749.  As another illustration of secondary results, the effects on a solution of acetate of potassa, may be quoted.  When a very strong solution was used, more gas was evolved at the anode than at the cathode, in the proportion of 4 to 3 nearly:  that from the anode was a mixture of carbonic oxide and carbonic acid; that from the cathode pure hydrogen.  When a much weaker solution was used, less gas was evolved at the anode than at the cathode; and it now contained carburetted hydrogen, as well as carbonic oxide and carbonic acid.  This result of carburetted hydrogen at the positive electrode has a very anomalous appearance, if considered as an immediate consequence of the decomposing power of the current.  It, however, as well as the carbonic oxide and acid, is only a secondary result; for it is the water alone which suffers electro-decomposition, and it is the oxygen eliminated at the anode which, reacting on the acetic acid, in the midst of which it is evolved, produces those substances that finally appear there.  This is fully proved by experiments with the volta-electrometer (707.); for then the hydrogen evolved from the acetate at the cathode is always found to be definite, being exactly proportionate to the electricity which has passed through the solution, and, in quantity, the same as the hydrogen evolved in the volta-electrometer itself.  The appearance of the carbon in combination with the hydrogen at the positive electrode, and its non-appearance at the negative electrode, are in curious contrast with the results which might have been expected from the law usually accepted respecting the final places of the elements.

750.  If the salt in solution be an acetate of lead, then the results at both electrodes are secondary, and cannot be used to estimate or express the amount of electro-chemical action, except by a circuitous process (843.).  In place of oxygen or even the gases already described (749.), peroxide of lead now appears at the positive, and lead itself at the negative electrode.  When other metallic solutions are used, containing, for instance, peroxides, as that of copper, combined with this or any other decomposable acid, still more complicated results will be obtained; which, viewed as direct results of the electro-chemical action, will, in their proportions, present nothing but confusion, but will appear perfectly harmonious and simple if they be considered as secondary results, and will accord in their proportions with the oxygen and hydrogen evolved from water by the action of a definite quantity of electricity.

751.  I have experimented upon many bodies, with a view to determine whether the results were primary or secondary.  I have been surprised to find how many of them, in ordinary cases, are of the latter class, and how frequently water is the only body electrolyzed in instances where other substances have been supposed to give way.  Some of these results I will give in as few words as possible.