is a
case in point, however, and shows most clearly that the sulphuric acid and
magnesia contributed to each other's transfer and final evolution, exactly
as the same acid and soda affected each other in the results just given
(527, &c.); and that so soon as the magnesia advanced beyond the reach of
the acid, and found no other substance with which it could combine, it
appeared in its proper character, and was no longer able to continue its
progress towards the negative pole.

* * * * *

534. The theory I have ventured to put forth appears to me to explain all
the prominent features of electro-chemical decomposition in a satisfactory
manner.

535. In the first place, it explains why, in all ordinary cases, the
evolved substances _appear only at the poles_; for the poles are the
limiting surfaces of the decomposing substance, and except at them, every
particle finds other particles having a contrary tendency with which it can
combine.

536. Then it explains why, in numerous cases, the elements or evolved
substances are not _retained_ by the poles; and this is no small difficulty
in those theories which refer the decomposing effect directly to the
attractive power of the poles. If, in accordance with the usual theory, a
piece of platina be supposed to have sufficient power to attract a particle
of hydrogen from the particle of oxygen with which it was the instant
before combined, there seems no sufficient reason, nor any fact, except
those to be explained, which show why it should not, according to analogy
with all ordinary attractive forces, as those of gravitation, magnetism,
cohesion, chemical affinity, &c. _retain_ that particle which it had just
before taken from a distance and from previous combination. Yet it does not
do so, but allows it to escape freely. Nor does this depend upon its
assuming the gaseous state, for acids and alkalies, &c. are left equally at
liberty to diffuse themselves through the fluid surrounding the pole, and
show no particular tendency to combine with or adhere to the latter. And
though there are plenty of cases where combination with the pole does take
place, they do not at all explain the instances of non-combination, and do
not therefore in their particular action reveal the general principle of
decomposition.

537. But in the theory that I have just given, the effect appears to be a
natural consequence of the action: the evolved substances are _expel