part of the molecule of the dissolved salt
with an attendant atmosphere of solvent round it. The conductivity of a
salt solution depends on two factors--(1) the fraction of the salt
ionized; (2) the velocity with which the ions, when free from each
other, move under the electric forces.[12] When a solution is heated,
both these factors may change. The coefficient of ionization usually,
though not always, decreases; the specific ionic velocities increase.
Now the rate of increase with temperature of these ionic velocities is
very nearly identical with the rate of decrease of the viscosity of the
liquid. If the curves obtained by observations at ordinary temperatures
be carried on they indicate a zero of fluidity and a zero of ionic
velocity about the same point, 38.5 deg. C. below the freezing point of
water (Kohlrausch, _Sitz. preuss. Akad. Wiss._, 1901, 42, p. 1026). Such
relations suggest that the frictional resistance to the motion of an ion
is due to the ordinary viscosity of the liquid, and that the ion is
analogous to a body of some size urged through a viscous medium rather
than to a particle of molecular dimensions finding its way through a
crowd of molecules of similar magnitude. From this point of view W. K.
Bousfield has calculated the sizes of ions on the assumption that
Stokes's theory of the motion of a small sphere through a viscous medium
might be applied (_Zeits. phys. Chem._, 1905, 53, p. 257; _Phil. Trans._
A, 1906, 206, p. 101). The radius of the potassium or chlorine ion with
its envelope of water appears to be about 1.2 X 10^-8 centimetres.

For the bibliography of electrolytic conduction see ELECTROLYSIS. The
books which deal more especially with the particular subject of the
present article are _Das Leitvermogen der Elektrolyte_, by F.
Kohlrausch and L. Holborn (Leipzig, 1898), and _The Theory of Solution
and Electrolysis_, by W. C. D. Whetham (Cambridge, 1902).
(W. C. D. W.)

III. ELECTRIC CONDUCTION THROUGH GASES

A gas such as air when it is under normal conditions conducts
electricity to a small but only to a very small extent, however small
the electric force acting on the gas may be. The electrical conductivity
of gases not exposed to special conditions is so small that it was only
definitely established in the early years of the 20th century, although
it had engaged the attention of physicists for more than a hundred
years. It had been known for a long time that a bo