tion should be
proportional to the quotient of the current by the velocity acquired by
an ion under unit potential gradient. Now this velocity is inversely
proportional to the pressure, so that the reaction should on this view
be directly proportional to the pressure. This agrees with Arrhenius'
results when the point is positive. Again, the velocities of an ion in
hydrogen, air and carbonic acid at the same pressure are approximately
inversely proportional to the square roots of their molecular weights,
so that the reaction should be directly proportional to this quantity.
This also agrees with Arrhenius' results for the discharge from a
positive point. The velocity of the negative ion is greater than that of
a positive one under the same potential gradient, so that the reaction
for the negative point should be less than that for a positive one, but
the excess of the positive reaction over the negative is much greater
than that of the velocity of the negative ion over the velocity of the
positive. There is, however, reason to believe that a considerable
condensation takes place around the negative ion as a nucleus after it
is formed, so that the velocity of the negative ion under a given
potential gradient will be greater immediately after the ion is formed
than when it has existed for some time. The measurements which have been
made of the velocities of the ions relate to those which have been some
time in existence, but a large part of the reaction will be due to the
newly-formed ions moving with a greater velocity, and thus giving a
smaller reaction than that calculated from the observed velocity.

With a given potential difference between the point and the neighbouring
conductor the current issuing from the point is greater when the point
is negative than when it is positive, except in oxygen, when it is less.
Warburg (_Sitz. Akad. d. Wissensch. zu Berlin_, 1899, 50, p. 770) has
shown that the addition of a small quantity of oxygen to nitrogen
produces a great diminution in the current from a negative point, but
has very little effect on the discharge from a positive point. Thus the
removal of a trace of oxygen made a leak from a negative point 50 times
what it was before. Experiments with hydrogen and helium showed that
impurities in these gases had a great effect on the current when the
point was negative, and but little when it was positive. This suggests
that the impurities, by condensing round the negative ions