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 conducting power, its miscibility with most of the substances acted upon, and its general relation to them in respect to chemical affinity.  It consists of elements, which in their electrical and chemical relations are directly and powerfully opposed, yet combining to produce a body more neutral in its character than any other.  So that there are but few substances which do not come into relation, by chemical affinity, with water or one of its elements; and therefore either the water or its elements are transferred and assist in transferring the infinite variety of bodies which, in association with it, can be placed in the course of the electric current.  Hence the reason why it so rarely happens that the evolved substances rest at the first surface of the water, and why it therefore does not exhibit the ordinary action of a pole.

559.  Air, however, and some gases are free from the latter objection, and may be used as poles in many cases (461, &c.); but, in consequence of the extremely low degree of conducting power belonging to them, they cannot be employed with the voltaic apparatus.  This limits their use; for the voltaic apparatus is the only one as yet discovered which supplies sufficient quantity of electricity (371. 376.) to effect electro-chemical decomposition with facility.