214	Faraday's Researches
and yet involve no contradiction to the law of definite electro-
lytic action, is perfectly consistent. All the facts and also the
theory I have ventured to put forth, tend to show that the act
of decomposition opposes a certain force to the passage of the
electric current; and, that this obstruction should be overcome
more or less readily, in proportion to the greater or less intensity
of the decomposing current, is in perfect consistency with all
our notions of the electric agent.
 732.	I have elsewhere (682) distinguished the chemical action
of zinc and dilute sulphuric acid into two portions; that which,
acting effectually on the zinc, evolves hydrogen at once upon
its surface, and that which, producing an arrangement of the
chemical forces throughout the electrolyte present (in this case
water), tends to take oxygen from it, but cannot do so unless
the electric current consequent thereon can have free passage,
and the hydrogen be delivered elsewhere than against the zinc.
The electric current depends altogether upon the second of
these; but when the current can pass, by favouring the electro-
lytic action it tends to diminish the former and increase the
latter portion.
 733.	It is evident, therefore, that when ordinary zinc is used
in a voltaic arrangement, there is an enormous waste of that
power which it is the object to throw into the form of an electric
current;   a consequence which is put in its strongest point of
view when it is considered that three ounces and a half of zinc,
properly oxydised, can circulate enough electricity to decompose
nearly one ounce of water, and cause the evolution of about
2400 cubic inches of hydrogen gas.    This loss of power not
only takes place during the time the electrodes of the battery
are in communication, being then proportionate to the quantity
of hydrogen evolved against the surface of any one of the zinc
plates, but includes also all the chemical action which goes on
when the extremities of the pile are not in communication.
 734.	This loss is far greater with ordinary zinc than with the
pure metal, as M. de la Rive has shown.1   The cause is, that
when ordinary zinc is acted upon by dilute sulphuric acid,
portions of copper, lead, cadmium, or other metals which it
may contain, are set free upon its surface; and these, being in
contact with the zinc, form small but very active voltaic circles,
which cause great destruction of the zinc and evolution of
hydrogen, apparently upon the zinc surface, but really upon the
1 Quarterly Journal of Science, 1831, p. 388;  or Bibliotheque Universelle,
1830, p. 391.