Convection patterns in ethanol-water mixtures with negative psi are st
udied when the fluid is heated from above. Although the linear analysi
s predicts that the instability occurs at zero wave number, a large wa
ve number pattern is observed. The onset is supercritical with a thres
hold that is experimentally indistinguishable from zero. The convectio
n amplitude exhibits damped oscillations for sudden change in the forc
ing parameter. At the constant Rayleigh number the patterns first coar
sen, then exhibit growth of narrow plumes. The instability appears to
be related to salt fingering.