyed or suspended throughout the whole of the circuit, excited the utmost astonishment.  But if I be right in the view I have taken of the effects, it will appear, that that which made the wonder, is in fact the essential condition of transfer and decomposition, and that the more alkali there is in the course of an acid, the more will the transfer of that acid be facilitated from pole to pole; and perhaps a better illustration of the difference between the theory I have ventured, and those previously existing, cannot be offered than the views they respectively give of such facts as these.

  [A] Philosophical Transactions, 1807, p. 1.

  [B] Ibid. p, 24, &c.

551.  The instances in which sulphuric acid could not be passed though baryta, or baryta through sulphuric acid[A], because of the precipitation of sulphate of baryta, enter within the pale of the law already described (380. 412.), by which liquidity is so generally required for conduction and decomposition.  In assuming the solid state of sulphate of baryta, these bodies became virtually non-conductors to electricity of so low a tension as that of the voltaic battery, and the power of the latter over them was almost infinitely diminished.

  [A] Philosophical Transactions, 1807, p. 25, &c.