ne, and illustrates in a
beautiful manner their progress in opposite directions, parallel to the
electric current, which is for the time giving a uniform general direction
to their mutual affinities (524.).

542. According to my theory, an element or a substance not decomposable
under the circumstances of the experiment, (as for instance, a dilute acid
or alkali,) should not be transferred, or pass from pole to pole, unless it
be in chemical relation to some other element or substance tending to pass
in the opposite direction, for the effect is considered as essentially due
to the mutual relation of such particles. But the theories attributing the
determination of the elements to the attractions and repulsions of the
poles require no such condition, i.e. there is no reason apparent why the
attraction of the positive pole, and the repulsion of the negative pole,
upon a particle of free acid, placed in water between them, should not
(with equal currents of electricity) be as strong as if that particle were
previously combined with alkali; but, on the contrary, as they have not a
powerful chemical affinity to overcome, there is every reason to suppose
they would be stronger, and would sooner bring the acid to rest at the
positive pole[A]. Yet such is not the case, as has been shown by the
experiments on free and combined acid (526. 528.).

[A] Even Sir Humphry Davy considered the attraction of the pole as
being communicated from one particle to another of the _same_ kind
(483.).

543. Neither does M. de la Rive's theory, as I understand it, _require_
that the particles should be in combination: it does not even admit, where
there are two sets of particles capable of combining with and passing by
each other, that they do combine, but supposes that they travel as separate
compounds of matter and electricity. Yet in fact the free substance
_cannot_ travel, the combined one _can_.

544. It is very difficult to find cases amongst solutions or fluids which
shall illustrate this point, because of the difficulty of finding two
fluids which shall conduct, shall not mingle, and in which an element
evolved from one shall not find a combinable element in the other.
_Solutions_ of acids or alkalies will not answer, because they exist by
virtue of an attraction; and increasing the solubility of a body in one
direction, and diminishing it in the opposite, is just as good a reason for
transfer, as modifying the affinity between t