n the electric
force in the uniform part of the discharge, when the force is
comparatively insignificant. This consideration explains a very
striking phenomenon discovered many years ago by Hittorf, who found
that if he put a wire carrying a bead of a volatile salt into the
flame, it produced little effect upon the current, unless it were
placed close to the cathode where it gave rise to an enormous increase
in the current, sometimes increasing the current more than a
hundredfold. The introduction of the salt increases very largely the
number of ions produced, so that q is much greater for a salted flame
than for a plain one. Thus Hittorf's result coincides with the
conclusions we have drawn from the theory of this class of conduction.

The fall of potential at the cathode is proportional to i - i0, where
i0 is the stream of negative electricity which comes from the cathode
itself, thus as i0 increases the fall of potential at the cathode
diminishes and the current sent by a given potential difference
through the gas increases. Now all metals give out negative particles
when heated, at a rate which increases very rapidly with the
temperature, but at the same temperature some metals give out more
than others. If the cathode is made of a metal which emits large
quantities of negative particles, (i - i0) will for a given value of i
be smaller than if the metal only emitted a small number of
particles; thus the cathode fall will be smaller for the metal with
the greater emissitivity, and the relation between the potential
difference and the current will be different in the two cases. These
considerations are confirmed by experience, for it has been found that
the current between electrodes immersed in a flame depends to a great
extent upon the metal of which the electrodes are made. Thus
Pettinelli (_Acc. dei Lincei_ [5], v. p. 118) found that, _ceteris
paribus_, the current between two carbon electrodes was about 500
times that between two iron ones. If one electrode was carbon and the
other iron, the current when the carbon was cathode and the iron anode
was more than 100 times the current when the electrodes were reversed.
The emission of negative particles by some metallic oxides, notably
those of calcium and barium, has been shown by Wehnelt (_Ann. der
Phys._ 11, p. 425) to be far greater than that of any known metal, and
the increase of current pr