  It was recently found that in sulphur-induced nuclear collisions at 200 A GeV
the observed strange hadron abundances can be explained within a thermodynamic
model where baryons and mesons separately are in a state of relative chemical
equilibrium, with overall strangeness being slightly undersaturated, but
distributed among the strange hadron channels according to relative chemical
equilibrium with a vanishing strange quark chemical potential. We develop a
consistent thermodynamic formulation of the concept of relative chemical
equilibrium and show how to introduce into the partition function deviations
from absolute chemical equilibrium, e.~g.~an undersaturation of overall
strangeness or the breaking of chemical equilibrium between mesons and baryons.
We then proceed to test on the available data the hypothesis that the strange
quark chemical potential vanishes everywhere, and that the rapidity
distributions of all the observed hadrons can be explained in terms of one
common, rapidity-dependent function $\mu_{\rm q}(\eta)$ for the baryon chemical
potential only. The aim of this study is to shed light on the observed strong
rapidity dependence of the strange baryon ratios in the NA36 experiment.
