d is concentrated
in patches, it is evident that the ions will recombine more quickly in
the second case than in the first, and that the value of [alpha] will
be different in the two cases. This probably explains the large values
of [alpha] obtained by Retschinsky, who ionized the gas by the [alpha]
rays from radium, a method which produces very patchy ionization.

_Variation of [alpha] with the Pressure of the Gas._--All observers
agree that there is little variation in [alpha] with the pressures for
pressures of between 5 and 1 atmospheres; at lower pressures, however,
the value of [alpha] seems to diminish with the pressure: thus
Langevin (_Ann. chim. phys._, 1903, 28, p. 287) found that at a
pressure of 1/5 of an atmosphere the value of [alpha] was about 1/5 of
its value at atmospheric pressure.

_Variation of [alpha] with the Temperature._--Erikson (_Phil. Mag._,
Aug. 1909) has shown that the value of [alpha] for air increases as
the temperature diminishes, and that at the temperature of liquid air
-180 deg. C., it is more than twice as great as at +12 deg. C.

Since, as we have seen, the recombination is due to the coming
together of the positive and negative ions under the influence of the
electrical attraction between them, it follows that a large electric
force sufficient to overcome this attraction would keep the ions apart
and hence diminish the coefficient of recombination. Simple
considerations, however, will show that it would require exceedingly
strong electric fields to produce an appreciable effect. The value of
[alpha] indicates that for two oppositely charged ions to unite they
must come within a distance of about 1.5 X 10^-6 centimetres; at this
distance the attraction between them is e^2 X 10^12/2.25, and if X is
the external electric force, the force tending to pull them apart
cannot be greater than Xe; if this is to be comparable with the
attraction, X must be comparable with e X 10^12/2.25, or putting e = 4
X 10^-10, with 1.8 X 10^2; this is 54,000 volts per centimetre, a
force which could not be applied to gas at atmospheric pressure
without producing a spark.

_Diffusion of the Ions._--The ionized gas acts like a mixture of
gases, the ions corresponding to two different gases, the non-ionized
gas to a third. If the concentration of the ions is not uniform, they
will diffuse through the non-ionized gas in such a way as to