  Atomic vapors are systems well suited for nonlinear optics studies but very
few direct measurements of their nonlinear refractive index have been reported.
Here we use the z-scan technique to measure the Kerr coefficient, $n_2$, for a
Cs vapor. Our results are analyzed through a four-level model, and we show that
coherence between excited levels as well as cross-population effects contribute
to the Kerr-nonlinearity.
