Experiments on the magnetic inhibition of thermal convection in horizontal layers of mercury heated from below are described. A large 36½ in. cyclotron magnet reconditioned for hydromagnetic studies was used in these experiments. By using layers of mercury of depth 3 to 6 cm and magnetic fields of strength 500 to 8000 gauss, it has been possible to determine the dependence of the critical Rayleigh number for the onset of instability on the parameter
Q
1
( =
σH
2
d
2
/
π
2
ρν
, where
H
denotes the strength of the field,
σ
the electrical conductivity,
ν
the coefficient of kinematic viscosity,
ρ
the density and
d
the depth of the layer) for
Q
1
varying between 40 and 1·6 × 10
6
. The experiments fully confirm the theoretical relation derived by Chandrasekhar.