.

The preceding tables show that the negative ions diffuse more rapidly
than the positive, especially in dry gases. The superior mobility of
the negative ions was observed first by Zeleny (_Phil. Mag._, 1898
[5], 46, p. 120), who showed that the velocity of the negative ions
under an electric force is greater than that of the positive. It will
be noticed that the difference between the mobility of the negative
and the positive ions is much more pronounced in dry gases than in
moist. The difference in the rates of diffusion of the positive and
negative ions is the reason why ionized gas, in which, to begin with,
the positive and negative charges were of equal amounts, sometimes
becomes electrified even although the gas is not acted upon by
electric forces. Thus, for example, if such gas be blown through
narrow tubes, it will be positively electrified when it comes out, for
since the negative ions diffuse more rapidly than the positive, the
gas in its passage through the tubes will lose by diffusion more
negative than positive ions and hence will emerge positively
electrified. Zeleny snowed that this effect does not occur when, as in
carbonic acid gas, the positive and negative ions diffuse at the same
rates. Townsend (loc. cit.) showed that the coefficient of diffusion
of the ions is the same whether the ionization is produced by Rontgen
rays, radioactive substances, ultra-violet light, or electric sparks.
The ions produced by chemical reactions and in flames are much less
mobile; thus, for example, Bloch (_Ann. chim. phys._, 1905 [8], 4, p.
25) found that for the ions produced by drawing air over phosphorus
the value of [alpha]/e was between 1 and 6 instead of over 3000, the
value when the air was ionized by Rontgen rays.

_Velocity of Ions in an Electric Field._--The velocity of ions in an
electric field, which is of fundamental importance in conduction, is
very closely related to the coefficient of diffusion. Measurements of
this velocity for ions produced by Rontgen rays have been made by
Rutherford (_Phil. Mag._ [5], 44, p. 422), Zeleny (_Phil. Mag._ [5], 46,
p. 120), Langevin (_Ann. Chim. Phys._, 1903, 28, p. 289), Phillips
(_Proc. Roy. Soc._ 78, A, p. 167), and Wellisch (_Phil. Trans._, 1909,
209, p. 249). The ions produced by radioactive substance have been
investigated by Rutherford (_Phil. Mag._ [5], 47, p. 109) and by Franck
and Pohl (_Verh. deutsch. phys.