he acids and alkalies
themselves[A]. Nevertheless the case of sulphate of magnesia is in point
(494. 495.), and shows that _one element or principle only_ has no power of
transference or of passing towards either pole.

[A] See the note to (670.).--_Dec. 1838._

545. Many of the metals, however, in their solid state, offer very fair
instances of the kind required. Thus, if a plate of platina be used as the
positive pole in a solution of sulphuric acid, oxygen will pass towards it,
and so will acid; but these are not substances having such chemical
relation to the platina as, even under the favourable condition
superinduced by the current (518. 524.), to combine with it; the platina
therefore remains where it was first placed, and has no tendency to pass
towards the negative pole. But if a plate of iron, zinc or copper, be
substituted for the platina, then the oxygen and acid can combine with
these, and the metal immediately begins to travel (as an oxide) to the
opposite pole, and is finally deposited there. Or if, retaining the platina
pole, a fused chloride, as of lead, zinc, silver, &c., be substituted for
the sulphuric acid, then, as the platina finds an element it can combine
with, it enters into union, acts as other elements do in cases of voltaic
decomposition, is rapidly transferred across the melted matter, and
expelled at the negative pole.

546. I can see but little reason in the theories referring the
electro-chemical decomposition to the attractions and repulsions of the
poles, and I can perceive none in M. de la Rive's theory, why the metal of
the positive pole should not be transferred across the intervening
conductor, and deposited at the negative pole, even when it cannot act
chemically upon the element of the fluid surrounding it. It cannot be
referred to the attraction of cohesion preventing such an effect; for if
the pole be made of the lightest spongy platina, the effect is the same. Or
if gold precipitated by sulphate of iron be diffused through the solution,
still accumulation of it at the negative pole will not take place; and yet
the attraction of cohesion is almost perfectly overcome, the particles are
in it so small as to remain for hours in suspension, and are perfectly free
to move by the slightest impulse towards either pole; and _if in relation_
by chemical affinity to any substance present, are powerfully determined to
the negative pole[A].

[A] In making this experiment, care must