i 34	Faraday's Researches
the oxygen and hydrogen from water are primary results; and
so also are the acid and alkali (themselves compound bodies)
evolved from sulphate of soda. But when the substances
separated by the current are changed at the electrodes before
their appearance, then they give rise to secondary results,
although in many cases the bodies evolved are elementary.
 479.	These secondary results occur in two ways, being some-
times due to the mutual action of the evolved substance and
the matter of the electrode, and sometimes to its action upon
the substances contained in the body itself under decomposition.
Thus, when carbon is made the positive electrode in dilute
sulphuric acid, carbonic oxide and carbonic acid occasionally
appear there instead of oxygen; for the latter, acting upon the
matter of the electrode, produces these secondary results.   Or
if the positive electrode, in a solution of nitrate or acetate of
lead, be platina, then peroxide of lead appears there, equally
a secondary result with the former, but now depending upon an
action of the oxygen on a substance in the solution.   Again,
when ammonia is decomposed by platina electrodes, nitrogen
appears at the anode ; x but though an elementary body, it is a
secondary result in this case, being derived from the chemical
action  of  the  oxygen  electrically  evolved  there, upon the
ammonia  in  the  surrounding  solution (290).   In the  same
manner when aqueous solutions of metallic salts are decomposed
by the current, the metals evolved at the cathode, though
elements, are always secondary results, and not immediate
consequences of the decomposing power of the electric current.
 480.	Many of these secondary results are extremely valuable;
for instance, all the interesting compounds which M. Becquerel
has obtained by feeble electric currents are of this nature; but
they are essentially chemical, and must, in the theory of elec-
trolytic action, be carefully distinguished from those which are
directly due to the action of the electric current.
 481.	The nature of the substances evolved will often lead to
a correct judgment of their primary or secondary character, but
is not sufficient alone to establish that point.   Thus, nitrogen
is said to be attracted sometimes by the positive and sometimes
by the negative electrode, according to the bodies with which
it may be combined (290, 291), and it is on such occasions
evidently viewed as a primary result;2 but I think I shall show
that, when it appears at the positive electrode, or rather at the
anode, it is a secondary result (483).   Thus, also, Sir Humphry
1 Annales de Chimie, 1804, torn. li. p. 167.	2 Ibid. p. 172.