Small‐scale experiments have been carried out for determining the minimum vertical thermal gradient which is required to cause convection in liquids entrapped in porous media. Observations relative to the onset of convective flow in unconsolidated sands indicate that the present theories predict minimum gradients which are excessive by considerable amounts, possibly because they neglect the temperature‐dependence of viscosity. The ratio of theoretical to observed gradients is found to be roughlyR=(Kh2&mgr;AV/k&rgr;&agr;)0.51,whereh2is the thermal diffusivity, &mgr;Avis average viscosity,kis flow‐permeability, &rgr; is density of liquid, &agr; is the coefficient of cubical expansion of the liquid, and whereK= 10−3sec.2per cm2°C for c.g.s. units. By extrapolation, it is possible to strengthen the earlier conclusion that convection occurs in the Woodbine sand near the Mexia fault zone.