56 RAYS OF POSITIVE ELECTRICITY

pass presumably, since it is a line spectrum, originates from
atoms and not molecules. These atoms cannot be moving with
velocities at all comparable with those of the particles in the
positive rays, for otherwise there would be an appreciable
broadening of these lines. Wien (" Ann. der Phys.," 43,
p. 955) investigated this point for lines given out by mercury
and helium and came to the conclusion that there was no
perceptible broadening. Fie could have detected easily the
effect if the atoms giving out the light had possessed velocities
of the order they would have acquired by collision with the
positive ray particles provided these collisions had been like
those between elastic spheres. Hence we conclude that those
collisions which result in the absorption of the positive rays
do not split up the molecules of the gas into charged atoms.
This is in accordance with the conclusions we drew (p. 54)
from Seeliger's measurements of the ionization produced by
positive rays.

On the other hand the collisions which result in a loss or
gain of charge by the positive ray particles, where, as we have
seen, the transference of energy from the particles to the
molecules is exceedingly small, not only ionize the gas but
split the molecules up into atoms,

We should expect that the particle would not be able to
lose or gain a charge unless its velocity exceeded a certain
critical value, for either of these charges involves the
tearing of an electron out of an atom or molecule. When the
particle loses its charge the electron is torn from the mole-
cules of the gas through which it is moving, when it regains
its charge the electron has to be torn from the particle.

To tear an electron from an atom or molecule requires
a finite amount of work, and in the case we are considering
this work must be supplied by the moving particle during
a collision with the molecule. Since there is little change of