eight of dissolved substance,
that is, to the concentration of the solution.

~Electrolysis of solutions.~ Pure water does not appreciably conduct the
electric current. If, however, certain substances such as common salt
are dissolved in the water, the resulting solutions are found to be
conductors of electricity. Such solutions are called _electrolytes_.
When the current passes through an electrolyte some chemical change
always takes place. This change is called _electrolysis_.

[Illustration: Fig. 31]

The general method used in the electrolysis of a solution is illustrated
in Fig. 31. The vessel D contains the electrolyte. Two plates or rods,
A and B, made of suitable material, are connected with the wires
from a battery (or dynamo) and dipped into the electrolyte, as shown in
the figure. These plates or rods are called _electrodes_. The electrode
connected with the zinc plate of the battery is the negative electrode
or _cathode_, while that connected with the carbon plate is the positive
electrode or _anode_.

~Theory of electrolytic dissociation.~ The facts which have just been
described in connection with solutions, together with many others, have
led chemists to adopt a theory of solutions called _the theory of
electrolytic dissociation_. The main assumptions in this theory are the
following.

1. _Formation of ions._ Many compounds when dissolved in water undergo
an important change. A portion of their molecules fall apart, or
_dissociate_, into two or more parts, called _ions_. Thus sodium nitrate
(NaNO_{3}) dissociates into the ions Na and NO_{3}; sodium chloride,
into the ions Na and Cl. These ions are free to move about in the
solution independently of each other like independent molecules, and for
this reason were given the name ion, which signifies a wanderer.

2. _The electrical charge of ions._ Each ion carries a heavy electrical
charge, and in this respect differs from an atom or molecule. It is
evident that the sodium in the form of an ion must differ in some
important way from ordinary sodium, for sodium ions, formed from sodium
nitrate, give no visible evidence of their presence in water, whereas
metallic sodium at once decomposes the water. The electrical charge,
therefore, greatly modifies the usual chemical properties of the
element.

3. _The positive charges equal the negative charges._ The ions formed by
the dissociation of any molecule are of two kinds. One kind is charged
with positive