either pole; but in consequence of the numerous compounds which it forms,
some of which pass to one pole, and some to the other, I have not always
found it easy to determine the true circumstances of its appearance. A pure
strong solution of ammonia is so bad a conductor of electricity that it is
scarcely more decomposable than pure water; but if sulphate of ammonia be
dissolved in it, then decomposition takes place very well; nitrogen almost
pure, and in some cases quite, is evolved at the positive pole, and
hydrogen at the negative pole.

555. On the other hand, if a strong solution of nitrate of ammonia be
decomposed, oxygen appears at the positive pole, and hydrogen, with
sometimes nitrogen, at the negative pole. If fused nitrate of ammonia be
employed, hydrogen appears at the negative pole, mingled with a little
nitrogen. Strong nitric acid yields plenty of oxygen at the positive pole,
but no gas (only nitrous acid) at the negative pole. Weak nitric acid
yields the oxygen and hydrogen of the water present, the acid apparently
remaining unchanged. Strong nitric acid with nitrate of ammonia dissolved
in it, yields a gas at the negative pole, of which the greater part is
hydrogen, but apparently a little nitrogen is present. I believe, that in
some of these cases a little nitrogen appeared at the negative pole. I
suspect, however, that in all these, and in all former cases, the
appearance of the nitrogen at the positive or negative pole is entirely a
secondary effect, and not an immediate consequence of the decomposing power
of the electric current[A].

[A] Refer for proof of the truth of this supposition to 748, 752,
&c.--_Dec. 1838._

556. A few observations on what are called the _poles_ of the voltaic
battery now seem necessary. The poles are merely the surfaces or doors by
which the electricity enters into or passes out of the substance suffering
decomposition. They limit the extent of that substance in the course of the
electric current, being its _terminations_ in that direction: Hence the
elements evolved pass so far and no further.

557. Metals make admirable poles, in consequence of their high conducting
power, their immiscibility with the substances generally acted upon, their
solid form, and the opportunity afforded of selecting such as are not
chemically acted upon by ordinary substances.

558. Water makes a pole of difficult application, except in a few cases
(494.), because of its small conduct