e electrode; or else their combination with those bodies, which being
contained in, or associated with, the body suffering decomposition, are
necessarily present at the _anode_ and _cathode_. The complexity is
rendered still greater by the circumstance that two or more of these
actions may occur simultaneously, and also in variable proportions to each
other. But it may in a great measure be resolved by attention to the
principles already laid down (747.).

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 inte