sary, the junction line being visible owing to the difference in
the optical refractive indices of two colourless solutions. Once the
boundary is formed, too, no gelatine is necessary, and the motion can be
watched through liquid aqueous solutions (see R. B. Denison and B. D.
Steele, _Phil. Trans._, 1906).

All the direct measurements which have been made on simple binary
electrolytes agree with Kohlrausch's results within the limits of
experimental error. His theory, therefore, probably holds good in such
cases, whatever be the solvent, if the proper values are given to the
ionic velocities, i.e. the values expressing the velocities with which
the ions actually move in the solution of the strength taken, and under
the conditions of the experiment. If we know the specific velocity of
any one ion, we can deduce, from the conductivity of very dilute
solutions, the velocity of any other ion with which it may be
associated, a proceeding which does not involve the difficult task of
determining the migration constant of the compound. Thus, taking the
specific ionic velocity of hydrogen as 0.00032 cm. per second, we can
find, by determining the conductivity of dilute solutions of any acid,
the specific velocity of the acid radicle involved. Or again, since we
know the specific velocity of silver, we can find the velocities of a
series of acid radicles at great dilution by measuring the conductivity
of their silver salts.

By such methods W. Ostwald, G. Bredig and other observers have found
the specific velocities of many ions both of inorganic and organic
compounds, and examined the relation between constitution and ionic
velocity. The velocity of elementary ions is found to be a periodic
function of the atomic weight, similar elements lying on corresponding
portions of a curve drawn to express the relation between these two
properties. Such a curve much resembles that giving the relation
between atomic weight and viscosity in solution. For complex ions the
velocity is largely an additive property; to a continuous additive
change in the composition of the ion corresponds a continuous but
decreasing change in the velocity. The following table gives Ostwald's
results for the formic acid series:--

Table XII.

+----------------------+----------+---------------------+
| | Velocity.| Difference for CH2. |
+----------------------+----------+---------------------+