tion, and the polarization caused by the first current on the
surface of the electrodes is destroyed before it rises to an appreciable
value. The polarization is also diminished in another way. The
electromotive force of polarization is due to the deposition of films of
the products of chemical decomposition on the surface of the electrodes,
and only reaches its full value when a continuous film is formed. If the
current be stopped before such a film is completed, the reverse
electromotive force is less than its full value. A given current flowing
for a given time deposits a definite amount of substance on the
electrodes, and therefore the amount per unit area is inversely
proportional to the area of the electrodes--to the area of contact, that
is, between the electrode and the liquid. Thus, by increasing the area
of the electrodes, the polarization due to a given current is decreased.
Now the area of free surface of a platinum plate can be increased
enormously by coating the plate with platinum black, which is metallic
platinum in a spongy state, and with such a plate as electrode the
effects of polarization are diminished to a very marked extent. The
coating is effected by passing an electric current first one way and
then the other between two platinum plates immersed in a 3% solution of
platinum chloride to which a trace of lead acetate is sometimes added.
The platinized plates thus obtained are quite satisfactory for the
investigation of strong solutions. They have the power, however, of
absorbing a certain amount of salt from the solutions and of giving it
up again when water or more dilute solution is placed in contact with
them. The measurement of very dilute solutions is thus made difficult,
but, if the plates be heated to redness after being platinized, a grey
surface is obtained which possesses sufficient area for use with dilute
solutions and yet does not absorb an appreciable quantity of salt.

Any convenient source of alternating current may be used. The currents
from the secondary circuit of a small induction coil are satisfactory,
or the currents of an alternating electric light supply may be
transformed down to an electromotive force of one or two volts. With
such currents it is necessary to consider the effects of self-induction
in the circuit and of electrostatic capacity. In balancing the
resistance of the electrolyte, resistance coils may be used in which
self-induction and the capacity are reduced t