  A recently proposed world-volume equivalence principal involving the Boillat,
as opposed to the Einstein, metric is examined in the context of some colliding
wave solutions of the Born-Infeld equations for which two plane polarized
pulses pass through one another without distortion. They suffer a delay with
respect to the usual Einstein metric but not with respect to the Boillat
metric. Both metrics are flat in this case, and the closed string and open
string causal structures are interchanged by the Legendre transformation that
is used for solving the associated Monge-Amp\`ere equation. In 1+1 dimensions
the equations are known to be equivalent to the vorticity free motion of a
Chaplygin gas. The latter is shown to be described by the scalar Born-Infeld
equation in all dimensions and it is pointed out that the equation of state is
Hagedorn-like: there is an upper bound to the pressure and temperature.
