  Above a critical matter density the propagating modes of the neutral kaon
system are essentially eigenstates of strangeness, but below it they are almost
complete eigenstates of CP. We estimate the real and imaginary parts of the
energies of these modes and their mixing at all densities up to nuclear matter
density $2\times 10^{14}$ g/cm$^3$. In a heavy ion collision the strong
interactions create eigenstates of strangeness, and these propagate
adiabatically until the density has fallen to the critical value, whereupon the
system undergoes a sudden transition to (near) eigenstates of CP. We estimate
the critical density to be 20 g/cm$^3$, and that this density will be reached
about $2\times 10^5$ fm/c after the end of the collision.
