Evolution of Bodies at the Poles       75
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.
 270.	The theory I have ventured to put forth appears to me
to explain all the prominent features of electro-chemical decom-
position in a satisfactory manner.
 271.	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.
 272.	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 decompos-
ing 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 gravita-
tion, magnetism, cohesion, chemical affinity, etc., 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, etc., 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 there-
fore in their particular action reveal the general principle of
decomposition.
 273.	But in the theory that I have just given, the effect
appears to be a natural consequence of the action: the evolved
substances are expelled from the decomposing mass (254, 255),
not drawn out by an attraction which ceases to act on one
particle without any assignable reason, while it continues to act
on another of the same kind: and whether the poles be metal,
water, or air, still the substances are evolved, and are sometimes
set free, whilst at others they unite to the matter of the poles,
according to the chemical nature of the latter, i.e. their chemical