8 2	Faraday's Researches
a secondary effect, and not an immediate consequer
decomposing power of the electric current.1
 292.	A few observations on what are called the pc
voltaic battery now seem necessary.   The poles are r
surfaces or doors by which the electricity enters intc
out of the substance suffering decomposition.   They
extent of that substance in the course of the electri
being its terminations in that direction:   hence the
evolved pass so far and no further.
 293.	Metals make admirable poles, in consequenc
high conducting power, their immiscibility with the s
generally acted upon, their solid form, and the oj
afforded of selecting such as are not chemically actec
ordinary substances.
 294.	Water makes a pole of difficult application, e
few cases (230), because of its small conducting  •
miscibility with most of the substances acted upco
general relation to them in respect to chemical ai
consists of elements, which in their electrical and
relations are directly and powerfully opposed, yet
to produce a body more neutral in its character than
So that there are but few substances which do not
relation, by chemical affinity, with water or one of its
and therefore either the water or its elements are 1
and assist in transferring the infinite variety of bod
in association with it, can be placed in the course of 1
current.   Hence the reason why it so rarely happen
evolved substances rest at the first surface of the ^
why it therefore does not exhibit the ordinary action
 295.	Air, however, and some gases are free from
objection, and may be used as poles in many cases (
but, in consequence of the extremely low degree of <
power belonging to them, they cannot be employee
voltaic  apparatus.   This  limits  their  use;    for  t
apparatus is the only one as yet discovered whic
sufficient quantity of electricity (107, 112) to effe
chemical decomposition with facility.
296.	When the poles are liable to the chemical
the substances evolved, either simply in consequen
natural relation to them, or of that relation aided by tl:
1 Refer for proof of the truth of this supposition to 483,
December 1838.