778.  When aqueous solutions of bodies are used, secondary results are exceedingly frequent.  Even when the water is not present in large quantity, but is merely that of combination, still secondary results often ensue:  for instance, it is very possible that in Sir Humphry Davy’s decomposition of the hydrates of potassa and soda, a part of the potassium produced was the result of a secondary action.  Hence, also, a frequent cause for the disappearance of the oxygen and hydrogen which would otherwise be evolved:  and when hydrogen does not appear at the cathode in an aqueous solution, it perhaps always indicates that a secondary action has taken place there.  No exception to this rule has as yet occurred to my observation.

779.  Secondary actions are not confined to aqueous solutions, or cases where water is present.  For instance, various chlorides acted upon, when fused (402.), by platina electrodes, have the chlorine determined electrically to the anode.  In many cases, as with the chlorides of lead, potassium, barium, &c., the chlorine acts on the platina and forms a compound with it, which dissolves; but when protochloride of tin is used, the chlorine at the anode does not act upon the platina, but upon the chloride already there, forming a perchloride which rises in vapour (790. 804.).  These are, therefore, instances of secondary actions of both kinds, produced in bodies containing no water.

780.  The production of boron from fused borax (402. 417.) is also a case of secondary action; for boracic acid is not decomposable by electricity (408.), and it was the sodium evolved at the cathode which, re-acting on the boracic acid around it, took oxygen from it and set boron free in the experiments formerly described.

781.  Secondary actions have already, in the hands of M. Becquerel, produced many interesting results in the formation of compounds; some of them new, others imitations of those occurring naturally[A].  It is probable they may prove equally interesting in an opposite direction, i.e. as affording cases of analytic decomposition.  Much information regarding the composition, and perhaps even the arrangement, of the particles of such bodies as the vegetable acids and alkalies, and organic compounds generally, will probably be obtained by submitting them to the action of nascent oxygen, hydrogen, chlorine, &c. at the electrodes; and the action seems the more promising, because of the thorough command which we possess over attendant circumstances, such as the strength of the current, the size of the electrodes, the nature of the decomposing conductor, its strength, &c., all of which may be expected to have their corresponding influence upon the final result.

782.  It is to me a great satisfaction that the extreme variety of secondary results has presented nothing opposed to the doctrine of a constant and definite electro-chemical action, to the particular consideration of which I shall now proceed.