  In a space-time of two dimensions the overall effect of the collision of two
solitons is a time delay (or advance) of their final trajectories relative to
their initial trajectories. For the solitons of affine Toda field theories, the
space-time displacement of the trajectories is proportional to the logarithm of
a number $X$ depending only on the species of the colliding solitons and their
rapidity difference. $X$ is the factor arising in the normal ordering of the
product of the two vertex operators associated with the solitons. $X$ is shown
to take real values between $0$ and $1$. This means that, whenever the solitons
are distinguishable, so that transmission rather than reflection is the only
possible interpretation of the classical scattering process, the time delay is
negative and so an indication of attractive forces between the solitons.
