Just above the liquid-vapor critical point, a fluid's large compressib
ility causes a stable stratification in which the density varies by as
much as 10% in 1 cm. This stratification supports internal gravity wa
ves which we observed with an oscillator immersed in a near-critical x
enon sample. We found the number and frequencies of the observable mod
es depended on the sample cell's orientation, with only two modes seen
in the horizontal cell. The frequencies of the two modes had differen
t temperature dependences: with decreasing temperature, the higher fre
quency increased monotonically from 0.7 to 2.8 Hz, but the lower frequ
ency varied nonmonotonically, with a maximum of 1.0 Hz at 20 mK above
the critical temperature. These temperature dependences continued to 2
0 mK below the critical temperature, where the xenon was separated int
o liquid and vapor phases. We calculated these two frequencies by solv
ing the eigenvalue problem of internal waves in a box containing a str
atified fluid. The fluid's density profile was obtained from xenon's e
quation of state. The calculated and measured frequencies agree to wit
hin 15%. Analytical calculations based on simple approximations of the
density profile provide insight into the observed temperature depende
nces. (C) 1996 American Institute of Physics.